Inter-epidemic Rift Valley fever virus seropositivity in an irrigation scheme in Bura, southeast Kenya

Rift Valley Fever (RVF) is an acute mosquito-borne viral zoonosis that causes cyclical epidemics in sub-Saharan Africa, with recent incursions into the Arabian Peninsula. Outbreaks of RVF are associated with above average rainfall and flooding, in which large numbers of vectors emerge. Recent studies into the interepidemic maintenance of RVF virus (RVFV) suggest that both vertical transmission in vectors and direct transmission between hosts, in combination with predisposing ecosystems, are responsible for persistence of the virus. A longitudinal survey was carried out in Tana River County, Kenya, in three ecosystems (irrigated, riverine and pastoral) from September 2014 – June 2015. The objectives of the study were to investigate possibility of low-level RVFV transmission during an inter-epidemic period, to examine variation in RVFV seroprevalence in sheep and goats across these ecosystems and to determine the risk factors for RVFV transmission. 316 small ruminants were selected and tested for immunoglobulin G antibodies against RVFV nucleoprotein using a competitive ELISA during 6 visits. Data on potential risk factors was also captured. The overall RVF virus seropositivity was 12.3% (95%CI = 9%-16.6%) by the end of the study. This varied across the sampling sites with the pastoral ecosystem having a significantly higher seroprevalence at 26.1% (95%CI = 16.6%-38.3%, p0.05) in the irrigated area (7) and in the riverine area (11). Seroconversions were significantly associated with presence of water either due to rainfall or irrigation (p<0.05). This study indicates that RVFV is circulating in the studied area. It also highlights the risk posed by land use changes, such as the creation and expansion of irrigation schemes, in increasing the risk of inter-epidemic RVFV transmission. This is through provision of necessary environmental conditions for vegetative growth thus providing vectors with more breeding grounds, shade and resting places, which further prolong their lifespans thereby propagating the virus. The findings provide policy makers with the evidence needed for vector and RVF prevention and control within this changing environment

Inter-epidemic Rift Valley fever virus seroconversions in an irrigation scheme in Bura, south-east Kenya

Rift Valley fever (RVF) is an acute mosquito-borne viral zoonosis whose outbreaks are often associated with prolonged rainfall and flooding, during which large numbers of vectors emerge. Recent studies into the inter-epidemic maintenance of RVF virus (RVFV) suggest that both vertical transmission in vectors and direct transmission between hosts act in combination with predisposing factors for persistence of the virus. A comparative longitudinal survey was carried out in Tana River County, Kenya, in irrigated, riverine and pastoral ecosystems from September 2014–June 2015. The objectives were to investigate the possibility of low-level RVFV transmission in these ecosystems during an inter-epidemic period (IEP), examine variations in RVFV seroprevalence in sheep and goats and determine the risk factors for transmission. Three hundred and sixteen small ruminants were selected and tested for immunoglobulin G antibodies against RVFV nucleoprotein using a competitive ELISA during six visits. Data on potential risk factors were also captured. Inter-epidemic RVFV transmission was evidenced by 15 seroconversions within the irrigated and riverine villages. The number of seroconversions was not significantly different (OR = 0.66, CI = 0.19–2.17, p = .59) between irrigated and riverine areas. No seroconversions were detected in the pastoral ecosystem. This study highlights the increased risk of inter-epidemic RVFV transmission posed by irrigation, through provision of necessary environmental conditions that enable vectors access to more breeding grounds, resting places and shade, which favour their breeding and survival