There are three forms of leishmaniasis. The most severe is called kala-azar. It causes fever, weight loss and immune system suppression. Mortality is high, but can be cut to 5 percent with good care and a 30-day course of treatment with an intravenous drug. The most common form of leishmaniasis produces long-lasting skin ulcers but is not fatal. A third and relatively rare form causes chronic, disfiguring infections of the nose and mouth. About a dozen species of Leishmania parasite cause one or more forms of the disease.
The parasites that cause leishmaniasis undergo profound changes during the course of infection. A sand-fly bite transmits the bug in a slender, swimming form called a "promastigote". Symptoms occur only after the organism invades cells and transforms into an oval "amastigote" stage. As the organism changes form, different genes are turned on and off, leading to production of different proteins for each stage. In response, the human host makes one set of antibodies against the promastigote and another set against the amastigote.
The surest way of diagnosing any infection is to find the offending organism in the blood or elsewhere in the body of the sick person. An alternative method is to detect antibodies that the person's immune system has made against the invading bug. An organism's presence is more definitive than an antibody's. However, antibody tests are preferred when testing large populations, for they only require a blood sample.
The antibody test currently in use looks for a reaction between person's blood and an extract of promastigotes. In people with kala-azar, the body is teeming with Leishmania organisms and the test almost always is positive.
Checking many people for leishmaniasis is impractical. The only test with a good chance of finding it requires the removal of cells from the bone marrow, a painful and moderately expensive procedure.