The search for bluetongue viruses in Australia

In the 200 years that cattle and sheep have been present in Australia no evidence of bluetongue (BT) disease has ever been reported. The discovery in 1977 that a virus isolated from Culicoides species collected in 1975 was bluetongue virus (BTV) triggered a search for BT viruses and their vectors throughout the country. Between 1975 and 1983, 46 strains of 5 serotypes of BTV were isolated by various workers from the blood of subclinically infected sentinel cattle or from Culicoides species. The serotypes are 1, 20, 21 and 2 others not yet fully characterized. By testing stored sera from cattle and buffaloes, antibodies to serotype 21 were found in sera collected in 1958, but antibodies to serotypes 20 and 1 were not found earlier than 1973 and 1969 respectively. Discrepancies between the results of BTV group antibody and neutralizing antibody surveys indicate that other serotypes may yet be found. Neutralizing antibodies to BTV have been found in cattle, buffaloes, deer, goats and sheep, but mammals indigenous to Australia are apparently not being infected. In the course of the search, Ibaraki virus and 4 other viruses of the epizootic haemorrhagic disease (EHD) group were found. Infection with EHD viruses has produced heterotypic antibodies detected in tests for BT group antibodies. The prevalence of antibodies to BT and EHD viruses in cattle is greater in the northern 1/3 of the continent than the middle 1/3 while the southernmost 1/3 is free. By the sequential sampling of sentinel cattle it has been determined that infection with BT and EHD viruses occurs mainly in the latter part of summer, and autumn. The seasons of the year, and the locations from which BT and EHD viruses originated were consistent with the serological findings. Isolations of BT and EHD viruses have also been made from insect vectors, BTV serotype 1 from C. fulvus and C. brevitarsis, plus serotype 20 from a mixed pool of Culicoides species and 2 different EHD viruses from C. brevitarsis. The susceptibility of other Culicoides species to experimental infection with BT and EHD viruses suggests that they may be involved in the natural spread of these viruses. BT and EHD viruses appear to be non-pathogenic for cattle under experimental conditions, but mildly or moderately pathogenic for sheep. The clinical signs produced are consistent with those produced experimentally in other countries.