May 24, 2016
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Leishmaniasis (cont.)

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What causes leishmaniasis? How is leishmaniasis transmitted?

Leishmaniasis is caused by protozoal parasites from the Leishmania species. The organisms are microscopic in size. There are about 21 species of Leishmania that affect humans, including the L. donovani complex and the L. Mexicana complex, among others. The life cycle is relatively simple. When the sand fly bites a human, it injects small numbers of parasites which are rapidly taken up by mononuclear blood cells. This stage is called the promastigote stage. Once inside the human mononuclear cells, the parasite enters the amastigote stage and begins to multiply and infect other cells and tissues. Uninfected sand flies acquire the parasite by feeding on infected people or infected animals such as dogs, foxes, or rodents.

Figure 2: Life cycle of Leishmania
Figure 2: Life cycle of Leishmania

Less commonly, parasites may be transmitted by blood transfusion or through drug users sharing contaminated needles. Leishmania may also be transmitted from a pregnant mother to her fetus.

What are risk factors for leishmaniasis?

The major risk factor for leishmaniasis is being exposed to infected sand flies. The sand flies are most active after dusk and are more common in rural areas. Casual travelers do not usually visit these areas at night, so infection is more common in adventure travelers, Peace Corps workers, missionaries, soldiers, and those with occupational activities that require them to live in rural areas. In healthy people, the degree of immune response to leishmaniasis appears to be genetically determined. In visceral leishmaniasis, a weak immune response is associated with more severe disease. Factors that weaken the immune system include malnutrition and infection with the human immunodeficiency virus (HIV). However, in mucocutaneous leishmaniasis, the symptoms appear to be caused in part by an overactive immune response. Interestingly, the Leishmania parasite itself can be infected with a virus that may cause the parasite to be more dangerous by overstimulating the inflammatory response from the human immune system.

Leishmania may live quietly for years in the body and then begin to multiply (reactivate) if the person's immune system becomes suppressed. Thus, people who were born in a country with leishmaniasis and those who have had travel-related exposure are at risk if they become immunosuppressed by conditions such as chemotherapy, use of steroids, or infection with HIV. Patients who have previously had cutaneous leishmaniasis acquired in certain parts of the New World are at risk for mucocutaneous leishmaniasis.

Medically Reviewed by a Doctor on 11/18/2015


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