Leishmaniasis in the middle course of the Ethiopian Rift Valley: II. Entomological observations

As part of an epidemiological investigation of visceral leishmaniasis, entomological surveys were conducted in Ziway-Langano, Dimtu-Bilate and Wadjifo-Mirab Abaya areas located in the middle course of the Ethiopian Rift Valley between November 1994 and June 1996. A total of 4518 sand flies (Diptera: Psychodidae) were collected from the study areas using CDC light traps and oiled plastic sheets which were identified to nine species of Phlebotomus and 14 species of Sergentomyia. Most of the Phlebotomus species were from Ziway-Langano area and they included P. martini and P. orientalis, which are the principal vectors of visceral leishmaniasis in Ethiopia. Among 339 females of nine Phlebotomus species dissected, natural infections with flagellates were detected in 11 specimens (two P. duboscqi and nine P. rodhami) from Ziway-Langano area, but were different from mammalian Leishmania species by their position in the insect gut, morphology and behaviour in cultures. However, the reasons for the low level of leishmanin skin-test positivity in this portion of the Rift Valley are not immediately clear in view of the rich sand fly fauna and the presence of potential vectors.