How is VL spread by its natural sand fly vectors and mammalian hosts?

Paul Ready: Now, we’re going to consider how visceral leishmaniasis is spread by its natural sand fly vectors and mammalian hosts.

Some definitions– Anthroponotic transmission means the sand fly vector spreads the parasite from human to human. In comparison, zoonotic transmission involves the parasite being transmitted from a mammal– a reservoir host– to a human. Visceral leishmaniasis caused by Leishmania donovani, is anthroponotic on the Indian subcontinent, with the sand fly species, Phlebotomus argentipes, transmitting the parasite from one human to another.

Parasites have been detected in water buffalo and other domesticated mammals, but these have not been demonstrated to be reservoir hosts, meaning they are not proven to be a source of infection to humans.

Consequently, some of the more important risk factors for contracting anthroponotic visceral leishmaniasis

on the subcontinent are A: proximity to human cases, including those with PKDL.

B: proximity to high densities of adult females of Phlebotomus argentipes and the mammals providing these vectors with blood meals, such as cattle and water buffalo.

And C: proximity to larval breeding sites in soil or dung, before they become flooded towards the end of the monsoon season.

In contrast, visceral leishmaniasis caused by Leishmania donovani could be zoonotic as well as anthroponotic in Sub-Saharan Africa, although the wild mammals found infected, such as mongoose or other rodents, have not yet been proven to be reservoir hosts.

The more important risk factors include the time spent in the Acacia-Balanites savannah environment of the sand fly, Phlebotomus orientalis– mostly in Sudan and Ethiopia, or near the termite mounds in the alternative savannah environments of Phlebotomus martini and related sand flies– mostly in Sudan, Ethiopia, and Kenya. Many visceral leishmaniasis cases result from migration into these environments because of drought and warfare, or the movements of pastoralists and their livestock herds following seasonal rains.

Visceral leishmaniasis caused by Leishmania infantum is usually zoonotic, with the domestic dog being the main reservoir host, both in the Mediterranean region and in Latin America. The Mediterranean region has many different sand fly vectors, mostly in the subgenus Phlebotomus larroussius. Whereas the predominant vector in Latin America is Lutzomyia longipalpis. This reflects the fact that visceral leishmaniasis was introduced to Latin America in dogs transported from the Mediterranean region, starting about 500 years ago. However, the vectors in both continents– Europe and Latin America– are usually most abundant in the yards of rural houses and farms, although Lutzomyia longipalpis and visceral leishmaniasis are reported to be increasing in suburban areas. The more important risk factors in such environments

are A: proximity to infected dogs, and B: proximity to high densities of the vectors. Leishmania infantum was named because many infections in the early 20th century were found in children. Adults are at high risk of infections, causing death in areas where transmission to children was interrupted by control measures. Many adults in such areas have not acquired immunity as children, and this can lead to more rapid disease progression if transmission resumes after control measures are terminated.