Leishmania Parasite Life Cycle

Leishmaniasis is a Parasitic Disease Caused by a Protozoan

Oct 9, 2007

Leishmania parasites cause a variety of similar diseases but in each case the organisms follow a similar life cycle in sand flies and humans.

Leishmania parasites are a group of closely related parasites that are transmitted to humans by the bite of a sand fly. They are protozoa belonging to the taxonomic genus Leishmania—tiny single celled parasites that cause leishmaniasis. Leishmaniasis takes different forms: cutaneous, visceral, and mucocutaneous. The latter two are particularly dangerous parasitic diseases. All of the species follow the same basic life cycle.

Leishmania Life Cycle

Leishmania parasites begin their trip to a new human host when a female sand fly (genus Phlebotomus, Sergentomyia, Lutzomyia, Brumptomyia, or Warileya) bites someone who is already infected:

When a sand fly takes a blood meal from someone with leishmaniasis, she ingests human cells that contain the tiny Leishman-Donovan bodies (or amastigotes), the vertebrate tissue stage of the parasite. Leishman-Donovan bodies are oval and microscopically tiny, about three to five one-thousanths of a millimeter long.
Inside the sand fly, amastigotes go to the middle or far end of the fly’s gut, where they transform into a larger stage, the promastigote, and attach to the gut wall. Promastigotes are about three times as long as the amastigotes and they have a long whip-like flagellum attached at one end.
Promastigotes multiply by reproducing their nuclear material and simply dividing in two, a process called binary fission. In this way, though the sand fly may have only ingested a small number of parasites, there are soon large numbers in the fly’s gut.
In less than a week, the promastigotes move forward again, lodging in the fly’s esophagus. When the fly feeds again, it attempts to clear out the promastigotes clogging its esophagus and expels them into the bite wound.
Promastigotes introduced into a fly bite invade host cells, often macrophages, cells that are involved in immunity. There, the promastigotes transform back into amastigotes and, again, multiply by binary fission.
When human macrophages become filled with amastigotes, they rupture and are destroyed. The parasites released invade new cells and begin to multiply again. Amastigotes – Leishman-Donovan bodies—inside host cells is the typical picture of leishmaniasis seen in laboratory samples.
Multiplying amastigotes spread to new cells and new tissues, causing lesions and tissue destruction.
Amastigotes present in the blood and tissues of an infected person are ingested by a biting sand fly and the life cycle repeats itself.

Cutaneous, Visceral, and Mucocutaneous Leishmaniasis.

Leishmaniasis is a disfiguring disease. In cutaneous leishmaniasis, the lesions are confined to the skin which, though unsightly and slow to heal, are not generally life threatening. Visceral leishmaniaisis often involves severe damage to the liver and spleen and immunity to infection is impaired. Untreated, many victims die within three years. Mucocutaneous leishmaniasis destroys mucous membranes—the linings of the nose and mouth—and is horribly disfiguring. Death due to complications is not uncommon.

Leishmania Species

Different species of leishmania parasites are found in different geographic locations and, in general, the different forms of leishmaniasis are associated with particular species. However, for reasons that are not well understood, any of the forms can appear with virtually any species from time to time. This, along with the severity of leishmaniasis and the strange ability of the parasite to evade the human immune system, actually multiplying within immune cells, has made leishmania the subject of continuing medical research.

Sources:

Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.

Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.

The copyright of the article Leishmania Parasite Life Cycle in Human Infections is owned by Rosemary Drisdelle. Permission to republish Leishmania Parasite Life Cycle must be granted by the author in writing.