Outbreaks of Rift Valley fever occur irregularly in East Africa, several years apart  When outbreaks do occur, they kill thousands of cattle  A livestock vaccine against the disease exists but is rarely used  It is too expensive for farmers to use routinely, and since it is not effective until a month after vaccination, administering it after an outbreak begins helps very little
Nevertheless, experts predict that use of the vaccine will increase significantly within the next few years
Rift Valley fever is spread by mosquitoes, but each outbreak is so widespread that it is impractical to control it by using insecticides.