Why floods raise the risk of Rift Valley fever in Kenya

Why does heavy rain raise the risk of Rift Valley fever? Because the virus lives in mosquitoes and livestock, and floodwater multiplies the mosquitoes. The World Health Organization (WHO) says Aedes mosquitoes are the main carriers in animals and can pass the virus to their offspring through eggs, and the US Centers for Disease Control and Prevention (CDC) says excessive rainfall lets more of those eggs hatch. Infected mosquitoes bite livestock, and people mostly catch the virus from the blood or organs of sick animals.

The Kenya Meteorological Service Authority (KMSA) forecast above-average rainfall over 80 per cent of the country for the October–December short rains, Capital FM reported on 26 August. KMSA's technical statement of 26 August said the Niño 3.4 index, a gauge of El Niño, reached +2.7°C for the week centred on 12 August, and that El Niño was "evolving toward a very strong event". A later weather outlook, dated 30 September, says most parts of the country will get above-average rain, the Kenya News Agency (KNA) reported in The Star, a Kenyan daily, on 5 October.

Rift Valley fever is a zoonosis, a disease that passes between animals and people. The virus was first identified in 1931 during an investigation into an epidemic among sheep in Kenya's Rift Valley.

Outbreak records tie the disease to floods. The WHO fact sheet says a major outbreak struck Kenya, Somalia and Tanzania in 1997–98, following an El Niño event and extensive flooding. The CDC says outbreaks typically occur in years of unusually heavy rainfall and flooding.

The International Livestock Research Institute (ILRI) describes a later epidemic. Heavy rain flooded East Africa at the end of 2006, and across the region an estimated 40,000 people fell sick. In Kenya alone, 158 people died, ILRI wrote in an article posted on 13 March 2026. The disease closed livestock trade to key Middle Eastern markets, costing Kenya US$32 million.

The WHO says outbreaks in animals precede human cases. Sheep and goats appear to be more susceptible than cattle or camels, and an outbreak in animals frequently shows up as a wave of unexplained abortions. ILRI wrote that in 2006 lambs and kids died and ewes and cows aborted.

In people, the incubation period is two to six days. Most infections cause no symptoms or a mild illness with sudden flu-like fever, muscle and joint pain and headache. The haemorrhagic form affects fewer than 1% of patients, with a case fatality of about 50%, and the eye form affects 0.5–2% of patients. Herders, farmers, slaughterhouse workers and veterinarians are at higher risk, and no human-to-human transmission has been documented.

The illness is hard to tell apart clinically from malaria, typhoid fever and yellow fever. An inactivated human vaccine has been developed but is not licensed or commercially available, the WHO says; the CDC says no vaccines are currently available for people. For livestock, animals must be vaccinated before an outbreak if an epizootic is to be prevented, and not once an outbreak has occurred.

The Ministry of Health listed Rift Valley fever with malaria and dengue among the risks of the coming rains, in an advisory dated 14 September, the Kenyan news website Kenyans.co.ke reported. The Standard reported on 15 September that "Increased cases of cholera, Rift Valley fever and malaria were also reported" during the September 2023 to February 2024 El Niño period. Kenyans.co.ke said the advisory named 21 counties, with Mandera, Wajir, Marsabit, Garissa, Kitui, Makueni and Machakos listed as high risk.

For households, The Standard reported that residents were advised to keep children away from floodwater, sleep under treated mosquito nets and remove stagnant water. The WHO advises hand hygiene and gloves when handling sick animals, and says that in epizootic regions all animal products (blood, meat and milk) should be thoroughly cooked before eating.

ILRI said Kenya's updated contingency plan for the disease was finalised and implemented in November 2025, after ILRI's team pioneered a risk map of the counties most exposed. Nairobi and Mombasa are among the counties where the short rains are expected to start in the second or third week of October, according to the 30 September outlook.