Understanding the Rift Valley fever exposure risk: A comparative perspective from a multi-country study in East and Central Africa, 2021-24

Abstract
Rift Valley fever (RVF) is a concern in East and Central Africa, particularly following periods of heavy rainfall and flooding. However no human outbreaks have been reported in the Democratic Republic of the Congo (DRC). To assess whether this reflects a true absence of virus circulation, we estimated RVF seroprevalence in Goma (eastern DRC) and examined context-specific risk factors, comparing the findings with data from outbreak-prone countries. A two-year longitudinal study, across six health facilities in DRC, Kenya and Uganda, enrolled febrile subjects aged ≥10 years. Human serum samples were analyzed for RVF virus and anti-RVF antibodies. Demographic, behavioral, occupational and environmental factors were evaluated. 4,806 participants were enrolled: 1,370 (28.5%) DRC, 1,468 (30.6%) Kenya and 1,968 (40.9%) Uganda. 253 participants (5.3%) tested positive for RVF by serological and/or molecular assays: 19 (1.4%) DRC, 29 (2.0%) Kenya and 205 (10.4%) Uganda (p < 0.001). Only in Uganda, subjects tested positive for RVF virus by PCR (10 subjects, 0.5%). Occupations and activities involving contact with livestock, predominated in Kenya and Uganda, whereas handling of raw meat was most common in DRC. No specific occupations or activities were significantly associated with RVF exposure in DRC while several significant factors were identified for Kenya and Uganda. Multivariate analysis across all three countries showed that being from Uganda, male, over 20 years of age, employed as butcher or crop farmer and engaging in animal-related activities, were independently associated with RVF positivity, as was contact with sheep. Despite a prevailing sense that RVF transmission does not occur in DRC, we found a seroprevalence of 1.4%, comparable to Kenya where RVF is well documented. Further research targeting high-risk human and animal populations in DRC is warranted. A One Health approach will contribute to defining the ecology of local transmission of RVF in DRC.

Author summary
Rift Valley fever (RVF) is a zoonotic arthropod-borne viral disease that threatens global health security. Although several outbreaks have been reported in East Africa and RVF virus circulation has previously been described in small ruminants and cattle in the Democratic Republic of Congo (DRC), to date there have been no reported outbreaks of RVF in humans in the DRC. We therefore investigated in Goma, eastern DRC, seroprevalence in humans and risk factors for exposure to RVF virus, and compared the results with those from neighbouring outbreak-prone countries, Kenya and Uganda. Despite a prevailing sense that RVF transmission does not occur in the DRC, we found a seroprevalence of 1.4%, similar to that observed in Kenya. No significant exposure factors were identified for the study population in the DRC alone, but joint analysis with Kenya and Uganda revealed several factors associated with previous exposure to the RVF virus, such as country and various types of contact with livestock. Further research targeting the most at-risk human and livestock populations in the DRC is needed to understand the underlying virus circulation mechanisms.