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 What is Known?
Rift Valley fever (RVF) virus was first isolated in 1931 from the blood of a newborn lamb and later from the blood of sheep and cattle. In the 1930s, one of the carriers for the virus was found to be the mosquito, as a result of the correlation of a series of events. In July 1930, Kenya was hit with very heavy rains that substantially increased the wetlands where mosquitoes bred. At the same time, occurrences of the disease increased. Those studying the disease made the connection between increased rains, wetlands, mosquitoes, and disease, and were eventually able to identify the virus associated with the disease. The RVF virus was isolated from mosquitoes belonging to several genera (Aedes, Culex, Mansonia, Anopheles, and Eretmapodites).

The graph below supports the theory that RVF outbreaks (shown by x's) generally appear at times when rainfall is above average. During unusually moist periods, the rate of incidence of Rift Valley fever tends to be much higher than normal.

Image of a graph showing the Outbreaks of RVF associated with rainfall oscillations. This image links to a more detailed image.
Reprinted with permission of S. Karger AG, Basel.

Much is still being learned about Rift Valley fever, and scientists have expanded their theories about how the disease spreads. Besides transmission by mosquitoes, it is believed that animals spread the virus directly to other animals, that animals spread it directly to humans, and that humans spread it directly to each other. It is believed that all three transmission modes operate during a major outbreak of this potentially lethal disease.

Scientists believe that the virus is also "maintained" between outbreaks by two possible means: (1) by drought-resistant mosquito eggs that can lie dormant for years waiting for adequate moisture in which to mature and (2) a "wild reservoir," i.e., the virus survives at a non-fatal, low-level of infection in some animal species. These animals maintain enough viral particles to cause major outbreaks.

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