Ngari, Bunyamwera, Ilesha, and Germiston viruses are among the mosquito-borne human pathogens in the
Orthobunyavirus genus, family Bunyaviridae, associated with febrile illness. Although the four orthobunyaviruses
have been isolated from mosquito and/or tick vectors sampled from different geographic regions in
Kenya, little is known of human exposure in such areas. We conducted a serologic investigation to determine
whether orthobunyaviruses commonly infect humans in Kenya. Orthobunyavirus-specific antibodies were detected
by plaque reduction neutralization tests in 89 (25.8%) of 345 persons tested. Multivariable analysis
revealed age and residence in northeastern Kenya as risk factors. Implementation of acute febrile illness
surveillance in northeastern Kenya will help to detect such infections.