Rift Valley fever is a viral disease of sheep, cattle, monkeys and rodents and is transmitted to humans by vector mosquito bites. It is not contagious and occurs chiefly in Africa. The symptoms are like those of dengue fever with headache, weakness, fever and sometimes nausea and vomiting. The illness is usually brief and complete recovery is the rule. The disease causes abortions, disease and death in infected herds or flocks (mainly of cattle and sheep).
RVF, a virus disease transmitted by mosquitoes, has been known in east and south Africa for some 50 years. The principal disease hosts in such areas have been cattle and sheep, with the imported exotic breeds proving more susceptible to disease than the indigenous animals. At an early stage, it was realized that man became infected whenever he has in close contact with diseased or dead animals. The numbers of human cases were small and in humans the disease was rarely fatal. Epizootics of RVF occurred after periods of unusually heavy rainfall, which were often separated by periods of up to 10-15 years when no case of Rift Valley fever was recognized. In East and South Africa the use of attenuated, neurotropic strains of RVF virus, developed in mice and shown to be immunogenic in the animal hosts, has proved to be a successful control measure. This situation was dramatically altered in 1977 when RVF was identified in Egypt in epizootic form.