NAMS task force report on vector borne diseases

Epidemiology

Over the last 20 years, the global incidence of dengue fever has significantly risen, spreading across 129 countries. It is endemic in over 100 nations within the World Health Organization (WHO), including regions of Africa, the Americas, the Eastern Mediterranean, South-East Asia, and the Western Pacific, leading to an estimated 100–400 million infections annually.¹⁶ The surge in dengue cases has been striking, increasing eightfold from year 2000 (505,430 cases) to year 2019 (5.2 million cases).¹⁷ India is a major contributor to the global dengue burden, grappling with a high prevalence of the disease. The country harbors multiple co-circulating serotypes as well as many genotypes.^15,18-20 Dengue has become hyper-endemic in the country.^20,21 According to the report by NCVBDC, as of September 17, 2023, 94,198 cases of dengue infection were reported in India, with 91 deaths.²² Kerala reported the highest number of cases (9770), more than double compared to the numbers seen in the last five years.^22,23 The escalation of dengue cases can be attributed to various factors such as enhanced global trade, increased travel, rapid urbanization, population growth, and climate variability. These elements create favorable conditions for the multiplication of Aedes mosquitoes as well as the survival of the virus.^24,25 Periodic outbreaks are now common in India, especially during the monsoon and post-monsoon seasons.^24,26

Clinical symptoms

Clinical signs and symptoms of dengue infection are usually asymptomatic or range from mild fever to severe bleeding, shock, and organ dysfunction. The incubation period varies from 4-6 (range 3-14) days. There are three clinical phases of dengue infection. The febrile phase is characterized by a high-grade fever (>38.5°C), which may be biphasic. This phase usually lasts for 2-7 days and may be associated with non-specific symptoms like vomiting, headache, myalgia, arthralgia, and maculopapular rash. Depending upon the severity of the disease, bleeding manifestations may occur during this phase. A few infected patients (5-10%) may progress to the critical phase. This is commonly reported among those with a positive previous history of dengue infection. This phase may last for 24-48 hours and is characterized by vasculopathy and coagulopathy, leading to bleeding, shock, and organ dysfunction. In the end, there is a convalescent-phase where the patient starts recovering. This usually lasts for 2-3 days. The clinical presentation in this phase is influenced by factors such as the infected person’s immunity, previous dengue infection, and the viral genotype. This highlights the importance of accurate categorization of patients into different phases of infection for further medical intervention. Symptomatic dengue infection could be either dengue without warning signs (mild dengue) or dengue with warning signs (moderate and severe dengue). Mild dengue is characterized by mild fever, though some individuals may remain asymptomatic. Moderate dengue encompasses additional symptoms such as abdominal tenderness, hepatomegaly, persistent vomiting, pleural effusion, ascites, positive tourniquet test, hypovolemia, hypoproteinemia, and signs of shock. Severe dengue, the most critical form, is marked by significant bleeding, profound shock, severe metabolic disorder, and organ involvement, including hepatic impairment and central nervous system (CNS) manifestations.¹⁵

Diagnosis and management

Accurate diagnostic techniques are pivotal in effectively identifying the infection, aiding in early detection, confirming cases, and distinguishing it from other infections. At present, various laboratory-based methods such as serological detection of non-structural protein 1 (NS1) Antigen, IgM antibodies, and nucleic acid amplification test are available.^27,28 The tourniquet test procedure has also been employed as a clinical method for dengue infection, though its utility is debatable.²⁹ Conventional diagnosis of active dengue infection is mostly done by laboratory-based tests. For the identification of infection during the acute phase (from day 1 of fever to 9^th day of infection), NS1 antigen detection is most commonly used. IgM antibody-based assays are also commonly utilized but only after five days of fever. Sometimes, IgM antibodies might persist beyond 1 to 2 months of infection, therefore, antigen-based assays are more specific for the identification of acute ongoing infection. Polymerase chain reaction (PCR) based assays are used only for epidemiological purposes or for serotyping/genotyping and should be done during the acute phase of infection, which is less than five days of fever. Many commercial enzyme-linked immunosorbent assays (ELISA), as well as rapid card-based assays for both NS1 antigen as well as IgM antibody detection, are available in the market with varying sensitivity and specificity^30,31. In fact, there is limited availability of good rapid card tests for the combined detection of NS1 antigen and IgM antibody, which are needed for the rapid diagnosis and decentralization of testing for dengue infections in public health settings.^32-34 IgG antibody-based assays are no longer used for the diagnosis of infections. Clinical management of dengue is mostly supportive care, such as fluid replacement and the use of analgesics, with few requiring hospitalization in case of severe dengue infection to prevent further complications.^35,36 Till date, there are no clinically approved antivirals present for the treatment of dengue infection.³⁷ A variety of antiviral drug candidates have been tried in the past, but most of the studies were either in-silico or in-vitro analyses; in-vivo studies using animal models are a major limitation in the development of any suitable candidate with good antiviral properties for dengue infection.³⁸ Several herbal preparations in the recent years have also been tried for the treatment of dengue infection, but none has been approved so far.^38,39

Preventive measures

In the absence of specific antiviral treatment, prevention is the only recourse to control this infection. Dengvaxia [chimeric yellow fever/dengue tetravalent dengue], a live attenuated vaccine (LAV), is the sole licensed vaccine at present available for dengue, but it is recommended for individuals with prior exposure to dengue infection.^40,41 Encouragingly, there are ongoing clinical trials for several new candidates for the dengue vaccine. Among these, QDenga (TAK-003), which is also an LAV vaccine candidate has shown encouraging results in both safety and efficacy against symptomatic dengue.⁴² It is the second vaccine to get pre-qualified by WHO (https://www.who.int/news/item/15-05-2024-who-prequalifies-new-dengue-vaccine). Three leading Indian vaccine producers — Serum Institute, Panacea Biotec, and Biological E—have obtained non-exclusive licenses for the clinical development and commercialization of TetraVax-DV, a dengue vaccine developed by the US National Institutes of Health.⁴³ Furthermore, Sun Pharma, in partnership with the Indian Council for Genetic Engineering and Biotechnology, has developed and acquired a license for the DSV4 vaccine. DSV4, a synthetic subunit vaccine incorporating dengue virus antigens, has shown effectiveness in stimulating both humoral and cellular immunity.⁴⁴

Bio-control measures

Various bio-control measures have been implemented to curb the proliferation of Aedes mosquitoes in India. An effective approach involves eradicating breeding sites of A. aegypti within human habitats.⁴⁵ Additionally, the introduction of larvivorous fish has proven successful in controlling the mosquito population by consuming mosquito larvae.⁴⁵ Biological control strategies such as herbal remedies and the release of sterile male Aedes have also been deployed.⁴⁶ India has also actively pursued the implementation of Wolbachia as a biocontrol strategy to manage mosquito breeding.⁴⁷ The efficacy of Wolbachia release programs depends on the adaptability of Wolbachia strains in their natural environment. The Indian Council for Medical Research (ICMR) has been pivotal in this effort, establishing two colonies of A. aegypti mosquitoes infected with the wMel and wAlbB Wolbachia strains. These colonies, designed to assess the impact of heat stress on Wolbachia, have shown promising outcomes.⁴⁸ To refine our understanding of the most efficient method for mosquito control as per our countries’ weather conditions, additional field studies are needed.

Challenges

India faces significant challenges in combating DENV infections, with the absence of specific antiviral treatments or an approved vaccine despite the disease’s growing status as a critical public health problem.²⁶ Moreover, there’s a struggle in utilizing the approved dengue vaccine, Dengvaxia, in the country. One major hurdle lies in the dynamic nature of dengue serotypes in India. Since 2000, all four dengue serotypes (DENV-1, DENV-2, DENV-3, and DENV-4) have been co-circulating, causing cyclical outbreaks every 3-4 years.^20,49 The circulating serotypes keep changing yearly and from region to region across different states in the country.²⁰ Before 2012, DENV-1 and DENV-3 were dominant, but after 2012, DENV-2 became the most prevalent serotype in most of the regions, but southern states in India saw the emergence of the DENV-4 serotype.⁴⁹ Few studies from North India highlighted DENV-2 as the most prevalent serotype in circulation, followed by DENV-3 and DENV-1, with DENV-4 being detected sporadically.^50-52

The currently available vaccine against dengue infection, Dengvaxia exhibited a skewed response towards protection against DENV-2 and DENV-4 serotypes, and this limited its use to subjects with prior exposure to dengue infection.^53,54 Moreover, information about its role in preventing severe outcomes like hemorrhagic manifestation was lacking.⁴³ Therefore, the current immunization program in the country does not support the implementation of the dengue vaccine. More research and extensive efforts should be done for the development of tailor-made vaccines suitable for the epidemiological needs of our country.

Additionally, the number of underdiagnosed cases contributes to the underestimated burden of DENV infection. Most of the studies are hospital-based assessments of infection. Therefore, large-scale multicentre studies combining prevalence, clinical spectra, and follow-ups should be executed to find out the actual state-wise disease burden. There is a need to initiate community-based cohort studies representing different geographic regions as well as different age groups to generate dengue seroprevalence data in the country. Challenges of dengue control in India involve periodic outbreaks and poor vector control.²⁶ These challenges have significant socioeconomic impacts during each outbreak, demanding efficient disease surveillance, early diagnosis, and accurate management to mitigate the illness’s severity and clinical consequences.^55,56 Therefore, effective disease surveillance, early detection, and proper management are critical to curb dengue’s impact in India.

Epidemiology

The CHIKV, initially identified in Tanzania during the 1950s, is endemic in various global regions. It’s categorized into three main genotypes: West African, East Central South African (ECSA), and Asian. The virus is predominantly found in tropical and subtropical areas, notably in Africa, Southeast Asia, the Indian subcontinent, and the Pacific Region.⁵⁹ The ECSA genotype of CHIKV migrated from Africa to the Indian Ocean islands in 2004, causing a substantial epidemic.⁵⁹ Similarly, the Asian genotype has been prevalent in several Asian countries and the ECSA genotype triggered a widespread outbreak in Malaysia in 2008.^60,61 As of November 30, 2023, approximately 460,000 CHIKV cases and over 360 deaths have been reported globally.⁶² CHIKV garnered attention due to its swift global spread. In India, CHIKV outbreaks are traced back to the early 20th century, with the first reported outbreak in Kolkata (Calcutta), West Bengal, in 1963.⁶³ After a long hiatus, CHIKV re-emerged in India in December 2005, affecting nearly 1.4 million people across 13 states, leading to significant economic losses.⁶⁴ The infection transcended state borders, reaching 28 out of 29 suspected states by 2018.⁶⁵ In 2019, although the number of affected states with CHIKV infection declined to 25, suspected cases increased from 57,813 to 81,914, and confirmed cases rose from the year 2018 (9756) to 2019 (12,205).⁶⁵ As per the NCVBDC published data in 2023 (till September 17), there were 93,455 suspected cases spread across 28 states, with only 3,711 confirmed cases in 25 states. The states with the highest number of confirmed cases were Karnataka, Maharashtra, and Tamil Nadu. Additionally, Andhra Pradesh, Gujarat, Haryana, Jharkhand, Punjab, Rajasthan, and Telangana reported a significant number of confirmed cases.⁶⁵

Clinical symptoms

Most common clinical features of CHIKV infection include fever with joint pain. Other symptoms are rash, headache, and muscle pain. Some individuals may experience debilitating joint pain that persists for months or even years. Treatment includes rest, fluids, and over-the-counter medications for pain and fever. Sometimes it can also cause severe disease and prolonged health problems, particularly in elderly patients and in patients with underlying medical conditions. India faces the challenge of overlapping seasons for chikungunya, dengue, and malaria, all of which are now endemic. Chikungunya and dengue share a common vector and exhibit similar clinical presentations. Therefore, differential diagnosis by clinicians becomes crucial, relying on varied clinical presentations and laboratory methods to initiate appropriate treatment and prevent complications like hemorrhage, acute respiratory distress syndrome renal failure, etc.⁶⁶

Diagnosis and management

Laboratory diagnosis for CHIKV involves serological methods such as CHIKV-specific IgM ELISA post five days of infection, and IgG antibody ELISA for sero-diagnosis using paired acute and convalescent phase serum samples. Cross-reactivity with other arboviruses affects the sensitivity of the serological assays.^67,68 PCR-based techniques, specifically reverse transcription PCR (RT-PCR) and virus isolation by culture-based techniques, are primarily used for epidemiological or research purposes.^69,70 After confirmation, symptomatic treatment remains the primary approach for management. Preventive methods include mosquito control measures. Despite the global availability of many vaccine candidates, that are in the pipeline, there are currently no ongoing trials for any of the vaccine in India.^71,72

Preventive measures

Multiple CHIKV vaccines are in development; the ground-breaking approval of the Ixchiq vaccine by the US Food and Drug Administration in November 2023 marks a significant milestone in combating this disease. This vaccine is a LAV developed by a French specialty vaccine company Valneva, in Saint-Herblain. The intended use of this vaccine is for individuals aged 18 years and above and at higher risk of exposure. The vaccine targets all three genotypes of the CHIKV.^71,73 Clinical trials have shown remarkable efficacy of this vaccine, with 98.9% of recipients meeting the zero response threshold in contrast to 0% in the placebo arm. This demonstrates a robust immune response triggered by the vaccine.⁷⁴ This approval signifies a breakthrough in combating chikungunya, offering a promising preventive measures against this debilitating illness.

Challenges

Despite being self-limiting and associated with low mortality, CHIKV poses a challenge due to the painful illness course and long-term sequelae, which could impact the quality of life. New diagnostic and clinical protocols to differentiate CHIKV from other similar illnesses like dengue and Zika virus are the need of the hour. In addition, any adaptive changes in the viral genome that could impact several intervention and control measures should be investigated. The NCVBDC data reveals that Karnataka consistently leads in confirmed cases of Chikungunya, followed by Maharashtra and Punjab.^65,75 A lack of specific case definitions to differentiate between co-existing arboviruses like dengue, Zika, etc., as well as a low proportion of lab-confirmed cases could contribute to either under- or over-reporting of cases. Well co-ordinated surveillance activities should be done for all the VBDs in the country. The absence of specific antiviral treatment and effective vaccination adds to the complexity of addressing the challenges of Chikungunya in the country.⁷¹ Although developing a CHIKV vaccine is technically less challenging than for dengue,⁷² factors such as genetic diversity, limited funding and resources, difficulties in performing vaccine efficacy studies due to unpredictable outbreak patterns, and a lack of long-term surveillance programs and corresponding data in Chikungunya-affected regions with sporadic transmission pose a significant challenge.^57,73,75 These challenges emphasize the critical need to address the rising prevalence of Chikungunya in India by enhancing public awareness, strengthening healthcare infrastructure, and mitigating the economic impact of the disease.

Epidemiology

JE traces its first isolation back to 1935, when it was identified in the brain of a person with fatal encephalitis in Japan.⁷⁸ In India, the first case of JE occurred in 1955 in Vellore, Tamil Nadu, with a major outbreak in 1973 reported in the Burdwan district of West Bengal. Since then, JE/AES occurrences have been documented across 327 districts in 24 states.⁷⁹ A substantial outbreak in the eastern UP in 2005 resulted in over 6,000 cases and 1,500 deaths.⁷⁷ Globally, estimations of JE cases and fatalities are varying. WHO’s 2018 surveillance report, referring to Campbell’s 2011 document, suggested around 68,000 clinical cases yearly, leading to approximately 13,600 to 20,400 deaths.^76,80 However, a 2023 Stat Pearls publication cited figures between 30,000 to 50,000 JE cases annually,⁸¹ conflicting with WHO’s year-wise reported cases, which indicated 1,877 cases globally in 2022 and 3,392 in 2011.⁸² This discrepancy raises questions regarding the surveillance or reporting accuracy and possible overestimation in disease estimations.

JE’s epidemiology varies among countries. WHO identified 24 countries in the South-East Asia and Western Pacific regions with JEV transmission risk, encompassing more than 3 billion individuals.⁸³ Predominantly reported cases stemmed from Asian countries, with India and China accounting for 95% of them⁸⁴ India, an endemic nation for JE, has witnessed outbreaks in 327 districts across 24 states, primarily in the north-eastern and northern regions.⁸⁵ The highest recorded cases were 6,000 in 2005 and 4,017 in 2006,^76,86 followed by 2,545 in 2019. The year 2022 recorded 1109 cases with 130 deaths. By September 30, 2023, NCVBDC documented 828 cases and 48 deaths due to AES/JE.^82,87 Specific regions in India, like eastern Uttar Pradesh, Karnataka, Andhra Pradesh, Uttar Pradesh, and West Bengal, are particularly susceptible to JE due to their terrain and annual flooding.⁸⁵

Clinical symptoms

Following a mosquito bite carrying the JE virus, an incubation period of 5-15 days begins. Most individuals infected with the JE virus either experience no symptoms or have mild symptoms such as fever, headache, and vomiting. However, a small percentage develops encephalitis, characterized by prodromal fever, headache, nausea, diarrhea, vomiting, weakness, and myalgia, lasting a few days. Less than 1% of those infected with the JE virus develop severe neurological illness, and approximately 1 in 4 of these cases result in fatalities. Although certain symptoms may improve following the acute phase, 30%-50% of individuals who survive encephalitis continue to endure neurological, cognitive, or psychiatric issues.^76,88

Diagnosis and management

The surveillance for fever and AES helps to identify suspected cases of JE, which can then be confirmed using laboratory tests. Confirmation of JE involves several diagnostic criteria: the presence of IgM antibodies in serum and/or cerebrospinal fluid, a four-fold difference in IgG antibody titer in paired sera, virus isolation from brain tissue, antigen detection through immunofluorescence, and nucleic acid detection via PCR. There is no specific anti-viral treatment available for the infection, and clinical management is only symptomatic.⁷⁷

Preventive measures

The JEV has five distinct genotypes (I, II, III, IV, and V). In India, the available JE vaccines primarily target the GT-III. JE vaccines available in India are the LAV, cell culture-derived SA 14-14-2, inactivated SA-14-14-2 vaccine (IC51) (IXIARO® by Intercel & JEEV® by Biological Evans India Ltd.), and inactivated vero cell culture-derived Kolar strain, 821564XY, JE vaccine (JENVAC® by Bharat Biotech). Vaccination is not recommended for general use but specifically for individuals residing in endemic areas.^89,90 LAV, Cell culture-derived SA 14-14-2 was introduced in the routine immunization under Universal immunization program (UIP) of government in the 297 endemic districts of 24 states of India.^79,91-93 All children above the age of 9 months must be offered the vaccine. This vaccination schedule involves two doses: one given alongside measles at 9 months and the second with the DPT booster at 16-24 months. Recently, the Government of India announced the introduction of one dose of JE vaccine for adults in 31 endemic districts.^79,92

Challenges

Addressing the challenges associated with JE requires a multifaceted approach encompassing heightened awareness, enhanced surveillance, improved rural healthcare accessibility, and continuous research efforts. Efforts should be directed towards increasing public awareness of JE, as the lack of understanding among the general population can result in delayed diagnosis and appropriate management. Concurrently, addressing the limitations in JE surveillance in India is crucial for accurately tracking the incidence and prevalence of the disease. In rural areas, where JE is more prevalent, there is a pressing need to expand access to healthcare facilities. Limited healthcare accessibility can impede the timely diagnosis and treatment of JE, underscoring the importance of bolstering healthcare infrastructure in these regions. Furthermore, ongoing research is paramount to evaluating the effectiveness of current vaccines against the evolving genotypes of the virus. The emergence of new genotypes, such as GT-I and GT-V, raises questions about the potential impact on existing vaccines.^93,94 Similarly, continuous monitoring of the genetic and antigenic variations among circulating JEV strains is essential, especially considering the expansion of GT-I JEV into different parts of India. Staying updated on ongoing research and recommendations is crucial for adapting vaccination strategies to the evolving genotypes of the virus. While JE vaccination is integrated into the UIP in India, the coverage remains limited. Many individuals in endemic areas remain unvaccinated, emphasizing the need for an expanded and inclusive vaccination strategy. Efforts should be intensified to overcome these challenges and ensure comprehensive protection against JE in India.^89,94,95 To conclude, enhanced surveillance, better vaccination access, and strengthened vector control measures are key to mitigating the impact of JE in India.

Epidemiology

Zika virus was initially identified in 1947 in Uganda, originating from a Rhesus macaque. Since then, it has spread across 89 countries and territories, triggering outbreaks in Africa, the Americas, Asia, and the Pacific regions.⁹⁰ In India, three confirmed cases of Zika virus disease were recorded on May 15, 2017, in the Bapu Nagar area of Ahmedabad, Gujarat.¹⁰¹ Subsequently, Zika virus presence has been documented in 16 states/UTs of India.⁶ The most recent cases were reported in Kerala and Maharashtra in July 2021.²⁵ Notably, the genetic sequences of the Zika virus obtained from cases in Ahmedabad showcased a close relationship to Asian strains, underlining its lineage in India.⁹⁶

Clinical symptoms

Zika virus primarily spreads through the bite of infected Aedes mosquitoes, but it’s also known to transmit from pregnant women to their fetuses or through sexual contact, blood transfusion, organ transplantation, or even laboratory exposure.^102,103 The incubation period of the disease typically ranges from 3 to 14 days. Interestingly, most infected individuals are asymptomatic. When symptoms do arise, they’re generally mild, characterized by fever, rash, conjunctivitis, muscle and joint pain, malaise, and headache, lasting for around 2 to 7 days.

Pregnant women infected with the Zika virus face severe risks as it can cause microcephaly and other birth defects in the developing fetus and newborn. Additionally, it leads to complications like fetal loss, stillbirth, and preterm birth. Furthermore, Zika virus infection is associated with triggering Guillain-Barré syndrome, neuropathy, and myelitis, particularly in adults and older children.¹⁰⁴

Diagnosis and management

In India, Zika virus is typically diagnosed using PCR and virus isolation from blood samples, primarily conducted at the National Centre for Disease Control in Delhi and the National Institute of Virology in Pune. However, serological diagnosis can be challenging due to potential cross-reactivity with other flaviviruses like West Nile and Yellow fever.^103,105

As of now, there isn’t a specific treatment available for Zika virus infection or disease. Treatment mainly focuses on managing symptoms, using antipyretics and/or analgesics. It’s crucial to avoid nonsteroidal anti-inflammatory drugs until DENV infections are ruled out, to minimize the risk of bleeding. Pregnant women residing in areas with Zika transmission should promptly seek medical attention for laboratory testing, guidance, counselling, and appropriate clinical care. Though numerous potential vaccines for the Zika virus have successfully undergone either phase I or phase II clinical trials; currently, no vaccine is currently available.¹⁰⁶

Challenges

The challenges posed by the Zika virus in India are extensive and multifaceted, encompassing several critical aspects. First, the 2021 outbreak of the Zika virus underscores its circulation in the SouthEast Asian region, indicating the potential for new outbreaks in the future.²⁵ The omnipresence of the Aedes mosquito vector in India poses a formidable challenge in curbing the virus’s spread. A study conducted in Jaipur, India, detailed the first Zika Virus outbreak, analyzing its clinico-epidemiological and genomic profile. The study emphasized the urgent requirement for enhanced surveillance and control measures. The spread of the Zika virus to multiple states in India highlights the pressing need for improved surveillance, particularly targeting pregnant women. Additionally, public health education is imperative to mitigate risks during pregnancy.¹⁰² The other major challenges are co-infections and cross-reactivity with other viruses. Zika virus shares similarities with other mosquito-borne diseases like dengue and chikungunya, leading to concerns about co-infections and cross-reactivity. This similarity complicates case detection and surveillance, primarily due to the limited availability of diagnostic tests and the challenges in interpreting serologic test results owing to the known cross-reactivity with related flaviviruses, notably DENV.¹⁰⁷

Epidemiology

KFD was discovered in 1957 after a sick monkey in Karnataka’s Kyasanur forest tested positive.¹⁰⁴ Initially confined to specific districts in Karnataka, recent years have seen reported cases emerging in Kerala, Goa, and Maharashtra.¹⁰⁸ This geographical expansion beyond the endemic regions has triggered significant public health concerns over the past 15 years.¹¹⁰ KFD’s fatality rate ranges from 3% to 10%, affecting approximately 400 to 500 individuals annually.¹¹¹ Its peak prevalence occurs from January to May, particularly in forested areas, heightening the risk of transmission during this period. Those regularly exposed to forest environments, such as forest guards, shepherds, and hunters, face a higher vulnerability to contracting KFD, which is transmitted through bites of infected hard ticks.¹¹²

Symptoms

The clinical manifestation of KFD typically follows a biphasic pattern, although it’s occasionally described in four stages. In the initial phase, patients commonly exhibit abrupt onset symptoms, including fever, chills, severe muscle pain, headache, vomiting, gastrointestinal issues, and bleeding complications. If the second phase occurs, which is observed in around 10-20% of patients, it emerges at the onset of the third week, termed the biphasic presentation. During this phase, patients may experience another wave of symptoms, featuring fever and neurological manifestations like severe headaches, mental disturbances, tremors, and vision deficits.¹¹²

Diagnosis and management

During the early stages of illness, molecular detection via Real time-PCR, Nested PCR, or qPCR, along with virus isolation from blood, can be utilized for diagnosis. Subsequently, serologic testing using ELISA to detect anti-KFD IgM and IgG antibodies becomes feasible. Timely diagnosis of KFD holds immense significance for initiating prompt treatment and managing the disease. While there’s no specific treatment available, early hospitalization coupled with supportive therapy remains crucial.¹⁰⁸

Preventive measures

An existing KFD vaccine, developed in the 1960s in India, comprises a formalin-inactivated tissue culture vaccine.¹⁰⁶ However, concerns have been raised about its efficacy, given reported cases of the disease in vaccinated individuals. This might be attributed to virus variants in circulation or low vaccine coverage.^113,114 To enhance the prevention and control of KFD, further research is imperative, aiming to develop more efficacious vaccines.^111,113

Challenges

Most cases of KFD remain underreported due to misdiagnosis or lack of reporting, attributed partly to its similarity to other diseases.¹¹⁵ Additionally, limited access to serological testing and challenges in implementing Real time-PCR-based diagnostic tools in remote areas contribute to this underreporting.¹¹⁵ The absence of comprehensive surveillance and screening at the community level hampers accurate disease burden estimation, affecting policy formulation by stakeholders and policymakers. There is a scarcity of national data surveillance strategies for this disease in our country. Deforestation and habitat loss pose significant challenges,¹¹⁵ as they push human habitation closer to the natural reservoirs of the virus found in infected monkeys and small mammals. This disruption to the ecosystem alters the behavior and distribution of tick vectors and their hosts, influencing virus transmission dynamics. The current vaccination strategy, limited to a 5 km radius around reported KFD areas, may need a relook, as cases have appeared beyond this range. Phylogenetic studies have traced the virus’s dispersal from Karnataka to Goa and Maharashtra, highlighting the need for improved vaccination and surveillance strategies.¹¹⁶ Further research on KFDV’s genotyping and molecular epidemiology is necessary for a deeper understanding of its transmission dynamics.^117,118 Novel control strategies, like the development of edible vaccines, have been proposed as preventive measures for KFD.¹¹¹ Increasing awareness of emerging zoonotic infections, enhancing laboratory capacities, and implementing One Health programs are crucial aspects of public health intervention. Addressing these challenges requires comprehensive measures, including improved access to serologic/molecular testing in remote areas, bolstered surveillance and communication systems, and interdisciplinary collaboration to enhance KFD diagnosis and management. Beyond individual health impacts, these diseases strain healthcare facilities, reduce productivity due to illness, and disproportionately affect marginalized communities, incurring substantial economic burdens from healthcare costs and lost productivity.

Transmission

The proposed vectors of the virus are female sandflies, mosquitoes, and ticks. Phlebotomus papatasi (a sandfly) is reported to be the vector of CHPV disease in Gujarat. Human-to-human transmission without vector involvement is not known to occur, Treatment and prevention: No specific treatment or vaccine is available for the virus. The transmission is supportive to control any complication.¹²⁰

Chandipura outbreaks in India

In 2003, southern India (Andhra Pradesh) experienced a CHPV outbreak in which ∼350 children developed AES and ∼200 died. Since then, CHPV has been frequently reported in adjoining states in central India and thus poses a serious threat of an epidemic outbreak and significant public health concerns in the Indian subcontinent. Since sandfly vectors are abundant in South-East Asia and African regions, CHPV may be present in other countries of Asia and Africa. The most serious concern about CHPV is that it can cause high case-fatality ratios, ranging from 56% to 75% as reported during previous outbreaks in India. The disease manifests with a febrile illness that may progress to convulsions and coma and can lead to high mortality within 48 to 72 hours of the onset of symptoms typically represented by acute encephalitis. Given these forewarnings, it is of paramount importance that CHPV biology can be understood comprehensively. A lack of vaccine or effective therapy stresses the immediate and urgent need for antiviral therapeutics.¹²¹

The Ministry of Health and Family Welfare of the Government of India, reported about 245 cases of AES including 82 deaths with 64 confirmed cases of CHPV infections between early June and 15 August 2024. The CHPV outbreak of 2024 is estimated to be the largest in the past 20 years and calls for early access to care and intensive supportive care of patients for better survival. The recommendations from WHO include vector control and protection against sandfly, mosquito, and tick bites to prevent the further spread of CHPV. It is a big relief, though, that human-to-human transmission of CHPV has not been observed, yet the recent outbreak poses several challenges in preparedness and response against possible future CHPV outbreaks, which include

• i.

  Lack of treatment: There is no specific treatment, vaccine, or antiviral therapy for CHPV.

• ii.

  Lack of diagnosis: We need better diagnostic tests/kits for rapid (on-site) detection of CHPV besides PCR based-tests

• iii.

  More research on CHPV: CHPV receptors on the host cell surfaces remains unknown. Knowledge of such receptor (s) may be instrumental in designing much needed antivirals.

  Likewise, better knowledge of the CHPV life cycle may help design/develop specific antiviral drugs.

• iv.

  Need for the expansion of the healthcare system and awareness among the people

Epidemiology

The first confirmed case of CCHF in India occurred in January 2011 during a nosocomial outbreak in Ahmedabad, Gujarat. Since then, outbreaks and sporadic cases have been reported, predominantly in Gujarat and Rajasthan. One case was detected in Kerala in 2016 and traced to a slaughterhouse worker who had traveled from Oman. Seroprevalence studies in Gujarat revealed that a substantial proportion of domestic animals, especially cattle, sheep, and goats, had been exposed to CCHF virus. Males are more commonly affected, and proximity to a confirmed CCHF case increases the likelihood of contracting the disease.¹²³

Clinical symptoms

CCHF presents with sudden onset symptoms, including high fever, muscle aches, dizziness, headache, neck pain, and sensitivity to light. Gastrointestinal symptoms such as nausea, vomiting, and abdominal pain are common, followed by mood changes and confusion. Hemorrhagic symptoms, including bleeding from the gums, petechiae, and larger bruises (ecchymoses), may develop, leading to multi-organ failure. The disease can be fatal, with a mortality rate of 30–50%, and most deaths occur within the second week of illness. Survivors usually begin to recover after the 9th or 10th day.^122,123

Diagnosis and management

Clinical diagnosing CCHF is challenging because its symptoms mimic those of other hemorrhagic fevers, such as dengue and leptospirosis. Laboratory tests such as ELISA and RT-PCR are used for diagnosis. There is no specific treatment for CCHF. Supportive care, including fluid replacement and treatment of symptoms is the primary approach. Ribavirin, an antiviral drug, has shown some effectiveness in reducing the severity of the disease. In the absence of a vaccine, prevention strategies focus on controlling tick populations and reducing human contact with infected animals and patients.^122,124

Challenges

There are several challenges in controlling CCHF. Due to the absence of a vaccine for both animals and humans, disease control relies on vector control and public awareness. Healthcare workers are at a significant risk of contracting the virus through contact with infected patients, and nosocomial outbreaks remain a concern. Diagnosing CCHF can be problematic due to the similarity of its symptoms to other viral hemorrhagic fevers. Improving diagnostic capacity, especially in regions where the disease is endemic, is crucial for timely identification and containment of outbreaks.¹²⁵

National government initiatives on current preventive measures

• a)

  Vector control programs: These initiatives concentrate on diminishing mosquito breeding sites, employing insecticides, and fostering community engagement.

• b)

  Public awareness and education: Empowering communities with knowledge regarding preventive measures and early symptom recognition is a pivotal component of the strategy.

• c)

  Development of guidelines and operational manuals: Technical guidance is provided to states and other stakeholders for the effective implementation of programs. All guidelines are accessible on the NVBDCP website.¹¹⁴

• d)

  Establishment of sentinel surveillance hospitals (SSH): Hospitals equipped with laboratory support have been established to enhance diagnostic facilities for diseases like dengue in endemic states since 2007. The number of these hospitals has increased to 805 in 2023. All these hospitals are linked with 17 apex referral laboratories (ARL), which offer advanced diagnostic support.¹¹⁴

• e)

  Ensuring functional diagnostic facilities: The responsibility for maintaining functional diagnostic facilities (SSH/ARL) and ensuring the availability of kits lies with the respective State Programme Officers of NCVBDC. The government has provided technical guidelines for prevention and control, case management, and effective community participation to the states for implementation.¹¹⁴

Malaria burden and trends of malaria reduction

In 2022, malaria remained a significant global health challenge, with an estimated 249 million cases reported across 85 malaria-endemic countries and territories, including French Guiana. This marked an increase of 5 million cases compared to 2021.¹³⁰ Within the WHO South-East Asia Region, which comprised nine malaria-endemic countries, there were 5.2 million estimated cases, contributing to 2% of the global malaria burden. Notably, India alone accounted for approximately 65.7% of all malaria cases in the region. Of the total cases reported in the region, nearly 46% were attributed to Plasmodium vivax infection.¹²⁷ Despite significant progress, malaria remains a substantial global health concern, as highlighted by the World Malaria Report 2021, with an estimated 241 million cases and 627,000 deaths in 2020, with African countries carrying a majority of the burden. Over the past two decades, the South-East Asia Region (SEAR) has witnessed a remarkable decline in malaria incidence, dropping from an estimated 23 million cases in 2000 to 5 million in 2020.¹³¹ India, with the greatest absolute decline in the region from around 20 million cases in 2000 to approximately 4.1 million in 2020, still accounted for 83% of malaria cases and 82% of fatalities in SEAR.¹³² The period between 2001 and 2015 marked a significant global reduction in malaria incidence by 30%, along with a 47% decrease in the fatality rate, preventing an estimated 4.3 million deaths.¹³³ This achievement prompted the WHO to adopt the global technical strategy (GTS) for Malaria 2016–2030 in 2015, aiming to provide technical support for scaling up malaria responses and working toward the ambitious goal of a 90% reduction in the global malaria burden and interrupting transmission in at least 35 countries by 2030.¹³³

Despite these efforts, the context of malaria reduction highlights the ongoing challenges, with SEAR and India playing pivotal roles in the global malaria landscape. Continuous commitment to interventions, innovative strategies, and international collaboration remains crucial to achieving the GTS targets and ultimately eliminating malaria as a public health threat.

Tools for malaria control

Diagnostics: Enhanced diagnostics are imperative for monitoring changes in malaria infection rates, ensuring the quality of medicines and insecticides, and assessing factors influencing treatment decisions.^134,135 Current rapid diagnostic tests (RDTs) are effective for diagnosing symptomatic malaria with higher parasite densities, expanding access beyond health facilities. However, a critical gap persists as these tests cannot detect low-level blood-stage infections of any malaria species or dormant liver stages of Plasmodium vivax and P. ovale.¹³⁶ There’s a pressing need for a highly sensitive point-of-care field test capable of rapidly detecting low-density parasitemia and identifying all infected individuals for immediate treatment.

Point-of-care G6PD deficiency test: To safely treat P. vivax, detection of G6PD enzyme deficiency is essential due to the risk of acute hemolytic anemia with primaquine.¹³⁷ Currently limited to specific facilities, rapid testing for enzyme deficiency, coupled with easy data access, will facilitate the broader implementation of effective P. vivax treatments to reduce malaria transmission.

Malaria microscopy: Malaria microscopy, a cornerstone in the diagnosis of the disease, is not without its challenges, which encompass technical, logistical, and operational aspects. One primary challenge lies in the need for highly skilled microscopists capable of accurately identifying and differentiating various Plasmodium species and their different life stages.¹³⁸ Achieving proficiency in malaria microscopy demands extensive training and experience, and maintaining a cadre of skilled microscopists can be a logistical challenge in resource-constrained settings. Furthermore, the requirement for microscopic examination of blood smears is time-consuming and labor-intensive, posing operational challenges, particularly in regions facing high caseloads or during disease outbreaks. Additionally, the reliance on microscopy limits access to accurate diagnosis in remote or underserved areas, where skilled professionals and laboratory infrastructure may be lacking. To address these challenges and enhance the effectiveness of malaria microscopy, several solutions are being implemented. Technological advancements have led to the development of automated or semi-automated microscopy systems that assist in parasite detection and species identification, reducing the dependency on manual examination.¹²⁶ Additionally, the use of quality assurance programs and proficiency testing helps ensure the ongoing competency of microscopists.¹³⁹ Efforts are being made to strengthen laboratory infrastructure and capacity in malaria-endemic regions, including the recruitment and training of laboratory personnel. Moreover, innovative strategies such as telemedicine and remote consultation are being explored to connect microscopists in remote areas with experienced experts for real-time guidance and quality assurance. In the quest for improved malaria diagnostics, molecular techniques, like PCR, are being integrated into diagnostic algorithms to complement microscopy, especially in detecting low-density infections and differentiating between parasite species.^140,141 ICMR-National Institute of Malaria Research has developed India’s first malaria slide bank to strengthen the diagnostic capability and training programs in India.^139,140,142 Emphasizing a comprehensive approach that combines technological innovation, capacity building, and strategic partnerships can overcome the challenges associated with malaria microscopy, ensuring accurate and timely diagnosis in the global effort to combat malaria.

Molecular methods: As countries move towards malaria elimination, the rise in low-density and asymptomatic infections necessitates new reference standards. Molecular methods like loop-mediated isothermal amplification and PCR play a crucial role in detecting low-density parasitemia, with PCR distinguishing between local and imported cases.¹⁴⁰ While their field applicability is expanding, decreasing costs and complexity enhance their potential for wider adoption.

Insecticide quantification kits: To ensure the quality of insecticide formulations and assess the effectiveness of indoor residual spraying (IRS), straightforward point-of-use kits have been developed. These kits serve as practical alternatives to insecticide bioassay methods, which, although recognized as the gold standard, are often impractical in resource-limited public health systems and are rarely employed in operational programs.⁷

Insecticide bioassay methods involve exposing live mosquitoes to treated surfaces and measuring the knockdown and mortality rates to determine insecticide efficacy. While accurate, this approach is resource-intensive, time-consuming, and requires specialized skills and facilities, making it challenging for routine use in operational settings. The development of point-of-use kits addresses these limitations. These kits are designed to provide a more practical and accessible means of assessing insecticide quality and IRS efficacy. They offer a simplified, user-friendly approach that can be implemented in diverse settings, including those with limited resources and infrastructure.¹⁴³ By replacing the traditional bioassay methods with these kits, public health programs can more feasibly and routinely monitor the quality of insecticide formulations and the performance of IRS interventions. This shift towards user-friendly tools enhances the practicality and efficiency of quality assurance measures, contributing to the overall success of malaria control and prevention efforts.

Health education and awareness: Public awareness campaigns are conducted to educate communities about the importance of preventive measures such as allowing house spraying, using bed nets, wearing protective clothing, and seeking timely diagnosis and treatment for fever.¹⁴⁴

Community engagement: Engaging communities and involving them in malaria control efforts can enhance the effectiveness and sustainability of interventions.^145,146 However, this is not easy. This can include training community health workers, involving local leaders, and mobilizing community participation in vector control activities. Continued commitment from the government, sustained funding, and innovative approaches are essential to further progress in malaria control in India.¹⁴⁵

Integration with other health programs

Malaria control efforts are often integrated with other health programs, such as maternal and child health, as well as with broader development initiatives addressing poverty, sanitation, and access to clean water.¹⁴⁷

Research and innovation

Research initiatives focus on developing new tools for malaria control, including better diagnostics, drugs, and vaccines. Collaboration with international partners and research institutions would help in leveraging expertise and resources and build partnerships within the country and from overseas.¹⁴⁸

Current challenges

Antimalarial drug resistance: Various tools are employed to assess antimalarial drug resistance, with genetic changes linked to reduced drug sensitivity serving as key indicators. For instance, specific mutations in the P. falciparum Kelch13 (PfKelch13) gene are associated with delayed parasite clearance following artemisinin-based treatments.¹⁴⁹ Monitoring surveys of these mutations offer insights into the emergence and dissemination of artemisinin partial resistance, characterized by delayed clearance post-treatment. Sulfadoxine-pyrimethamine, utilized as an artemisinin-based combination therapy partner and for chemoprevention, has its resistance monitored through the identification of mutations in the dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps) genes of P. falciparum. Mefloquine resistance is associated with an upsurge in Pfmdr1 copy numbers, while piperaquine resistance is linked to increased Pf plasmepsin 2/3 copy numbers and mutations in the P. falciparum chloroquine resistance transporter (PfCRT). It is noteworthy that certain mutations are validated as resistance markers solely in parasite strains from specific geographic regions. These genetic assessments play a crucial role in monitoring and understanding antimalarial drug resistance, informing strategies for effective malaria treatment and control.^150-152

Diagnostic: Accurate diagnosis plays a pivotal role in achieving the goal of malaria elimination. While P. falciparum and P. vivax contribute to most cases among the five Plasmodium species, the varied distribution of both mono-infection and mixed infections complicates the diagnostic process. Microscopy, traditionally considered the gold standard, requires highly skilled microscopists well-versed in recognizing different Plasmodium species stages capable of detecting even low-density parasitemia. Meeting such requirements in rural India poses a significant challenge, resulting in more than a quarter of malaria cases being overlooked by microscopy.^7,135 In areas where microscopy is impractical, RDTs are employed. P. falciparum histidine-rich protein 2 (PfHRP2) antigen is a prominent target in over 90% of malaria RDTs.¹⁵³ Nevertheless, challenges such as deletions in the Pfhrp2 gene, fluctuations in plasmodium lactic dehydrogenase (pLDH) expression levels, and the prozone phenomenon pose significant hurdles, leading to inaccurate diagnoses of Plasmodium species. Addressing these challenges is imperative for ensuring the reliability of malaria diagnoses, particularly in regions where microscopy may not be feasible

The problem of insecticide resistance in malaria vector: Between 2010 and 2020, out of the 88 countries providing insecticide resistance monitoring data to WHO, 78 countries confirmed resistance to at least one insecticide in a malaria vector species at a specific mosquito collection site. Among these, 29 countries reported resistance to four insecticide classes—pyrethroids, organophosphates, carbamates, and organochlorines—in at least one malaria vector species across different locations within the country.^127,143 Within this group, 19 countries identified at least one site where resistance was confirmed for all four classes in a local vector. Globally, resistance to pyrethroids was observed in 87% of countries and 68% of sites, to organochlorines in 82% of countries and 64% of sites, to carbamates in 69% of countries and 34% of sites, and to organophosphates in 60% of countries and 28% of sites. Resistance to these four insecticide classes was found in all WHO regions, although the geographical distribution of resistance varied widely among regions. In India and various other locations, insecticides have been a primary component of vector control strategies. Presently, 83 vector control products have received prequalification from the WHO. In India, a range of insecticides, including dichloro-diphenyl-trichloroethane (DDT), malathion, and various pyrethroids, have been deployed for adult malaria vector control. Additionally, larval control involves the use of temephos, bacterial pesticides, and insect growth regulators, while space spray includes malathion and formulations of synthetic pyrethroids like deltamethrin and cyphenothrin, as well as natural pyrethrum extract.¹²⁷

Proper utilization of LLINs as vector control tool: LLINs today stand as the globally favored tools for vector control. Ensuring their sustained impact and timely replacement due to wear and ineffectiveness requires diligent monitoring and evaluation. In mid-2017, on distribution by the NCVBDC in high malaria-endemic districts of Odisha, the LLINs demonstrated efficacy, achieving 100% mortality in Anopheles jeyporiens is mosquitoes 30 months post-distribution.¹³¹ LLINs are a cornerstone in malaria vector control, primarily distributed through national malaria control programs worldwide, including India’s NVBDCP. Despite being a primary channel for net distribution, challenges persist in achieving optimal coverage, attributed to factors like population growth, timely procurement, replacement delays, emergencies (such as floods), and logistical constraints. To enhance access, India must explore complementary avenues, such as making nets available in the private sector at affordable prices. Although barriers like affordability, economic viability for manufacturers, and regulatory issues exist, they can be pragmatically addressed. The commercialization of nets in the private market is imperative for India to achieve its malaria elimination goal by 2030.^127,148

Climate changes: The WHO has identified climate change as the most significant health threat to humanity. This impact is disproportionately felt by populations in low-income countries, even though they contribute minimally to the causes of climate change. Many of these nations also grapple with malaria as a significant public health concern. The implications of climate change extend beyond direct environmental consequences and pose a substantial risk to global health progress. These risks include disruptions to livelihoods, increased exposure to harmful particulates, pathogens, and diseases, strain on health systems, and the exacerbation of existing social and economic inequalities. Therefore, climate change is not merely an isolated threat but a substantial amplifier of various health-related challenges. Malaria transmission is significantly influenced by the distinct and interconnected impacts of temperature, rainfall, and humidity. These elements not only delineate the geographical boundaries of the disease but also govern its seasonal patterns and intensity within those boundaries. As a result, these factors shape the epidemiology of malaria, determine the disease burden, and impact the effectiveness of various interventions. These considerations form the foundation for the development of national malaria strategic plans.¹²⁶

Modern day developments and malaria: Several modern developments have contributed to fostering malaria transmission. Rapid urbanization, especially in cities like Mumbai, Delhi, and Bangalore, has outpaced the development of proper infrastructure. Poorly planned construction projects, slums with inadequate drainage systems, and stagnant water in construction sites create ideal breeding grounds for Anopheles mosquitoes. High levels of migration from high endemic areas for labor, particularly from rural, malaria-endemic states, contribute to the spread of malaria. Large-scale irrigation projects and dam construction, such as those on the Narmada and Godavari rivers, have also increased malaria risk in nearby areas. The creation of stagnant water in reservoirs, canals, and irrigation systems provides ideal breeding conditions for mosquitoes, leading to higher transmission in surrounding villages and towns.^154,155

Zoonotic malaria

Until recently, four human malaria parasites, with P. falciparum and P. vivax being the most prevalent, were recognized. However, a fifth parasite, P. knowlesi, has emerged as a significant challenge for malaria control, particularly in South-East Asia. Initially identified in monkeys, this zoonotic parasite poses a substantial threat, characterized by a human fatality rate of 1–2% and the rapid onset of severe disease. First detected in Malaysia in 2004, P. knowlesi has since spread extensively across Southeast Asia and globally through travel and tourism, except for Timor-Leste. Despite a global decline of 24.2% in P. knowlesi cases in 2022 (2768 cases), notable increases occurred in Indonesia and Thailand. Alarmingly, P. knowlesi was responsible for all malaria-related deaths in Malaysia and Thailand that year. The spread of P. knowlesi complicates malaria elimination efforts and impacts the WHO’s certification process for malaria-free status, which traditionally focused on four human malaria parasite species. In response to the rise of P. knowlesi, WHO is reassessing the criteria for certification.^127,156

Future perspectives in malaria elimination

Prospects in malaria research in India involve a multidisciplinary approach aimed at addressing various challenges associated with malaria control and elimination. Here are some potential areas of focus for future research:

Development of novel control tools: Research efforts can focus on developing innovative vector control tools such as new insecticides, repellents, and traps. Emphasis should be placed on environmentally friendly and sustainable interventions to combat insecticide resistance.^8,157

Drug discovery and resistance management: Continued research into antimalarial drug discovery is crucial, including the development of new compounds and strategies to overcome drug resistance. Understanding the mechanisms of drug resistance and developing combination therapies can help prolong the efficacy of existing drugs.^158,159

Vaccine development: Despite progress, an effective malaria vaccine remains elusive. Research in India can contribute to vaccine development efforts by identifying suitable vaccine candidates, optimizing vaccine delivery strategies, and conducting clinical trials to assess vaccine safety and efficacy.^160,161

Genomic studies: Genomic studies of malaria parasites and mosquito vectors can provide valuable insights into their biology, evolution, and transmission dynamics. This knowledge can inform the development of targeted interventions and help track the spread of drug-resistant parasites and insecticide-resistant mosquitoes.

Health systems research: Research on health systems and delivery mechanisms is essential for optimizing the implementation of malaria control interventions. This includes assessing the effectiveness of community health worker programs, strengthening surveillance systems, and improving access to diagnosis and treatment services, particularly in remote and underserved areas.^146,162

Climate change and environmental factors: Climate change can influence the distribution and transmission of malaria. Research on the impact of climate change on malaria transmission patterns, as well as the role of environmental factors such as deforestation and urbanization, can inform adaptive strategies for malaria control and elimination.^127,162

Behavioral and socioeconomic research: Understanding the social and behavioral determinants of malaria transmission is crucial for designing effective interventions suiting the local conditions. Research can explore factors influencing mosquito breeding habitats, community perceptions of malaria, health-seeking behaviors, and adherence to preventive measures.¹⁶³

Integration with other health programs: Research can investigate the potential synergies between malaria control efforts and other health programs such as maternal and child health, nutrition, and water, sanitation, and hygiene (WASH). Integrated approaches can maximize health impact and resource efficiency.¹⁶⁴

Partnerships and collaboration: Collaboration between researchers, government agencies, NGOs, and international partners is essential for advancing malaria research and translating findings into policy and practice. Collaborative research networks can facilitate knowledge sharing, capacity building, and the mobilization of resources.¹⁶⁵

Summary

Despite these challenges, India has made significant progress in malaria control through various interventions, including vector control measures, prompt diagnosis and treatment, community engagement, and research initiatives. The government, along with partners at the national and international levels, continues to prioritize malaria control and elimination efforts to further reduce the burden of this disease in India.

Historical aspects

The recognition of Leishmania parasites as the causative agents of leishmaniasis began in the 20th century, a disease transmitted by sandflies (vector). Historical evidence of leishmaniasis dates to 2,500 B.C., with primitive descriptions found in ancient archaeological materials and writings. Molecular findings confirm the presence of Leishmania in ancient Egyptian and Christian Nubian mummies dating back to 2,000 B.C. In northern Sudan, Leishmanial DNA was reported in 1,500 B.C., and in Peru, a 6-year-old girl mummy infected with Leishmania was recorded in 800 B.C. In the 18th century, Russell (1756) provided the first detailed clinical examination of leishmaniasis.^166,167

Kala-azar symptoms were first described by Indian physicians in the 17th century, with sand flies playing a role in disease transmission. In 1900, William Boog Leishman identified ovoid bodies in splenic smears,¹⁶⁸ leading to the term “Dum-dum fever.” Charles Donovan and Ronald Ross later identified a new parasite. Further research proved sandfly as the vector after discovering Leishmania promastigotes in the sandfly gut and its various species were reported.^169,170

Epidemiology of Leishmaniasis

Regarding the burden of leishmaniasis, six countries are regarded as important ones. More than 90% of visceral leishmaniasis (VL) cases are reported from these nations: Bangladesh, Brazil, Ethiopia, India, South Sudan, and Sudan. Ten nations make up the majority of cutaneous leishmaniasis (CL) cases, with more than 70% of them involving CL. These nations, which account for more than 70% of cases, are the Islamic Republic of Iran, Ethiopia, Algeria, Brazil, Colombia, Costa Rica, Peru, Sudan, and the Syrian Arab Republic. According to the WHO, the populations of these endemic countries are 431 million and 616 million vulnerable to CL and VL, respectively. According to the WHO (2015), there were 1.3 million leishmaniasis cases reported in 2015. These instances included a million cases of CL, 300,000 cases of VL, and around 20,000–50,000 cases of fatal VL.

Geographical distribution of vl

Visceral leishmaniasis is endemic in 75 countries of African, Asian, and American continents, of which 95% of VL cases occur in 10 developing countries. VL predominantly affects five countries: Brazil, East Africa, Bangladesh, Nepal, and India, with approximately 50,000 deaths reported annually to the WHO. VL remains a significant global health concern due to migration, insufficient control measures, and HIV-VL co-infection, which contributes to its increasing incidence. In 2019, ten countries, including Brazil, Eritrea, Ethiopia, India, Iraq, Kenya, Nepal, Somalia, South Sudan, and Sudan, accounted for over 90% of new VL cases (WHO). Globally, 0.7 to 1 million new cases of VL and more than 1 million new cases of CL are reported worldwide. India, Bangladesh, and Nepal bear the 50% burden of all VL cases.¹⁷¹ In India, 54 districts spread over four nearby states namely Bihar, Jharkhand, West Bengal, and Uttar Pradesh. Bihar alone is responsible for 80-90% of all VL cases reported in India and 40% of the global burden.¹⁷² Bihar has consistently been the center of VL infections over a century; concurrently, the rate of new HIV infections is rising.¹⁷³ Sporadic cases of VL are also now reported from other Indian states. Factors such as crowded housing, which facilitates sandfly breeding and resting sites, malnutrition, and population mobility, increase the risk of VL transmission.

Epidemiology of Post-kala-azar dermal leishmaniasis (PKDL) in the Indian Subcontinent

PKDL is endemic in regions where VL is/was prevalent, such as the Indian and sub-Saharan subcontinent. It typically occurs months to years after the apparent cure of VL or without the earlier treatment of VL cases, making surveillance and early detection essential for effective control strategies.¹⁷⁴ In the Indian subcontinent, PKDL predominantly affects children and young adults, with a male predominance noted in some studies. The incidence of PKDL varies geographically within endemic regions, with hotspots identified in certain districts and states.¹⁷⁵ PKDL, especially (mixed infection) papulo-nodular form serves as a reservoir for Leishmania parasites, contributing to the persistence and transmission of the disease. The parasite load in PKDL lesions is lower than in VL, but patients remain infectious to sandfly vectors, thus perpetuating the transmission cycle.

Leishmaniasis

Leishmaniasis is a parasitic VBD transmitted by the infected female sand flies. It is not a simple disease but a disease complex. The three primary types of leishmaniasis are PKDL, mucocutaneous, and VL. Other names for VL include “Kala-azar, poor man’s disease, and black fever”.¹⁷⁶ It is one of the top 20 neglected tropical illnesses as determined by the WHO and a significant global public health concern. Fever usually lasts more than 2 weeks; weight loss, hepatosplenomegaly, and pancytopenia are its defining symptoms.

Pathogenesis

Establishing any parasitic infection in a host, involves a series of molecular and cellular interactions and events.^194,196-200 This also applies to leishmanial infection, where the parasite’s ability to evade killing, enter host macrophages, and survive and multiply inside the cell determines the host’s initial susceptibility to leishmanial challenge.^194,196 Leishmania must withstand the activation of the host’s complement system¹⁹⁹ and the effects of natural killer cells upon entering the host.²⁰¹

While attaching to DCs the leishmania parasites typically adhere to host macrophages. Although promastigotes of distinct leishmanial species preferentially attach to macrophages using the polar flagellum, the initial adhesion of Leishmania may be mediated by the pole of the organism.¹⁹⁹ Factors such as the parasite surface charge and surface free energy may impact Leishmania’s binding to macrophages.²⁰² It is likely that the initial attachment of promastigotes is mediated by a small ligand, causing clustering of gp63 or lipophosphoglycan (LPG) molecules at a few locations on the parasite’s surface, ultimately leading to the parasite’s engulfment through ligand-receptor contact.²⁰³ The ability of L. mexicana mutants lacking either gp63 or LPG to infect macrophages suggests the presence of other ligands.^203,204 Numerous receptors, including the mannose-fucose receptor, fibronectin receptors, C-reactive protein receptors, and receptors recognizing specific amino acid sequences like asp-gly-asp (RGD sequence) or integrin type receptors, are involved in parasite binding to the host cell surface.²⁰⁴ Complement receptors (CR1 and CR3) are particularly significant under physiological settings.²⁰⁵

Following complement-dependent adhesion, promastigotes are engulfed by coiled or tubular pseudopods resembling funnels.^206,207 Upon ingestion, the parasitophorous vacuole (PV) or phagolysosome 200 forms when promastigote-containing phagosomes fuse with endocytic organelles. The ability to thrive in the acidic environment of the PV, withstand destruction by lysosomal enzymes and other metabolic products, and reproduce are crucial for parasite intracellular survival.^208,209 The development of the PV may be delayed by the parasite’s LPG molecules, allowing promastigotes to convert into amastigotes, which are better suited to the enzymes and acidic pH of the PV.²⁰⁵ A tightly packed glycocalyx, primarily composed of low molecular weight glycoinositolphospholipids (GIPLs) and glycosphingolipids derived from the host, covers the surface of Leishmania amastigotes.²¹⁰ This layer may protect the parasite surface from the hostile environment of the PV. Several enzymes involved in the synthesis of glycocalyx components appear crucial for parasite survival or act as potent virulence factors in amastigotes.^211,212 The parasite inside the PV is shielded from intra-phagolysosomal destruction by the proteolytic activity of gp63.^213.214

Several protective factors, including cysteine proteases, acid phosphatase,²¹⁵ magnesium-dependent adenosine triphosphatase (ATPase),²¹⁵ and others, are released by parasites in soluble form and are associated with LPG.²¹⁶ Amastigotes multiply within infected cells before escaping to infect additional host cells. The non-filamentous proteophosphoglycan (aPPG) secreted by amastigotes is believed to protect the parasite from complement-mediated destruction and facilitate its entry into macrophages.²¹⁷ Amastigotes may employ receptors distinct from those used by promastigotes for their entry into host macrophages.^218-221

The survival of intracellular parasites within the host cell depends on factors controlling the biochemistry and physiology of the host cell. These factors likely determine the host cell’s ability to eliminate intracellular parasites through respiratory bursts and the production of degradative enzymes or reactive metabolites such as superoxide anion (O^2-), H₂O₂, and nitric oxide (NO). Inducible nitric oxide synthase (iNOS or NOS2) catalyzes the synthesis of NO from L-arginine and molecular oxygen in macrophages, while the release of reactive oxygen intermediates is mediated by the nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) oxidase complex.^194,222,223 The ability of the host cell to generate reactive oxygen and nitrogen intermediates is diminished when murine or human macrophages are infected with L. donovani.^194,224,225

Similar outcomes were observed with pure parasite surface elements such as GIPL, LPG, or gp63, which may contribute to the parasite’s survival inside macrophages. Leishmanial GIPL administered to cells before stimulation with gamma interferon (IFNγ) or lipopolysaccharides inhibited the activation of iNOS in murine macrophages.^226-228 LPG or GIPL, on the other hand, synergize with IFNγ to induce iNOS.²²⁷

Promastigotes of L. donovani inhibit the phosphorylation and activation of c-Jun-N-terminal kinase (JNK), p38, and p42/44 MAPK, as well as mitogen-activated protein kinases (MAPKs) in host cells. LPG mutant parasite strains are significantly less capable of these actions.¹⁹⁷ Early in the leishmanial infection phase, these events result in the inhibition of the iNOS pathway and NO production.²⁰⁵

Survival of the parasite under stress

To avoid reactive oxygen and nitrogen species, parasites have developed defenses against toxic metabolites. Trypanothione and polyamines, which provide thiols, play a vital role in some of these pathways.^{214,226,228-232} L. donovani reduces macrophage protein kinase C (PKC) activity, prevents phagosome-lysosome fusion, and inhibits IFNγ-induced tyrosine kinase activation, impairing signaling via interleukin 12 (IL-12) and NO generation. IL-1 and tumor necrosis factor-alpha (TNF-alpha) production are downregulated in macrophages infected with L. donovani.^227,232,233 Leishmanial infection increases the production of deactivating cytokines such as IL-10 and transforming growth factor-beta (TGFβ).^232-236 Some parasite proteins, such as EL-1α and members of the lysosomal cysteine proteinase B family, are transported to the macrophage’s cytoplasm where they interact with host signaling pathways.²³⁷ Amastigotes internalize and degrade class II major histocompatibility complex molecules, suppressing macrophages’ ability to present antigens.²³⁸ All of these effects and signals differentially influence macrophage activity and its interaction with other cells, contributing to parasite survival in vivo.

Proteome changes in L. donovaniis associated with oxidative and nitrosative stress: A study on the proteome changes associated with L. donovani promastigotes’ adaptation to oxidative and nitrosative stress conditions revealed that the parasite must effectively counteract these stresses through adaptive changes in its proteome. Key proteins involved in stress response and adaptation were identified, including the up-regulation of antioxidant enzymes in response to oxidative stress and alterations in the expression of proteins associated with the detoxification of NO and repair of damaged biomolecules. The identified changes in the expression of metabolic enzymes and transport proteins suggest a reprogramming of cellular metabolism to meet stress adaptation demands and maintain cellular homeostasis under adverse conditions. These findings provide valuable insights into the molecular mechanisms underlying L. donovani adaptation and suggest potential targets for therapeutic intervention against leishmaniasis.²³⁹

Metabolic reconfiguration of central glucose metabolism in L. donovaniis crucial for survival during oxidative stress: The Leishmania parasite adapts its central glucose metabolism to counteract oxidative stress, ensuring its survival. This involves glycolysis, the pentose phosphate pathway (PPP), and the tricarboxylic acid (TCA) cycle. During oxidative stress, the parasite upregulates the PPP to generate NADPH, essential for reducing equivalents and antioxidant defenses. The TCA cycle also undergoes modulation to meet increased energy demands and support antioxidant defense mechanisms. The reconfiguration of central glucose metabolism is tightly regulated at multiple levels, with transcription factors like cAMP-responsive element-binding protein playing a crucial role in this adaptation.²⁴⁰

Glucose-6-phosphate dehydrogenase and trypanothione reductase interaction protects leishmania donovani from metalloid-mediated oxidative stress: Protozoan parasite L. donovani faces oxidative stress induced by metalloids during host invasion. The interaction between glucose-6-phosphate dehydrogenase (G6PD) and trypanothione reductase (TR) is crucial in sustaining the antioxidant defense deployed by the parasite against oxidative stress. G6PD catalyzes the PPP, producing NADPH, while TR reduces oxidized trypanothione, a key antioxidant molecule in Leishmania parasites. Metalloid exposure induces oxidative stress, leading to reactive oxygen species (ROS) and oxidative damage. The G6PD-TR complex facilitates the efficient transfer of reducing equivalents between NADPH and trypanothione pools, enhancing the parasite’s ability to neutralize ROS and maintain redox homeostasis. The interaction is tightly regulated to optimize antioxidant activity in L. donovani, providing insights into parasite adaptation to environmental stress and potential therapeutic strategies against leishmaniasis.²⁴¹

Phosphorylation of Translation Initiation Factor 2-Alpha plays a role in Leishmania donovani’s survival under stress: Under stress conditions, phosphorylation of eIF2α inhibits global protein synthesis, allowing cells to conserve energy and redirect resources towards stress response mechanisms. The fact was investigated that how phosphorylation of eIF2α influences protein synthesis and cellular responses in L. donovani under stress. By inhibiting translation initiation, phosphorylated eIF2α orchestrates a coordinated stress response, promoting parasite survival under adverse conditions. The molecular mechanisms underlying eIF2α phosphorylation in L. donovani explores the role of stress-responsive kinases, including eIF2α kinases (e.g., PKR-like endoplasmic reticulum kinase-PERK), in phosphorylating eIF2α in response to specific stress stimuli encountered by the parasite. Phosphorylation of LdeIF2α enhances L. donovani tolerance to stress by reprogramming gene expression and promoting the synthesis of stress-responsive proteins. It was discussed how eIF2α phosphorylation-mediated translational control enables the parasite to adapt to adverse conditions and survive within the host environment. Overall, the findings highlight the importance of eIF2α phosphorylation in regulating L. donovani survival under stress conditions encountered during infection. By modulating protein synthesis and cellular responses, phosphorylated eIF2α enables the parasite to adapt to adverse environments and persist within the host.²⁴² Further, in the context of host-pathogen interactions, it was explored that L. donovani infection delays the host’s apoptotic process by inducing unfolded protein response in macrophages to ensure its survival and infection establishment.²⁴³

Protective role of Mevalonate Kinase in Leishmania donovani against oxidative stress: Mevalonate kinase (MVK) enzyme of the mevalonate pathway helps the Leishmania parasite to survive under oxidative stress. The mevalonate pathway is an important metabolic pathway in trypanosomatids, which leads to the synthesis of various isoprenoid precursors and are crucial for trypanosomatid viability.²⁴⁴ Inhibition of MVK leads to decreased viability and altered morphology. MVK expression was found to be up-regulated under oxidative stress conditions, establishing the role of leishmania donovani mevalonate kinase (LdMVK) in stress tolerance. Ergosterol is a crucial isoprenoid obtained through the mevalonate pathway, which are essentially required for the Leishmania parasites as they cannot survive without it.²⁴⁵ Apart from its role in maintaining cellular integrity, it has also been proven to be essential to overcome the variety of stress such as heat, osmotic, and oxidative stress.^246,247 Recent studies showed that L. donovani increases MVK-dependent ergosterol biosynthesis and decreases the peroxidation of cellular lipids when exposed to oxidative stress. LdMVK was found to be associated with maintaining plasma membrane integrity and also in preventing the peroxidation of cellular lipids when exposed to oxidative stress.²⁴⁸ This finding highlights the protective role of MVK in L. donovani against oxidative stress through the modulation of ergosterol biosynthesis. By enhancing the parasite’s antioxidant capacity, ergosterol contributes to its resilience to oxidative damage and promotes survival within the host environment.

Adenylate Cyclase mediated Antioxidant Defense Mechanisms in L. donovani: L. donovani, causing the VL disease in humans, applies its sensory system comprising adenylate cyclase (ACs) to modulate its own as well as the host’s machinery. These ACs localize diversely, such as membrane-bound to a cytosolic location in the cell, and catalyze adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP). L.donovani possibly manages the oxidative environments and its cytotoxic effects exerted either exogenously or endogenously by utilizing both the membrane and cytosol localized adenylate cyclases. Localization of ACs decides the stress responses and cellular activity during the parasite’s developmental stages and drug-exposed stages. Apart from the membrane-bound ACs, the cytosolic AC (LdHemAC) is soluble, and hence, not only preferably controls the PKA-cAMP activity but also induces the alignment of anti-oxidant machineries against the intracellular stress. Unlike membrane bound ACs, which are activated by extracellular signals, its activation occurs by internal factors such as oxidative stress. The recent study using the overexpression construct of LdHemAC signifies its role in infectivity and amastigote survival in murine macrophage. At protein level (LdHemAC), it plays a role in aligning the cAMP-PKA signaling axis to control substrate proteins phosphorylated by PKA under oxidative exposures, and modulates host Heme-oxygeenase-1 (HO-1) to counter oxidative stress.²⁴⁹ Studies focusing in this context are appreciated for new findings and their application for therapeutics or diagnosis value.

Hybrid forms: Hybrid forms in Leishmaniasis are the result of genetic exchange between different strains or species of Leishmania parasites. This phenomenon can occur through genetic recombination, which plays a crucial role in the evolution of Leishmania parasites, contributing to their diversity and potential adaptability to different environments or hosts. It can contribute to increased drug resistance and increased virulence.²⁵⁰

Immunology and host immune response in visceral leishmaniasis

Understanding the host immune response to Leishmania infection is crucial for developing effective therapeutic interventions and vaccines against this disease. The overview of the host immune response to Leishmania infection is summarized below:

Innate immune response: Upon infection, Leishmania promastigotes are phagocytosed by macrophages, DCs, and neutrophils. Recognition of leishmania by pattern recognition receptors (PRRs) such as toll-like receptors (TLRs) and C-type lectin receptors (CLRs) triggers the activation of innate immune responses.^251,252 Recent studies have highlighted the role of various signaling pathways, including nuclear factor-κ-B (NF-κB) and MAPK pathways, in modulating the innate immune response to Leishmania infection.²⁵³

Adaptive immune response: The adaptive immune response plays a critical role in controlling Leishmania infection. CD4+ T cells, particularly Th1 cells, are pivotal in activating macrophages to kill intracellular Leishmania parasites through the production of IFN-γ and TNF-α.²⁴⁴ Conversely, a Th2-polarized response is associated with disease progression, characterized by IL-4, IL-10, and TGF-β production, which inhibit macrophage activation and promote parasite survival.²⁵⁴ Recent studies have elucidated the role of CD8+ T cells, γδ T cells, and regulatory T cells (Tregs) in modulating the adaptive immune response to Leishmania infection.²⁵⁵ CD8+ T cells contribute to parasite clearance through cytotoxic activity, while γδ T cells exhibit both protective and regulatory functions.²⁵⁶ Tregs, on the other hand, suppress effector T cell responses, thereby promoting parasite persistence and disease progression.

Cytokine and chemokine signaling: The balance between pro-inflammatory and anti-inflammatory cytokines and chemokines determines the outcome of Leishmania infection.²⁵⁷ Recent studies have highlighted the role of chemokines such as C-X-C motif chemokine ligand (CXCL10) and chemokine (C-C motif) ligand (CCL2) in recruiting immune cells to the site of infection and shaping the immune response.^258,259 Deregulation of cytokine and chemokine signaling contributes to immune evasion mechanisms employed by Leishmania parasites, facilitating their survival and dissemination within the host.²⁶⁰

Immunomodulatory mechanisms: Leishmania parasites employ various strategies to subvert host immune responses and establish chronic infections. These include inhibition of macrophage activation, interference with antigen presentation, induction of host cell apoptosis, and modulation of host signaling pathways.²⁶¹ Recent advances in understanding the molecular mechanisms underlying parasite-host interactions have identified potential targets for therapeutic intervention.

Genetic susceptibility to visceral leishmaniasis

Genetic factors play a significant role in determining individual susceptibility to VL. Genome-wide association studies have identified polymorphisms in genes involved in immune regulation, such as cytokines, chemokines, and their receptors, that influence susceptibility to Leishmania infection and disease severity.²⁶² Understanding the genetic basis of susceptibility to VL may aid in the development of personalized therapeutic strategies and vaccination approaches. In conclusion, the host immune response to Leishmania infection is complex and multifaceted, involving innate and adaptive immune mechanisms. Recent studies have provided insights into the cellular and molecular processes underlying immune regulation during VL. Further research aimed at deciphering host-parasite interactions, identifying biomarkers of disease progression, and developing novel therapeutic interventions and vaccines is warranted to combat this neglected tropical disease effectively.

Therapeutic interventions and vaccine development

Current treatment options for VL primarily rely on antimonial drugs, amphotericin B and miltefosine, which have limitations such as toxicity, drug resistance, and high cost. Therefore, there is an urgent need for the development of safer, more effective therapeutic interventions and vaccines against VL. Recent advances in vaccine development include recombinant protein vaccines, DNA vaccines, and live-attenuated vaccines targeting Leishmania antigens.²⁶³

First generation vaccine

The first generation of leishmaniasis vaccines involved inoculating live Leishmania promastigotes in attempts to create effective immunization against the disease.¹¹⁴ However, due to the complex and unpredictable nature of the parasite’s antigens, modern vaccine development faces significant challenges. In the 1940s, Brazil initiated the development of these first-generation vaccines, which comprised the entire crude antigen of Leishmania. Despite these efforts, their efficacy was limited.²⁶⁴ Some formulations included autoclaved whole cell lysate, either alone or combined with Bacillus Calmette-Guérin (BCG) as an adjuvant, as the antigen source.²⁶⁵

Second-generation vaccine

Treatment of VL

Challenges

Treatment of VL-HIV, drug resistance with first-line treatment, and concerns about reduced efficacy of other treatment options, such as miltefosine and amphotericin B, clearly highlight the urgent need for alternative therapies and surveillance of drug resistance patterns.

Background

Lymphatic filariasis is a neglected tropical disease that poses a significant public health burden worldwide. It is primarily transmitted through the bites of infected mosquitoes carrying parasites of the Wuchereria bancrofi, Brugia malayi, and B. timori species. Some species of Anopheles, Culex, Aedes, and Mansonia genera are implicated in the transmission of human filarial parasites. Among these, W. bancrofi accounts for more than 90% of lymphatic filariasis cases globally. Many species of several mosquito genera transmit these parasites, viz. Anopheles, Aedes, Culex, and Mansonia. Adult parasites are primarily found in the lymphatic system of humans and produce several thousands of microfilariae (MF) released in the bloodstream.³¹⁹ The mosquito vector picks up the MF when feeding on an infected human host. They undergo further development in the mosquito host molting to become infective larvae (L3 stage). When this infected mosquito takes the next blood meal, the infective larvae find their way into another human and develop to become adults. In this way, the life cycle of the parasite continues. The impact of lymphatic filariasis on affected individuals and communities is severe. The disease causes chronic and debilitating conditions such as lymphedema, elephantiasis, and hydrocele. These conditions not only lead to physical disability but also result in social stigma, economic hardship, and reduced productivity. The burden of the disease falls disproportionately on marginalized populations in low-resource settings, further exacerbating health inequities.³²⁰

To address this global health challenge, the WHO launched the Global Program to Eliminate Lymphatic Filariasis (GPELF) in 2000. The program aimed to eliminate lymphatic filariasis as a public health problem by 2024 and this timeline for elimination has been extended to 2030 now.³²¹ GPELF adopted a comprehensive approach that combined mass drug administration (MDA) with other preventive measures such as vector control, morbidity management, and disability prevention. MDA involves the administration of antiflarial drugs, including Albendazole, Ivermectin, and Diethylcarbamazine (DEC), to entire at-risk eligible populations in endemic areas. The MDA-ineligible section of the population includes children below a certain age (<2 years in case of DA-MDA and <5 years in case of IDA-MDA), pregnant women, and severely ill.³²² These drugs target the MFs, the larval stage of the parasites circulating in the bloodstream, and aim to reduce their numbers, prevent transmission, and alleviate symptoms. The success of MDA campaigns in reducing disease burden has been significant, and several countries have made remarkable progress toward elimination goals. However, challenges remain on the path toward the complete elimination of lymphatic filariasis.³²³ One of the primary challenges is the persistence of adult worms in infected individuals for years, as the currently available drugs primarily target MF. These adult worms can continue to produce MF, perpetuating the transmission cycle of the disease. At present, there is no potent macrofilaricide drug that can penetrate lymphatics and kill adult worms.^324,325

Additionally there are concerns about the emergence of drug resistance in the parasite populations. There have been some reports of poor responses to DEC treatment as well as to Ivermectin. Moreover, a mutation associated with resistance to Albendazole has been detected in W. bancrofti populations. Such challenges could impact the effectiveness of MDA interventions. Innovative strategies are needed to overcome these challenges and achieve sustained elimination.^323,326-328

The burden and global programme to eliminate lymphatic filariasis (GPELF)

LF is endemic in 73 countries and territories of the tropical and subtropical regions of the globe, with 1.39 billion people at risk of developing this debilitating disease. In India, LF continues to be an important public health problem contributing about 44.3% to the global burden. To eliminate LF, combined drug regimens of antifilarial drugs are being mass administered to populations living in at least 48 endemic countries. At least 36 million people were suffering from chronic disease manifestations, and among them, 25 million men were affected with hydrocele and 15 million with lymphedema. In India, over 670 million people are at risk of infection by W. bancrofti and B. malayi parasites in 339 districts. Infection due to W. bancrofti is widespread in 21 states and UTs, contributing to about 99.4 percent of the total cases. The remaining cases are due to B. malayi, being limited to small pockets in six endemic States. About 0.48 million cases of lymphoedema and 0.18 million cases of hydrocele are line-listed in the endemic districts of the country.³²⁴

In 1997, the WHO, via the World Health Assembly Resolution, WHA 50.29, pledged to eliminate LF as a public health problem by 2020. The GPELF, launched in the year 2000, has two principal aims: (i) to interrupt LF transmission and (ii) to manage morbidity and prevent disability. The overwhelming success of the GPELF, so far, has led to a significant decrease in MF and antigenemia prevalence in several countries that have accomplished several rounds of MDA.^324,325

The current LF elimination strategy adopted in India

The Government of India prioritized the elimination of LF through the annual MDA program in 2004 and continued with a single dose of diethylcarbamazine citrate (DEC), 6 mg/kg of body weight, plus albendazole annually over 5-6 years. The GOI had set the target to achieve LF elimination by 2015 and now by 2030. In India, as many as 138 districts out of 339 endemic districts have stopped MDA in Gujarat, Goa, Puducherry, Dadra Nagar Haveli, Daman & Diu, Tamil Nadu, Lakshadweep, Andaman & Nicobar Islands. An additional 30 districts are under TAS evaluation. In 171 districts where MDA is implemented, 108 are under DA-MDA, while 63 are under IDA-MDA. In 2023, to accelerate the elimination of LF, the Govt. divided the country such that roughly half of the endemic districts will receive MDA on February 10^th, coinciding with National Deworming Day and the remaining districts on August 10th. This division is expected to manage the supply chain and implementation of MDA more effectively.^329,330

Despite several rounds of MDA, the progress of LF elimination in the country so far has been suboptimal. Much remains to be achieved as about 50.4% of the total 339 endemic districts are still under MDA. The major challenge in implementing MDA is poor compliance and drug consumption. It is necessary to have a critical look at the causes of slow progress and provide alternatives to address many challenges that are impeding LF elimination in India.³³¹

The challenges and impediments to LF elimination

The Govt. of India is making concerted efforts with the help of various stakeholders, drug donors, and funding partners to eliminate LF by the targeted year 2030. Some of the challenges have been mentioned below and steps have been suggested for consideration by the Govt. to further accelerate the elimination efforts.

Delineation of endemic blocks/districts of the country

From time to time, the addition of the new LF endemic districts to the existing pool is worrisome. That suggests a need to scientifically delineate all the endemic districts in which the status of LF is uncertain in the country. To identify them, especially those bordering the existing MDA districts, systematic surveys to line list the hydrocele and lymphedema cases in the villages and the cities are necessary through Wellness Centres. Additionally, Night blood surveys carried out will inform the prevalence of infection (MF rate) in different blocks of the districts. When above the 1% threshold, the MF rate will also be an indicator of ongoing transmission of the disease and the inclusion of these blocks for the MDA. As currently, block is the intervention unit it is emphasized that the focus of MF surveys should be the blocks, especially those in which a sizable number of chronic cases are present.^329,330,332

The challenges with diagnosis and diagnostics

This is a very important issue. Diagnostics conforming to different use cases with high sensitivity and specificity are the cornerstones for determining LF status during different phases of elimination and during sustenance monitoring in the post-elimination phase. The following are the diagnostics and their use cases.

• Microscopy – detection of MF in the night blood sample – a very useful measure of the prevalence of the source of infection for monitoring the impact of interventions and for active case detection and treatment.

• Diagnostic test to detect circulating filarial antigens – a sensitive and operationally feasible tool useful for mapping, monitoring, evaluating the impact, stopping decisions, and post-elimination surveillance.

• An antibody assay that can detect filarial antibody as a marker of exposure to infection – useful to demonstrate active transmission.

Microscopy for examining stained night blood smears is considered a gold standard method to detect MF. However, there are challenges with night blood surveys, which usually start after 9 PM and normally end by midnight when the MF appears in the peripheral blood and is captured in the blood smears. The timing for making a blood smear is critical, which is close to midnight to determine the true MF prevalence. Equipping the wellness center with a microscope, staining facility, and trained staff in all the endemic blocks is a basic requirement in every endemic block in the country.^333-335

As for an antigen test, currently, an Alere Filariasis Test Strip (FTS^TM) is imported and costs about Rs. 400 per test. The total cost is very high considering the large number of tests needed in our country for post-MDA monitoring and Evaluation and for making stop-MDA decisions. Hence, concerted efforts are needed either to import this technology and manufacture antigen detection tests under a license in India or to take steps to develop indigenous tests.³²⁹

A typical filarial antibody detection test is an IgG/IgM Rapid Test Cassette (Whole Blood/Serum/Plasma) that is a rapid chromatographic immunoassay for the qualitative detection of IgG and IgM antibodies to Filariasis parasites (W. Bancrofti and B. Malayi) in whole blood, serum, or plasma to aid in the diagnosis of Filariasis infection. These tests should have a sensitivity and specificity of >95%. Antibody tests are useful as exposure markers in the detection of new disease foci and useful during the post-elimination validation surveys. Indigenous development of these tests should be encouraged.³³⁵

Mass Drug Administration (MDA): Challenges to meet the elimination thresholds and stopping MDA strategy

Currently, MDA is the main LF elimination strategy globally and in the country. Two types of MDAs have been rolled out. In 108 endemic districts, a combination of DEC and Albendazole (DA) combination is being used during MDA campaigns, while in 63 districts Ivermectin+Diethylcarbamazine+Albendazole (IDA) combination is being administered. For DA and IDA to be effective, a minimum of 65% and 85% consumption of drugs must be achieved during the MDA campaigns every time. As per the modeling studies, 5-6 rounds of DA and 2-3 rounds of IDA are enough to achieve the LF elimination targets, i.e., <1% Mf rate and <2% filarial antigenemia provided the above coverage is achieved. This is followed by Transmission Assessment Surveys (TAS) in DA-MDA districts wherein antigenemia in children of 6-7 years of age who have lived in areas with 5-6 MDA rounds. The TAS is conducted in the schools following a statistical sampling tool. To stop MDA, the district or block must pass three consecutive transmission assessment surveys as per the guidelines of WHO and NCVBDC.^329,336-338 By 2020, as many as 98, 87, and 42 endemic districts had cleared 1^st, 2^nd, and 3^rd transmission assessments, respectively, and stopped MDA. However, the districts that failed TAS 2 and 3 after having passed TAS 1 & 2, respectively, were reverted to MDA. It is noteworthy that DA-MDA was started in 2004. Hence, some districts have undergone 18-19 rounds of MDA and yet not passed TAS suggesting that the desired level of drug consumption targets during these MDA campaigns were not achieved. For IDA-MDA an effective monitoring and evaluation strategy is needed as per WHO.³²⁹ WHO recommends the FTS for all areas endemic to W. bancrofti and Brugia Rapid Test for all areas endemic to Brugia spp. The FTS, which measures circulating filarial antigen (CFA), is used in all steps of the GPELF strategy. However, CFA can take 12 months or more to appear after infection and persists several years after adult worms can no longer reproduce or have died. New tools are needed to detect, ideally, the presence of viable worms or MF following the introduction of IDA. WHO suggests community surveys and hence all levels of the population for post-MDA assessment in case of IDA-MDA.^338,339

Space for molecular xenomonitoring (MX)

The success of the GPELF through MDA hinges on the availability of sensitive tools to monitor the outcome of the elimination efforts. The current challenge is to have appropriate diagnostic tools and time for cessation of MDA. The decision to stop MDA is complex, and multiple decision support tools have been reported for arriving at the decision. The interruption of transmission of infection is presently being monitored using human infection measures such as MF loads, antigenemia, anti-filarial antibodies in humans, and infection in vectors. Transmission, a key parameter in this regard is a function of both the prevalence of mosquitoes with infective-stage larvae and the man-biting rate and can be monitored by measuring changes in infection status of either vectors or humans. It has two components: transmission from man to mosquito vector (infection) and transmission from the mosquito vector to the human host (infectivity). Direct detection of MF/larval stages of the parasite in the vector is indicative of both the presence of patent (circulating microfilariae) infections in humans and transmission of the infection from humans to the vector.³⁴¹ Detection of larvae of filarial parasites in vectors by dissection and microscopic examination has been the gold standard method to estimate the transmission levels in endemic settings. However, this method is cumbersome, subjective, has low throughput and is not applicable in areas with ultra-low parasite prevalence in vectors. These attributes make this method unsuitable for use in assessing large-scale programs, such as GPELF. MX and PCR detection of filarial DNA in mosquito vectors are highly sensitive and less invasive tools that help indirectly detect filarial infection in communities. Hence, efforts have been made to develop PCR-based methods for the detection of infection and infective (L3) stage larvae of W. bancrofti and B. malayi. A simple and inexpensive technique for the isolation of DNA from infected mosquitoes has been developed in the ICMR-Vector Control Centre (VCRC) and found to be useful in xenomonitoring of LF. Further, the MX for W. bancrofti developed at VCRC has been evaluated in several geographic locations in India along with other parameters (microfilaria, circulating filarial antigen, and antibody) and found to be 2-3 times more sensitive than the mf survey. The technique utilizes a simple and inexpensive buffer which is routinely used in all molecular biology laboratories and bead beating. The cost of isolation of DNA from vectors by employing this method is a small fraction (US $ 0.06) of that by the published method that utilizes commercial kits (US $ 5.0). This cost-saving will bring down the cost of xenomonitoring immensely which is vital for source-constrained endemic countries currently implementing global programmes like GPELF.^335,339 It may be noted that MX for LF has remained largely within the realm of research laboratories for almost 2 decades. The LF elimination programme, launched 2 decades ago, has now reached advanced stages in many countries with several rounds of MDA administered. Now it is the time to deploy the MX in the LF elimination programs globally, through establishing the assay for monitoring the transmission for stopping MDA in an endemic setting and finally proceed to certification of LF elimination in the country.³⁴⁰

DEC fortified salt as an alternate strategy

At this juncture, a well-researched approach, i.e. the use of DEC-fortified salt, also advocated by the WO, as a unique form of MDA, is proposed. As per this strategy, a low dose of DEC (0.2% w/w) is added to the cooking salt at the manufacturing facility of iodized salt and consumed by the LF-endemic communities for about two years. Many examples of successful use of this strategy for LF elimination in small- and large-scale trials have been documented in India and several other endemic countries in the world. Implementing DEC–iodine-fortified salt is a safe, less expensive, more efficient, and prompt approach for achieving the elimination of LF in India. Adverse effects are either absent or minor and self-limiting. The DEC-fortified salt strategy can easily piggyback on the existing countrywide deployment of iodized salt under the National Iodine Deficiency Disorders Control Programme (NIDDCP), which has achieved great success in reducing iodine-deficiency disorders such as hypothyroidism. This existing robust program can be leveraged to launch DEC-fortified salt for the community. If implemented appropriately, this strategy may ensure the complete cessation of LF transmission within two years from its introduction. A detailed and in-depth blueprint may be needed to implement this strategy considering the technical, ethnic, political, cultural, social, and ethical issues related to DEC-fortified salt introduction for LF elimination.^342-345

Prospects of a LF vaccine

Extensive research efforts have been dedicated to identifying suitable vaccine candidates using various approaches, including subtractive genomics and reverse vaccinology. Subtractive genomics involves comparing the genomes of the parasite with the host organisms to identify unique proteins or metabolic pathways that could be potential targets for interventions. This approach helps in prioritizing proteins essential for the survival or reproduction of the parasite, that could be explored to develop potential vaccines or new drug therapies. Reverse vaccinology complements subtractive genomics to predict immunogenic epitopes and design and evaluate multi-epitope vaccines using various immune-informatics tools. This computational approach aids in the selection of vaccine candidates with the highest potential for success, minimizing the need for extensive experimental testing. So far development of a vaccine against LF has been a neglected area, with practically little or no research being undertaken to develop an effective vaccine that will have far-reaching implications on disease prevention for billions at risk of LF.^343-348

Space for vector control

Vector Control in LF is another neglected area due to the lack of sufficient vector control capacity in the country. Most of the W. bancrofti infections are transmitted by the Culex quinquefasciatus which is a predominant mosquito species in both rural and urban India. This mosquito prefers polluted man made water bodies such as drains, septic tanks, and ditches. These sites can be managed by proper sanitation, routine cleaning, and drainage facilities. It is noted that in the villages, box-type concrete drains are being constructed, which lack gradients, as a result, the wastewater stagnating in them creates ideal conditions for Culex quinquefasciatus breeding. Hence, MDA campaigns are carried out to control the parasite, while little to no effort is directed at the control of vector mosquitoes, which unabatedly transmit the disease. Hence, there is a need for introducing integrated vector management (IVM) which is recommended by the WHO and advocated by the NVBDCP. This will need a multipronged approach, including identification of stubborn LF pockets needing IVM, making species sanitation as part of the ‘Swachh Bharat Mission’ of Govt. of India and improvements in rural and urban drainage systems and advocating LLINs in the endemic blocks to prevent/minimize vector–human contact.^{332,334,339,349-352}

Epidemiology

Lyme disease is not endemic in India; sporadic cases have been identified, especially in the northern regions, such as Himachal Pradesh, where tick populations are abundant due to forested areas and livestock farming. The disease is more common in cooler climates where ticks thrive. Individuals working in forestry, agriculture, or animal husbandry are at higher risk of exposure to infected ticks.³⁵⁵

Clinical symptoms

Lyme disease progresses through three stages: early localized, early disseminated, and late disseminated.³⁵⁶ Early Localized Stage: The hallmark symptom is erythema migrans, a circular, “bullseye” rash that appears at the site of the tick bite. This occurs within 3 to 30 days after infection. Other early symptoms include fatigue, fever, headache, and muscle or joint pain. Early Disseminated Stage: If untreated, the infection spreads to other parts of the body. Symptoms may include multiple rashes, facial palsy (loss of muscle tone on one or both sides of the face), meningitis, and shooting pains. Late Disseminated Stage: Months to years after infection, untreated patients may develop severe arthritis, especially in the knees, and neurological complications such as neuropathy or cognitive dysfunction may appear.^356,357

Diagnosis and management

Lyme disease can be diagnosed through clinical observation, particularly the presence of erythema migrans, and confirmed using serological tests such as enzyme immunoassay (EIA) and Western blot to detect antibodies against Borrelia burgdorferi. Due to a lack of awareness, cases are often misdiagnosed as other tick-borne illnesses.³⁵⁸

Treatment in the early stages involves antibiotics like doxycycline or amoxicillin. In the later stages, intravenous antibiotics may be required to treat severe neurological or cardiac complications.³⁵⁸

Challenges

The lack of awareness among healthcare professionals and the public may lead to misdiagnosis, as the symptoms can resemble other febrile illnesses. The presence of Ixodes ticks in certain northern regions poses a potential public health threat, particularly as climate changes alter tick distribution patterns. Lack of prevention efforts, such as tick control and public education, can make it difficult to reduce exposure.³⁵⁹

Epidemiology

Scrub typhus is endemic in the “tsutsugamushi triangle,” which includes South and Southeast Asia, parts of China, and northern Australia. In India, the disease is widespread in rural and forested areas, particularly in Himachal Pradesh, Uttarakhand, Jammu & Kashmir, and Assam, as well as parts of southern India like Tamil Nadu and Kerala. The disease re-emerged as a major public health concern in India in the early 21st century, with outbreaks frequently reported during the monsoon and post-monsoon periods when chiggers are most active.^362,363

Clinical Symptoms

Symptoms typically begin 6 to 21 days after being bitten by an infected mite. The disease presents with high-grade fever, eschar, rash, and systemic involvement in severe cases. Scrub typhus can lead to complications such as pneumonia, myocarditis, meningoencephalitis, acute renal failure, and multi-organ dysfunction. Without treatment, the disease can progress to severe complications and can be fatal, with a mortality rate ranging from 7% to 30%.^363,364

Diagnosis and Management

Scrub typhus is diagnosed based on clinical symptoms, particularly in endemic areas, and confirmed by serological tests such as the Weil-Felix test, indirect immunofluorescence assay (IFA), or ELISA.³⁶⁵

Doxycycline is the treatment of choice and is highly effective if started early. In some cases, azithromycin is used as an alternative, especially in children and pregnant women. Prompt treatment with antibiotics significantly reduces the risk of complications and death.³⁶⁶

Challenges

Lack of awareness contributes to delayed diagnosis and treatment. Seasonal outbreaks are common, particularly during the monsoon when the vector population increases. Public health initiatives focused on vector control and educating rural populations on protective measures, such as avoiding areas with dense vegetation and using insect repellents, are essential to controlling the spread of scrub typhus.