An apparently tick-borne gastroenteritis of sheep and goats was identified by a Kenya veterinary officer among trade sheep at the Nairobi quarantine area in 1910. Nairobi sheep disease (NSD) is the most pathogenic virus disease known for sheep in East Africa and probably Somalia. Losses in the Nairobi quarantine areas to which NSD-susceptible sheep were brought approached 90%. Such losses have regularly been encountered following the movement of nonimmune sheep populations into endemic areas. Similar mortality rates were reported from northern Somalia. NSD virus (NSDV) is a member of the family Bunyaviridae and of the genus Nairovirus. Many members of this genus are transmitted by ixodid ticks, and some transovarially. The NSD group of this genus includes Ganjam and Dugbe viruses. Sheep are the principal disease hosts for NSDV, and goats, although susceptible, are generally less so.