Sequential Rift Valley Fever Outbreaks in Eastern Africa Caused by Multiple Lineages of the Virus

Methods. Following confirmation of the RVF epidemic in Kenya on 19 December 2006 and in Tanzania on 2 February 2007, teams were sent to the field for case finding. Human, livestock, and mosquito specimens were collected and viruses isolated. The World Health Organization response team in Kenya worked with the WHO's polio surveillance team inside Somalia to collect information and specimens from Somalia. Results. Seven geographical foci that reported hundreds of livestock and > 25 cases in humans between December 2006 and June 2007 were identified. The onset of RVF cases in each epidemic focus was preceded by heavy rainfall and flooding for at least 10 days. Full-length genome analysis of 16 RVF virus isolates recovered from humans, livestock, and mosquitoes in 5 of the 7 outbreak foci revealed 3 distinct lineages of the viruses within and across outbreak foci. Conclusion. The findings indicate that the sequential RVF epidemics in the region were caused by multiple lineages of the RVF virus, sometimes independently activated or introduced in distinct outbreak foci.