Epidemiological and molecular characterization of Rift Valley fever outbreak in livestock in Burundi, May - November 2022

An outbreak of Rift Valley fever (RVF) was officially reported for the first time in Burundi on 10 th May 2022. The outbreak originated in the northern provinces and progressively spread to other regions of the country. This study presents (i) epidemiological investigations that were carried out through a countrywide emergency response and (ii) the characterization of the genotype of the RVF virus that caused the outbreak through phylogenetic analyses. Field teams visited each affected farm, collected data on observed syndromes, species and number of animals affected, farm’s locations, and herd size. Blood, serum and tissue samples were collected from selected clinical cases. Epidemiological data were analyzed using R (version 4.4.2) to determine the spatiotemporal distribution of cases. Mixed effects Poisson regression models were fitted to the data to identify risk factors. A total of 1,739 clinical cases were recorded. Of 100 samples collected and screened using Reverse Transcription Polymerase Chain Reaction (RT-PCR), 36 tested positives. Phylogenetic analyses revealed that the outbreak was caused by an RVFV strain of lineage C, sub-clade C.2.2 of the dominant lineage that was circulating in East Africa, with a close relationship to RVFV that was isolated in Rwanda in 2022. Epidemiological analyses revealed the northeastern region as the epicenter of the outbreak. Multivariable analyses showed that increased RVF cases were significantly associated with high and persistent rainfall and an upsurge in the minimum temperatures that occurred 3–4 months earlier. The analyses conducted provided insights on the risk of RVF in the country. The results would help the development of risk maps and other decision support tools that would be used to manage future risks of the disease.