Rift Valley fever in Nigeria: infections in humans.

Between 1985 and 1989, a total of 3,121 human sera collected from different population groups in six ecological zones of Nigeria were tested for the presence of antibodies to Rift Valley fever (RVF) virus by the haemagglutination-inhibition test. All reactive sera were further tested by the plaque reduction neutralisation test and specific RVF immunoglobulin M (IgM) assay. A total of 461 sera (14.8%) demonstrated haemagglutination-inhibiting antibody and 390 of the 461 initially reactive sera (84.6%) revealed neutralising antibodies. A significantly higher exposure to the virus was found among livestock workers and wildlife rangers than in other categories of people tested. The rate of positive reactions was higher in adults of 30 years or more than in younger age groups. Of 461 sera tested for specific RVF IgM, 107 gave positive results (23.2%). The highest prevalence of RVF IgM was found among livestock and forestry workers. In the longitudinal survey, an RVF virus infection rate of 6.7% was demonstrated. The infection rate was significantly higher during the wet season than during the dry season of the same year.