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.