The native vertebrate reservoir host of Rifl Valley fever virus in sub-Saharan Africa has not been precisely identified but is presumably one or several species of wild ungulates. The virus survives in a silent enzootic cycle for many years and then, when there is exceptionally heavy rainfall, explodes in epizootics of great magnitude among domestic sheep and cattle. In such epizootics, Rift Valley fever virus is transmitted by many species of Culex and Aedes mosquitoes, which are very numerous after heavy rains or when improper irrigation techniques are used. The mosquitoes are infected when feeding on viremic sheep or cattle, which maintain a very high level of vir-emia for 3-5 days. This amplification of the transmission cycle, together with mechanical transmission by biting flies, results in infection and disease in a very high proportion of animals and humans at risk.

These epizootic cycles are started by infected mosquitoes occupying an unusual ecologic niche. Throughout the grassy plateau regions of sub-Saharan Africa there are dry depressions in which floodwater Aedes species live, surviving long periods of drying as eggs and emerging only when the depressions are filled by exceptional rainfall These mosquitoes are tran-sovarially infected with Rift Valley fever virus and are capable of transmitting it to a few sheep, cattle, and wild ruminants, thus starting epizootics which are maintained and amplified by other mosquito species.

In its epizootic cycles Rift Valley fever virus may also be spread directly by fomites, by direct contact, and mechanically by arthropods such as tabanid flies. Infected sheep have a very high level of viremia, and pregnant ewes and cows almost invariably abort; transmission to humans at the time of abortion via contaminated placenta and fetal and maternal blood is a particular problem. Abattoir workers, farmers, and veterinarians are often infected directly.

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