Visceral Leishmaniasis - Life-cycle of The Parasite

Life-cycle of The Parasite

Visceral leishmaniasis (Kala-azar) is spread through an insect vector, the sandfly of the Phlebotomus genus in the Old World and the Lutzomyia genus in the New World. Sandflies are tiny creatures, 3-6 millimeters long by 1.5-3 millimeters in diameter, and are found in tropical or temperate regions throughout the world. Sandfly larvae grow in warm, moist organic matter, such as old trees, house walls, or waste — making them hard to eradicate.

The adult female sand fly is a bloodsucker, usually feeding at night on sleeping prey. When the fly bites an animal infected with L. donovani, the pathogen is ingested along with the prey’s blood. At this point the protozoan is in the smaller of its two forms, called an amastigote — round, non-motile, and only three to seven micrometers in diameter.

Taken into the stomach of the sandfly, the amastigotes quickly transform into a second L. donovani form, called the promastigote. This form is spindle-shaped, triple the size of the amastigote, and has a single flagellum that allows for mobility. The promastigotes live extracellularly in the sandfly’s alimentary canal, reproducing asexually, then migrate to the proximal end of the gut where they become poised for a regurgitational transmission. This is their means of transmission back into a mammalian host, as the fly injects its saliva into prey when it bites. The promastigotes are introduced locally at the bite site along with the fly’s saliva.

Once inside the new host, promastigotes invade macrophages. Once inside, they transform back into the smaller amastigote form. As an amastigote, L. donovani can only reproduce intracellularly — and the amastigotes replicate in the most hostile part of the macrophage cell, inside the phagolysosome, whose normal defensive response they are able to prevent. After they have reproduced to a certain extent, the L. donovani lyse their host cell by sheer pressure of mass, but there is some recent speculation that they are able to leave the cell by triggering the exocytosis response of the macrophage. The daughter cell protozoans then migrate through the bloodstream to find new macrophage hosts. In time, L. donovani becomes a systemic infection, spreading to all the host’s organs, particularly the spleen and liver.