The epizootiology of bluetongue: the African situation.

Bluetongue virus is transmitted biologically by various species of Culicoides, notably C. pallidipennis and C. variipennis. Factors such as rainfall, temperature and relative altitude, which influence the breeding of the insect vectors also govern the incidence and distribution of the disease. The host range of bluetongue virus includes sheep, cattle, goats and various antelopes. Many other, as yet unidentified hosts could perhaps harbour the virus and influence the epizootiology of the disease. The close relationship between C. pallidipennis and cattle is indicated and the efficient mechanism for virus maintenance which this relationship constitutes is emphasised. It is further postulated that sheep are not essential for the continued survival of bluetongue virus, but merely function as accidental or indicator hosts.