Jarra, E. | bioRxiv Science

bioRxiv2025

Rift Valley fever virus dynamics in a transhumant cattle system in The Gambia

Rift Valley fever (RVF) is a zoonotic disease of global concern, driven by environmental conditions, vector activity, and livestock mobility. Although RVF has been reported in The Gambia, its epidemiology remains poorly understood. This study developed a compartmental model to study RVF dynamics in the cattle population of the country. The model incorporated seasonally dynamic transmission parameters reflecting transhumant movement and ecological differences between two distinct ecoclimatic regions: the Sahelian area and the Gambia river. Parameterised using serological data linked to household survey data, the model predicted endemic RVF virus (RVFV) circulation within The Gambia, and captured temporal infection trends that closely match empirical data. Weak decay rates of seropositivity were required to match predicted and observed age-seroprevalence. Results indicated sustained RVFV transmission during the dry season in the Gambia river eco-region, with a high risk of seasonal virus introductions to the Sahelian eco-region at the start of the wet season via the returning transhumant cattle. Our study highlighted the role of livestock mobility in RVFV epidemiology in The Gambia and the need for targeted control strategies that might include, for example, targeted cattle vaccination or application of topical insecticide treatments for transhumant herds.

ecology↗

bioRxiv2025

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.

ecology↗