Neglected tropical diseases (NTD) are a group of diverse communicable diseases that cause high morbidity and mortality in people from low/middle income countries, including India. The World Health Organization identified 20 NTDs and India leads the world in the total number of cases for at least 11 major NTDs. Lymphatic filariasis and visceral leishmaniasis are among the major NTDs and accounted for 29% and 45% of the global cases in 2016, respectively.
Lymphatic filariasis or elephantiasis, is a parasitic infection spread by mosquitos. In India, it is caused by the parasitic filarial worms Wuchereria bancrofti (>90% of the cases) and Brugia malayi. The adult worms reside in the lymphatic system of humans. Female worms produce microfilariae, which appear in the peripheral blood during the night and are ingested by mosquitoes along with a blood meal. Microfilariae undergo development within the mosquito to infective stage larvae, which are transmitted from mosquito to humans during blood feeding. The larvae move to lymphatic vessels and develop into adult worms. The parasites infect children in early life although symptoms appear later in their adult life.
Lymphatic filariasis infection causes asymptomatic, acute, and chronic conditions.Asymptomatic
Visceral leishmaniasis or kala-azar, is caused by protozoan parasites of different Leishmania species in distinct geographical areas and is spread by sandflies. In India, visceral leishmaniasis is caused by Leishmania donovani. Leishmania exists in two different morphological stages: the extracellular motile promastigote in the sand fly and the intracellular amastigote in humans. The sand flies inject the promastigotes into humans during blood meals. Promastigotes are ingested by phagocytic cells in humans where they transform into amastigotes and multiply. The infected cells eventually burst and release amastigotes, which in turn infect other phagocytic cells. Sand flies become infected by ingesting infected cells during blood meals. Amastigotes transform into promastigotes within the gut of sand flies and migrate to the proboscis for transmission during the next blood meal. The incubation period ranges between 2 weeks to 8 months. Visceral leishmaniasis is fatal within 2 years if left untreated.
Post-kala-azar dermal leishmaniasis (PKDL) is a nonlethal dermal condition that occurs in 10%-20% of patients in Asia after treatment of visceral leishmaniasis. Leishmania parasites reside within skin lesions and can be transmitted through the bite of sand flies.
Most infections are asymptomatic; symptoms appear in 2%-23% of infected individuals within a year.