Screening Mosquitoes for Rift Valley Fever Virus and Blood Meal Sources during the 2006/2007 Outbreak in Kenya

Rift Valley Fever (RVF) is a mosquito borne viral infection, first reported in the Rift Valley province of Kenya in 1912 and identified in 1931. Major outbreaks have been reported in Kenya in 1997/1998 and 2006/2007. Baringo, Garissa and Kilifi district of Kenya were hot spots in the last major RVF outbreak that occurred in the country in 2006/2007. Investigations were conducted during the outbreak to establish putative mosquito vectors and vertebrate host of RVF virus. Engorged female mosquitoes (Aedes, Anopheles, Culex, Hodgesia and Mansonia, genera) were selected from mosquitoes sampled outdoor in Baringo, Garissa and Kilifi districts using CO2-baited CDC light traps, and singly cryopreserved. Heads and abdomens of the individual mosquito samples were screened for the virus by cell culture and RT-PCR. Putative vertebrate hosts of RVF were determined by amplification and sequencing cytochrome b (cytb) and cytochrome c oxidase I (COI) genes from DNA extracted from the blood meals obtained from the mosquito abdomens. The cyt b and COI sequences were annotated through bioinformatic pipeline suite comprising of 1) BioEdit 2) Basic Local Alignment Search Tool (BLAST) and 3) Barcode of Life Data Systems (BOLD) database. A total of 33, 162 and 21 engorged mosquitoes from Baringo, Garissa, and Kilifi districts respectively were analysed. From the mosquito samples, RVF virus was detected in the heads of five Mansonia uniformis (Baringo district), one Aedes mcintoshi (Garissa district) and four Aedes ochraceous (Garissa district). Among the infected mosquitoes, Ae. mcintoshi had blood meals from donkey(s) (Equus asinus), Ae. ochraceous from sheep (Ovis aries), human (Homo sapiens) and Ma. uniformis from sheep and goat (Capra hircus). However, an Ae. ochraceous from Garissa and Hodgesia spp from Baringo and Ma. uniformis (Baringo district) had the RVF virus in the blood meals putatively from goat and two sheep respectively, but not in their head tissues. Other blood meals detected in the mosquitoes included those from, Bird (Milvago chimachima), blue duiker (Cephalophus monticola), cattle (Bos taurus), rat (Mus musculus), frog (Anura and Colostethus sp.) or cat (Felis catus). These findings incriminate Mansonia uniformis as putative vectors of RVF in Baringo, and Ae ochraceous and/or Ae. mcintoshi in Garissa districts. This finding suggests possible involvement of Donkey, goat, human and sheep in RVF virus transmission/amplification/maintenance in Baringo and Garissa. These vectors and hosts may have played a role in to the epidemiology of RVF in Baringo and Garissa districts in between 2006 and 2007. Further investigations are required to understand on interactions between these vectors, host and the virus in order to shed light on their specific roles in the epidemiology of RVF.