Rift Valley fever has been known for many years as a devastating disease of ruminants which breaks out every decade or so in East or South Africa, killing lambs and calves and inducing abortion in pregnant ewes and cows. At the time of such epizootics occasional cases of a nonlethal dengue-like illness were observed in people who came into contact with sick animals or handled their carcasses. Suddenly in 1977 an epizootic of unprecedented scale occurred in the delta and valley of the Nile, with many hundreds of thousands of cases in sheep and cattle, and for the first time large numbers of humans were affected. The disease broke out again in 1978. Over 200,000 Egyptians contracted the disease, and more than 600 died. The extent and severity of this epizootic/epidemic may have been due to the high population densities of fully susceptible animals and humans. In 1987-1988, further virus activity was detected in eastern Africa and in western Africa, with hundreds of human deaths in Senegal and Mauritania.
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