Mapping the risk of Rift Valley fever in Uganda using national seroprevalence data from cattle, sheep and goats
Authors: Tumusiime Dan , Isingoma Emmanuel , Tashoroora Optato B. , Ndumu Deo B. , Bahati Milton , Nantima Noelina , Mugizi Denis Rwabiita , Jost Christine , Bett Bernard 
Published: May 2022

Uganda has had repeated outbreaks of Rift Valley fever (RVF) since March 2016 when human and livestock cases were reported in Kabale after a long interval. The disease has a complex and poorly described transmission patterns which involves several mosquito vectors and mammalian hosts (including humans). We conducted a national serosurvey in livestock to determine RVF virus (RVFV) seroprevalence, risk factors, and to develop a risk map that could be used to guide risk-based surveillance and control measures. A total of 3,281 animals from 189 herds were sampled. Serum samples collected were screened at the National Animal Disease Diagnostics and Epidemiology Unit (NADDEC) using a competition multispecies anti-RVF IgG ELISA kit. Data obtained were analyzed using a Bayesian model that utilizes integrated nested Laplace approximation (INLA) and stochastic partial differential equation (SPDE) approaches to estimate posterior distributions of model parameters, and account for spatial autocorrelation that was detected in the data based on Moran’s I test. Variables considered included animal level factors (age, sex, species) and multiple environmental data including meteorological factors, soil types, and altitude. A risk map was produced by projecting fitted (mean) values, from a final model that had environmental factors onto a spatial grid that covered the entire domain. The overall RVFV seroprevalence was 11.39% (95% confidence interval: 10.35 – 12.51%). Higher RVFV seroprevalences were observed in older animals compared to the young, and cattle compared to sheep and goats. RVFV seroprevalence was also higher in areas that had (i) lower precipitation seasonality, (ii) haplic planosols, and (iii) lower cattle density. The risk map generated demonstrated that RVF virus was endemic in several regions including those that have not reported clinical outbreaks in the northeastern part of the country. This work has improved our understanding on spatial distribution of RVFV risk in the country as well as RVF burden in livestock.

Author summary

Outbreaks of Rift Valley fever occur periodically in Uganda in livestock and humans. An Initial outbreak was reported in Kabale in March 2016 after a long quiescent period. Factors that trigger these outbreaks have not been fully described, but above-normal precipitation and flooding are known to be important causes. This paper presents a study that was conducted in the country to analyse RVF virus seroprevalence and risk factors for exposure in livestock. The study also mapped spatial distribution of RVFV seroprevalence. Data used in the study were generated from a national cross-sectional serosurvey that involved cattle, sheep and goats. Results obtained showed that the national RVFV seroprevalence was 11.39%, with 95% confidence interval of 10.35 – 12.51%. Cattle had higher RVFV seroprevalence compared to sheep and goats. Environmental factors that were associated with increased seroprevalence were low precipitation seasonality, haplic planosols, and low cattle density. Prevalence maps generated showed that the southwestern, central and parts of the northeastern regions had higher seroprevalence compared to other regions of the country. These can be used to guide the surveillance of the disease in the country.