Emergencies preparedness, response

Rift Valley Fever

WHO/P. Formenty

Rift Valley Fever (RVF) is caused by a virus transmitted by mosquitoes and blood feeding flies that usually affects animals (commonly cattle and sheep) but can also involve humans. In humans the disease ranges from a mild flu-like illness to severe haemorrhagic fever that can be lethal. When livestock are infected the disease can cause significant economic losses due to high mortality rate in young animals and waves of abortions in pregnant females.

The virus was first identified in 1931 during an epidemic among sheep on a farm in the Rift Valley of Kenya. Since then, outbreaks have been reported in sub-Saharan Africa, North Africa, and in 2000 Saudi Arabia and Yemen, marking the first reported occurrence of the disease outside the African continent, raising concerns that it could extend to other parts of Asia and Europe.

The majority of human infections result from direct or indirect contact with the blood or organs of infected animals. The virus can be transmitted to humans through the handling of animal tissue during slaughtering or butchering, assisting with animal births, conducting veterinary procedures, or from the disposal of carcasses or fetuses. Certain occupational groups such as herders, farmers, slaughterhouse workers and veterinarians are therefore at higher risk of infection.

The virus infects humans through inoculation, for example via a wound from an infected knife or through contact with broken skin, or through inhalation of aerosols produced during the slaughter of infected animals.

There is some evidence that humans may also become infected with RVF by ingesting the unpasteurized or uncooked milk of infected animals. Human infections have also resulted from the bites of infected mosquitoes, most commonly the Aedes and Culex mosquitoes, as well as from bites of hematophagous (blood-feeding) flies.

Mild form of Rift Valley Fever in humans
The interval from infection to onset of symptoms for RVF varies from 2 to 6 days. Those infected either experience no detectable symptoms or develop a mild form of the disease characterized by a feverish syndrome with sudden onset of flu-like fever, muscle pain, joint pain and headache. Some patients develop neck stiffness, sensitivity to light, loss of appetite and vomiting; in these patients the disease, in its early stages, may be mistaken for meningitis. The symptoms of RVF usually last from 4 to 7 days.

Severe form of Rift Valley Fever in humans
While most human cases are relatively mild, a small percentage of patients develop a much more severe form of the disease. This usually appears as one or more of three distinct syndromes: ocular (eye) disease (0.5-2% of patients), meningoencephalitis (less than 1%) or haemorrhagic fever (less than 1%).

The total case fatality rate has varied widely between different epidemics but, overall, has been less than 1% in those documented. Most fatalities occur in patients who develop the haemorrhagic icterus form.

Because the symptoms of Rift Valley fever are varied and non-specific, clinical diagnosis is often difficult, especially early in the course of the disease. Rift Valley fever is difficult to distinguish from other viral haemorrhagic fevers as well as many other diseases that cause fever, including malaria, shigellosis, typhoid fever, and yellow fever.

Definitive diagnosis requires testing that is available only in reference laboratories. Laboratory specimens may be hazardous and must be handled with extreme care. Rift Valley fever virus infections can only be diagnosed definitively in the laboratory using the following tests:

1) reverse transcriptase polymerase chain reaction (RT-PCR) assay
2) IgG and IgM antibody enzyme-linked immunosorbent assay (ELISA)
3) virus isolation by cell culture.

As most human cases of RVF are relatively mild and of short duration, no specific treatment is required for these patients. For the more severe cases, the predominant treatment is general supportive therapy.

An inactivated vaccine has been developed for human use. However, this vaccine is not licensed and is not commercially available. It has been used experimentally to protect veterinary and laboratory personnel at high risk of exposure to RVF. Other candidate vaccines are under investigation.

During an outbreak of RVF, close contact with animals, particularly with their body fluids, either directly or via aerosols, has been identified as the most significant risk factor for RVF virus infection. Raising awareness of the risk factors of RVF infection as well as the protective measures individuals can take to prevent mosquito bites is the only way to reduce human infection and deaths.

Technical information

Outbreak forecasting models

RVF outbreaks in East Africa are closely associated with periods of heavy rainfall that occurs during the warm phase of the El Niño/Southern Oscillation (ENSO) phenomenon. These findings have enabled the successful development of forecasting models and early warning systems for RVF using satellite images and weather/climate forecasting data enabling authorities to implement measures to avert impending epidemics.

Country support

WHO Collaborating Centres work in close collaboration with WHO and Member States to investigate, confirm and control outbreaks, such as those of Rift Valley Fever.

General information

Maps and graphs

Geographic distribution of Rift Valley fever outbreaks (Data as of 2009).

Rift Valley fever virus ecological zones mapped using a combination of reported cases, serological evidence and potential risk of transmission based on the presence of the Aedes mosquito.