Visceral leishmaniasis in Aba-Roba, south-western Ethiopia: prevalence and incidence of active and subclinical infections.

Between August 1997 and February 2005, a prospective study of human visceral leishmaniasis (VL) was undertaken in two villages in the Konso district of south-western Ethiopia, to provide epidemiological indices of subclinical infection and active VL. Six cross-sectional surveys at 6-month intervals (ending in August 2000) were complemented by a single survey in February 2005. The prevalences and incidences of leishmanial infection and active VL, which were determined using leishmanin skin tests (LST), direct agglutination tests (DAT) and parasitological diagnosis, varied spatio-temporally and by age and gender. At baseline, when 1339 individuals were investigated, the overall prevalences of LST positivity, DAT positivity and active VL among the 1232 subjects who had not been treated previously were 30.0%, 5.4% and 0.49%, respectively. During the study, <10% of the subjects found DAT-positive at baseline progressed to active VL (with a mean of about nine cases of subclinical infection for every one of active VL). The median age of an incident VL case was 10.5 years. The highest rates of LST conversion occurred among the subjects aged 5-25 years. A subject who became LST-positive during the study was much less likely to develop active VL than the other subjects.