Sero-prevalence and spatial distribution of Rift Valley fever infection among agro-pastoral and pastoral communities during Interepidemic period in the Serengeti ecosystem, northern Tanzania

Background: In the past two decades, Rift Valley Fever (RVF) outbreaks have been reported twice in Tanzania, with the most recent outbreak occurring in 2006/07. Given the ecology and climatic factors that support mosquito vectors in the Serengeti ecosystem, we hypothesized a continued transmission of RVF virus (RVFV) during interepidemic periods. This study was carried out to determine sero-prevalence, spatial distribution and factors associated with RVF in at-risk agro-pastoral and pastoral communities in the Serengeti Ecosystem in northern Tanzania. Methods: A cross sectional study was carried out to establish the general exposure to RVFV by detecting anti-RVFV IgG and anti-RVFV IgM using ELISA techniques. The health facilities where human subjects were blood sampled concurrent with interviews included Bunda District Designated Hospital, Wasso DDH, Endulen hospital, Arash, Malambo, Olbabal, and Piyaya dispenaries (Ngorongoro district) and Nyerere DDH (Serengeti district) respectively. In addition, human subjects from Lamadi ward (Busega district) were recruited while receiving medical service at Bunda DDH. We conducted logistic regression to assess independent risk factor and mapped the hotspot areas for exposure to RVFV. Results: A total of 751 subjects (males = 41.5%; females = 58.5%) with a median age of 35.5 years were enrolled at out-patient clinics. Of them, 34 (4.5, 95% CI 3.3-6.3%) tested positive for anti-RVFV IgG. Of the 34 that tested positive for anti-RVFV IgG, six (17.6%) tested positive for anti-RVFV IgM. Odds of exposure were higher among pastoral communities (aOR 2.9, 95% C. I: 1.21-6.89, p < 0.01), and agro-pastoral communities residing in Ngorongoro District (aOR 1.8, 95% C. I 1.14-3.39, p = 0.03). Hotspot areas for exposure to RVFV were Malambo, Olbalbal and Piyaya wards in Ngorongoro district, and Lamadi ward in Busega district. Conclusions: The study found both previous and recent exposure of RVFV in humans residing in the Serengeti ecosystem as antibodies against both IgG and IgM were detected. Detection of anti-RVF IgM suggests an ongoing transmission of RVFV in humans during inter-epidemic periods. Residents of Ngorongoro district were most exposed to RVFV compared to Bunda and Serengeti districts. Therefore, the risk of exposure to RVFV was higher among pastoral communities compared to farming communities.