Rift Valley fever is transmitted by mosquito bites. The causative agent was isolated in 1931 from an infected sheep in Kenya's Rift Valley. In east Africa, outbreaks usually occur every 5 to 10 years, probably due to movement of the inter-tropical convergence zone. The many shallow depressions, "dambos" in east and southern Africa, filled with water during the rainy season are the main habitat for mosquito larva. Rift Valley fever was confined to the South of the Sahara until 1977 when a big outbreak occurred in Egypt. One of the factors believed responsible for the outbreak was the abundant water supply from canals of the newly constructed (Aswan? ) dam.