First field evidence for natural vertical transmission of West Nile virus in Culex univittatus complex mosquitoes from Rift Valley Province, Kenya

West Nile virus is a mosquito borne flavivirus endemic over a large geograp hic area including Africa, Asia, and the Middle East. Although the virus ge nerally causes a mild, self-limiting febrile illness in humans, it has spor adically caused central nervous system infections during epidemics. An isol ate of West Nile virus was obtained from a pool of four male Culex univitta tus complex mosquitoes while we were conducting an investigation of Rift Va lley fever along the Kenya-Uganda border in February-March 1998. This repre sents the first field isolation of West Nile virus from male mosquitoes and strongly suggests that vertical transmission of the virus occurs in the pr imary maintenance mosquito vector in Kenya. A phylogenetic analysis of the complete amino acid sequence of the viral envelope glycoprotein demonstrate d a sister relationship with a Culex pipiens mosquito isolate from Romania made in 1996, This unexpected finding probably reflects the role of migrato ry birds in disseminating West Nile virus between Africa and Europe.