Sero-epidemiology of Rift Valley fever virus in ruminant livestock in The Gambia

Rift Valley fever (RVF) virus is a globally significant zoonotic pathogen primarily transmitted by mosquitoes, causing abortion storms and neonatal mortality in ruminant livestock and haemorrhagic encephalitis in humans. Although localized RVF outbreaks occur frequently in the West Africa Sahel, the complex interplay between vectors, ruminant hosts, human behaviours, and ecoclimatic factors driving these outbreaks remains unclear. In 2022, we conducted a cross-sectional study across The Gambia that collected serum samples linked to questionnaire data from 202 randomly selected livestock-owning households to estimate population-level seroprevalence, investigate the patterns of, and identify risk factors associated with RVFV seropositivity in ruminant livestock. Design adjusted seroprevalence estimates were 36.8% (95% Confidence Interval [CI]: 31.6-40.0) for cattle (n = 1416), 2.1% (95% CI: 0.5-3.7) for goats (n = 1101) and 4.3% (95% CI: 1.2-7.4) for sheep (n = 1085) using the IDScreen(R) RVF c-ELISA assay. The intracluster correlation coefficients for RVFV seropositivity at the herd level were 0.13 (95% CI: 0.09-0.18) in cattle and 0.22 (95% CI: 0.06-0.31) in small ruminants on the logit scale. Risk factors associated with RVFV seropositivity were investigated using a LASSO regression, with transhumant movement to the Gambia river valley, increasing age, introduction of new ruminants, and proximity to water identified as most strongly associated with seropositivity. By fitting the serologic data to a catalytic force of infection (FOI) model, age-independent FOI estimates of 0.12 (95% Credible Interval [CrI]: 0.10-0.16) for cattle and 0.013 (95% CrI: 0.01-0.02) for small ruminants were obtained. The observed patterns of RVFV seropositivity indicate widespread and endemic viral circulation, with livestock management practices and the Gambia river floodplains likely sustaining transmission. The high seroprevalence in cattle raises concerns that sporadic outbreaks may have occurred unnoticed or unreported, underscoring the critical need for comprehensive One Health surveillance to identify potential clinical cases.