Vectors of bluetongue virus in Australia.

Two of the 5 serotypes of bluetongue virus (BTV) known from Australia have been isolated from field collected insects. Serotype 20 was isolated in 1975 from a mixed pool of 214 insects containing several Culicoides species. Serotype 1 has been isolated from C. (Avaritia) fulvus Sen & Das Gupta collected at Beatrice Hill in the Northern Territory and from C. (Avaritia) brevitarsis Kieffer collected at Peachester in southeast Queensland. All other isolates of bluetongue (BT) group viruses have been made from sentinel cattle. An additional 2 species of the subgenus Avaritia, C. wadai Kitaoka and C. actoni Smith, 1 species of the subgenus Culicoides, C. peregrinus Kieffer and 1 species of the Schultzei group, C. oxystoma Kieffer have been infected in the laboratory. Serotype 20 was transmitted from sheep to sheep by C. fulvus and serotype 1 by C. fulvus and C. actoni. The infection rates established for Culicoides fed on sheep infected with serotype 20 were C. fulvus 62%; C. wadai 11%; C. actoni 2% and C. brevitarsis 0.3%. All species of insects successfully infected are widely distributed in the Oriental region. Attempts to infect species that are restricted in range to the Australasian region have been unsuccessful. The 3 species with highest experimental infection rates: C. fulvus, C. wadai and C. actoni, are confined in Australia to areas with an annual summer rainfall in excess of 800 mm, and do not penetrate to the drier areas where sheep are commercially husbanded. C. brevitarsis is the vector responsible for transmission in the coastal dairying areas, and although it does occur where sheep are reared, it is an inefficient vector. It breeds in discrete cow dung pats on pasture and is more closely associated with cattle than with sheep when the 2 hosts occur together. More than 1 species of Culicoides is responsible for the transmission of BT group viruses in Australia and no BT disease of sheep has been recorded because the more efficient vector species are not present in sheep rearing areas.