Monday, May 07, 2007
Leishmaniasis in U.S. Soldiers
US soldiers in Iraq and Afghanistan are coming back home infected with Leishmanisis. Leishmanisis is a disease caused by infection with the protozoa Leishmania. It's transmitted by the bite of the sandfly Phlebotomus.
Leishmania promastigotes. This is the form that lives in the sandfly and is injected into humans or animals. Notice the flagella.
Leishmania amastigote. This is the form that lives in the human. Notice the lack of a flagella.
Phlebotomus (sandfly) is the insect vector of "Old World" Leishmania.
Leishmaniasis occurs in the "Old World" as well as the "New World". Both areas of the World have cutaneous (skin) and visceral (organ) forms of the disease caused by infection with different species of Leishmania.
The "Old World" forms are found in Europe, Africa and Asia. Cutaneous forms of the disease are due to infection with L. tropica or L. major with a "diffuse" cutaneous form caused by infection with L. aethiopica. The visceral form of Leishmaniasis is du to infection with L. donovani. Similar forms of the disease are found in the new world and are caused by different species of Leishmania. The different Leishmania species all look similar under a microscope and can best be identified using DNA or isoenzyme testing of the parasite. Otherwise, the geographic location and type of lesion can help one guess the species one is dealing with.
The life cycle of Leishmania is relatively simple. When a sandfly takes a blood meal, it injects Leishmania promastigotes into a human. The promastigote form of the parasite is taken up by macrophages which under normal circumstances (for most pathogens) would kill the promastigote. Unfortunately Leishmania protozoa have evolved to make the macrophage their home, inside a parasitophorous vacule. In the macrophage, the promastigote loses its tail and transforms into an amastigote. Amastigotes undergo rounds of asexual replication in the macrophage until eventually the macrophage ruptures releasing newly formed amastigotes to infect more host cells. Uninfected sandflies become infected when they take a bloodmeal from an infected individual. The sandfly ingests amastigotes which transform into promastigotes in the gut of the sandfly. Promastigotes reproduce by longitudinal binary fission, quickly reaching numbers that clog the anterior gut and pharynx of the sandfly. When the sandfly feeds again, promastigotes are dislodged and enter the human or animal host.
Cutaneous leishmaniasis is a somewhat mild skin disease that features a "crater-like" lesion. In humans the initial sign of infeciton is a nodule appearing at the site of a sandfly bite. The lesion eventually becomes ulcerated and then erupts.
A "dry sore" is found in those infected with L. minor and a "wet sore" in those infected with L. major. Most lesions tend to heal within a year if there are no complicating bacterial infections. Fluconazole, an anti-fungal drug, can be used to speed up healing of lesions.
Visceral leishmaniasis or "Kala-azar" is caused by infection with Leishmania donovani. Parasite-infected cells can be found in spleen, liver, lymph glands, intestinal mucosa and bone marrow. This can result in a very enlarged liver and spleen. This can be a fatal diseas of humans if not treated. Unfortunately, the treatment, pentavalent antimonials, is almost as bad as the disease. Treatment with Amphtericin B is also used also with harsh side effects.