Transmission of visceral leishmaniasis

• Reservoir hosts
• Insect vectors
• Parasite life cycle

Pictures of parasites

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Leishmaniasis is primarily a zoonotic disease in which wild and domestic animals such as the fox, jackal, rodents and wolves serve as reservoir hosts. Other animals in the surrounding areas can become infected and these are referred to as secondary or incidental hosts. Of all the potential animal hosts, domestic dogs by far play the most important role in harbouring and transmitting the disease to humans due to the close association between humans and dogs as pets (WHO expert committee report, 1991; Arias et al, 1996).

In anthroponotic VL due to L.d donovani such as in India and Sudan, man is the principal reservoir host. Asymptomatic carriers and PKDL patients are a particular source of infection for sandflies (WHO expert committee report, 1991).

Insect vectors

The only proven vector of the leishmania parasite is the blood-sucking female of the genus Phlebotomus in the old world and Lutzomyia in the new world. The insects are 2-3 mm long and are found through-out the tropical and temperate parts of the world. The sandfly larvae require organic matter, heat and humidity for development and so are commonly found in house-hold rubbish, bark of old trees, burrows of old trees and in cracks in house walls. The sandflies usually feed at night while the host is asleep (Arias et al, 1996). Only 30 or so of the over 500 species of Phlebotomine sandflies are known to transmit leishmania parasites, these include P.argentipes on the Indian sub-continent, P.martini and P.orientalis in Africa and the Mediterranean basin, P.chinensis and P.alexandri in china. In the new world Lutzomyia logipalpis is the only known vector of L.d chagasi (WHO WWW site, 1997).

The leishmania Life cycle

The sandfly vector becomes infected when feeding on the blood of an infected individual or an animal reservoir host. The leishmania parasites live in the macrophages as round, non-motile amastigotes (3-7 micrometers in diameter). The macrophages are ingested by the fly during the blood- meal and the amastigotes are relesed into the stomach of insect. Almost immediately the amastigotes transform in to the motile, elongated (10-20 micrometers), flagellate promastigote form. The promastigotes then migrate to the alimentary tract of the fly, where they live extracellularly and multiply by binary fission. Four to five days after feeding the promastigotes move forward to the oesophagus and the salivary glands of the insect. When the sandlfy next feeds on a mammalian host, it's proboscis pierces the skin and saliva containing anti-coagulant is injected into the wound to prevent the blood from clotting, the leishmania promastigotes are transferred to the host along with the saliva. Once in the host the promastigotes are taken up by the macrophages where they rapidly revert to the amastigote form. The leishmania are able to resist the microbiocidal action of the acid hydrolases release form the lysozymes and so survive and multiply inside the macrophages, eventually leading to the lysis of the macrophages. The released amastigotes are taken up by additional macrophages and so the cycle continues. Ultimately all the organs containing macrophages and phagocytes are infected, especially the spleen, liver and bone marrow.

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