Spotlight on Rift Valley fever virus: When Rain Brings Risk
Updated: Aug 10
By Amy Dighe
In 1930, unusually heavy rainfall and flooding in Kenya’s Rift Valley disrupted the normal rhythm of the seasons – delaying lambing from its usual peak in November to July the following year. When lambing finally began, a mysterious outbreak swept through sheep herds, causing exceptionally high mortality in lambs, as well as widespread abortions and deaths in ewes. Sheep herders and veterinary investigators in the area also began to fall ill with fever.
The cause of the outbreak was later identified as a zoonotic mosquito-borne phenuivirus, and named Rift Valley Fever virus (RVF virus). Studies of viral genetic sequences suggest that RVF virus emerged a few decades before it was first detected in the Rift Valley, coinciding with the introduction of large numbers of susceptible livestock by colonial settlers. Since its discovery, devastating RVF outbreaks have occurred episodically in sheep, goats, cattle and camels across Africa – often coinciding with flooding events – and spread to the Arabian Peninsula in the early 2000’s via the livestock trade.
The virus poses a significant threat not only to the economy but also to public health in livestock-rearing regions. While in early outbreaks, fatalities were largely confined to livestock, outbreaks since the 1970s have often resulted in thousands of human cases and hundreds of deaths. The most recent outbreak occurred just last year in West Africa, with 500 human cases and 31 deaths confirmed in Senegal at the end of 2025. With no RVF vaccines licensed for use in humans and strong climatic drivers, there is growing concern that the incidence and geographical range of outbreaks will increase with climate change.
The drivers of RVF transmission are likely to be complex and varied, but the relationship between flooding and inter-epidemic persistence within the mosquito population is thought to contribute to episodic outbreaks in some settings. RVF virus can be passed from female Aedes mosquitoes to their eggs through transovarial transmission. This is particularly consequential in floodwater Aedes species, whose desiccated eggs can survive in dry soil for months or even years. When heavy rainfall eventually occurs, these eggs can hatch en masse, producing large numbers of mosquitoes—many already infected with RVF virus.
Although mosquitoes are the primary route of transmission between livestock, most human RVF cases are thought to arise from direct contact with infected animals or their tissues, particularly during slaughter or veterinary care, placing livestock workers at higher risk. While most human cases involve mild febrile illness, a proportion progress to severe disease with potentially fatal complications affecting the eyes and brain.
Today, owing to their virulence and wide host range, the phenuivirus family to which RVF virus belongs is one of eleven viral families identified by the World Health Organisation (WHO) as posing high pandemic risk. Phenuiviruses are therefore a key surveillance target in the WaSPP network’s pilot wastewater surveillance programme for pandemic prevention. RVF virus, along with Severe fever with thrombocytopenia syndrome (SFTS) virus – a phenuivirus threatening public health in South East Asia – are both shed in human urine and stool, suggesting wastewater surveillance could help us understand and monitor the risk this viral family poses. Assessing the presence of RVF in wastewater during the WaSPP pilot in Ghana and DRC could be particularly informative.
Dr Michael Owusu, Head, Department of Medical Diagnostics at Kwame Nkrumah University of Science and Technology, Accra, who is leading the WaSPP pilot in Ghana says:
“Despite ecological and epidemiological conditions in Ghana that are conducive to RVF transmission, there remains limited data on human exposure, especially among high-risk occupational groups”.
As well as looking for RVF virus in wastewater, Dr Owusu’s team is conducting a cross-sectional sero-surveillance study to assess past exposure of slaughterhouse workers to the virus. About 258 slaughterhouse workers and community controls have been recruited and are being tested for IgG antibodies to RVF using in-house designed assays. He says:
“Such studies will inform key policy decisions by identifying at-risk populations and potential transmission pathways and offer public health interventions, including improved occupational safety practices, community awareness, and strengthened surveillance systems to enhance early detection and outbreak preparedness in Ghana and similar settings.”
About the Author
Amy Dighe is a Research Associate focused on modelling and analytics for WaSPP at Imperial College London
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