How is visceral leishmaniasis transmitted?

Paul Ready: Now we’re going to consider how visceral leishmaniasis is transmitted or spread. Visceral leishmaniasis (VL) is a human disease associated with a variety of symptoms. Some of or all of which can be caused by infections of various Leishmania species. Usually, the clinical symptoms include an irregular fever, anaemia, and enlarged spleen or liver. Leishmania are single-cell protozoan parasites and they occur in a form without a long flagellum in humans and other mammals. These are known as the amastigote stage. Amastigotes can be transmitted between humans by the sharing of hypodermic needles and syringes or by contaminated blood transfusions. These are avoidable accidents.

Less easily avoided is the natural transmission of Leishmania species by sand flies, which are the only proven transmitters or vectors of the disease-causing parasites. Specifically, the vectors are Phlebotomine sand flies (Diptera, Phlebotominae), which are small insects up to four millimetres long, recognisable by their hopping flight and half closed pair of wings when resting. Male sand flies are not vectors. Only the adult female has the piercing mouth parts or proboscis that permit blood feeding, during which the amastigote parasites are taken up from the infected skin or blood of a mammalian host and then passed on to another host during a later blood meal.

The parasites are not transmitted by other kinds of sand fly, such as the biting midges on the far right of the picture, which can be a nuisance on tropical sandy beaches. Not all sand fly species have females that are competent vectors, that is to say ones that are adapted to carry and transmit the parasite. To be a competent vector, the sand fly species has to permit,

A: the establishment of flagellated parasites of the promastigote stage in their mid guts

and sometimes hindguts, B: the development of other parasite growth stages that can move forward towards the foregut near the proboscis,

and C: the survival of the infective parasites or metacyclics in mammalian cells after regurgitation with sand fly saliva during a subsequent blood feeding.

Vector competence is necessary but not sufficient for establishing enough parasite transmission to maintain an endemic focus of visceral leishmaniasis. An endemic focus as a geographical region in which new cases appear annually without introductions of infected hosts from the outside.

Phlebotomine sand fly vectors of Leishmania species are tropical and sub-tropical insects. And so, visceral leishmaniasis is mostly found from about 30 degrees north to 30 degrees south, except in Europe and Asia where it reaches to about 45 degrees north.

The vector has to encounter the mammalian hosts of the parasite, and to do so sufficiently frequently, which introduces the importance of studying not only the overlapping ecological niches of the vectors and mammalian hosts, but also the spatial and seasonal dynamics of transmission. A study in a visceral leishmaniasis endemic area of Bihar, in North East India, looked at the possible relationship between seasonal variation in such things as rainfall and temperature, and if there were any trends in the numbers of VL cases seen, and also how the sand flies might be affected.

On the right of the slide, a study by Tiwary and colleagues showed that sand flies are highly prevalent during the rainy season and that more of them are infected with Leishmania donovani during November to December. Typically, the time from infected bite to disease is two to six months, which would lead to an increase in visceral leishmaniasis incidents in March to April the following year. This is borne out by the study on the left.