West Africa is currently grappling with an "unusually severe outbreak" of Rift Valley Fever (RVF), one of the world's deadliest diseases, which has claimed the lives of at least 42 individuals. Since its detection in late September, primarily in Senegal and Mauritania, a total of 404 human cases have been identified as of October 30. The World Health Organization (WHO) has highlighted the gravity of the situation, with experts deployed to strengthen disease surveillance, treatment protocols, and infection prevention and control within health facilities in the affected regions.

The initial alert for the outbreak came on September 20 when human cases first emerged in Senegal. The majority of subsequent cases have been concentrated in the country’s northern livestock-producing region. RVF is a viral disease predominantly observed in domesticated animals in sub-Saharan Africa, including cattle, buffalo, sheep, goats, and camels. However, it poses a significant zoonotic threat, capable of spreading to humans and has been linked to several fatal outbreaks over recent decades.

The WHO has expressed concern that the risk of further spread "remains high." This is attributed to prevailing warm, humid weather conditions and periods of heavy rainfall, which are conducive to mosquito breeding, as well as the movement of livestock, particularly towards Mali and Gambia for grazing and trade activities, which could facilitate the virus's surge.

Human infection typically occurs through mosquito bites or direct contact with the blood or organs of infected animals. This places specific occupational groups, such as herders, farmers, slaughterhouse workers, and veterinarians, at heightened risk. Significantly, there are no recorded instances of human-to-human transmission of RVF.

The clinical presentation of RVF in humans varies. Many individuals experience either no symptoms or a mild, cold-like illness characterized by fever, weakness, back pain, and dizziness. However, a high proportion of severe cases have been reported in the current outbreak, suggesting that the virus may often go undetected until a later, more difficult-to-treat stage. Rarer, severe manifestations can include debilitating eye damage, life-threatening brain swelling, or haemorrhagic fever, which can be fatal. In Senegal, over one in ten cases have presented with haemorrhagic symptoms, leading to 20 fatalities. Mauritania has reported an even more severe outcome, with approximately three in every ten cases proving fatal.

Given the severity and high fatality rates in the affected areas, the WHO has elevated the public health risk to "high" at a national level for Senegal and Mauritania, while maintaining a "low" assessment globally. The UN agency has previously identified RVF as a disease with the potential to spark a future pandemic. In 2018, RVF was indeed listed by the WHO as one of nine priority diseases that pose the biggest risk to public health, primarily due to a lack of effective treatments and its capacity to cause widespread epidemics.

Historically, RVF has been responsible for numerous devastating outbreaks. Notable past events include the 1998 outbreaks in Kenya and Somalia, which resulted in over 470 deaths. The virus then spread beyond Africa for the first time in 2000, causing over 200 deaths in Saudi Arabia and Yemen, raising international concerns about wider dissemination into Asia and Europe. Senegal's last major outbreak in the late 1980s claimed more than 200 lives in the country and neighboring Mauritania. To date, there have been no known cases of RVF ever recorded in the United Kingdom.