RVF virus belongs to the genus Phlebovirus and is a typical Bunyavirus. An enveloped spherical particle of 80–100 nm in diameter, it has a three-segmented, single-stranded, negative-sense RNA genome with a total length of ~11.9 kb. Each of the segments, L (large: 6.4 kb), M (medium: 3.9 kb), and S (small: 1.7 kb), is contained in a separate nucleocapsid within the virion. Remarkably little genetic diversity has been found between RVF virus isolates from many countries, and no significant antigenic differences have been demonstrated, but differences in pathogenicity are seen. The disease is endemic in many tropical and subtropical regions of Africa, Madagascar, and the Arabian Peninsula. Thought to have been originally confined to the Rift Valley region of eastern and southern Africa, the virus has recently expanded its range, with major outbreaks having occurred in Egypt since 1977, West Africa since 1987, Madagascar since 1990, and the Arabian Peninsula in 2000. Particularly large epidemics with large numbers of human cases occurred in Egypt in 1977–1978 and in Kenya in 2006–2007. It is considered a threat to regions further afield, where competent mosquito vectors are present. Sporadic, large epidemics have occurred at 5–10 yr intervals in drier areas of eastern Africa, and less frequently in southern Africa. Outbreaks are usually associated with periods of abnormally heavy rainfall or, in some cases, with localized flooding due to dam building or flood irrigation. During interepidemic periods, the virus may remain dormant in eggs of floodwater-breeding aedine mosquitoes in the dry soil of small, ephemeral wetlands (dambos or pans). In some areas, this transovarial transmission is believed to be the most important interepidemic survival strategy of the virus; however, inapparent cycling of the virus between vectors and wild or domestic mammalian hosts has been shown to occur in many areas. RVF virus may also spread by movement of viremic animals and possibly by wind-borne mosquitoes. When emergence of infected mosquitoes, or introduction of virus to an area, coincides with abnormally wet conditions and the presence of a highly susceptible host population, a large epidemic may ensue when the virus is amplified in ruminants and spread locally by many species of mosquitoes or mechanically by other insects. The incidence of RVF peaks during the late rainy season. In areas with cold winters, both the disease and vectors may disappear after the first frost. In warmer climates where insect vectors are present continuously, seasonality is not seen.
