Distribution of bluetongue and other arboviruses in northern Australia

Summary

Eight serotypes of bluetongue virus (BTV) have been isolated in Australia. The viruses are transmitted by biting midges of the genus Culicoides and studies have identified C. brevitarsis as the principal vector in eastern Australia. The viruses circulate in cattle in the absence of clinical symptoms but infection of sheep can result in bluetongue disease. Studies have shown BTV .23 to be the most virulent, killing 30 - 40% of infected sheep and BTV.3, 15 and 16 are moderately virulent killing up to 10% of infected sheep. BTV.1 appears of low virulence and BTV.9 is a virulent. The virulence of field isolates of BTV.20 and 21 has not been established. All 8 serotypes have been isolated from the Northern Territory but serological' evidence indicates that only 2 serotypes, BTV.1 and 21 have spread from there. Infection of sheep with both serotypes has been detected serologically in Queensland and New South Wales but no clinical disease has been reported.

The absence of clinical disease may be due to a combination of low virus virulence, the low density of vectors, a vector preference for cattle and limited vector competence to transmit the viruses. However, the presence of susceptible sheep, competent vectors and bluetongue viruses means a high probability that an outbreak of clinical bluetongue will occur in Australia at some time. The costs incurred due to stock losses would depend on the number of sheep infected but the loss of export earnings could be substantial.

The discovery of BTV in Australia in 1975 caused disruptions in the export of ruminant animals and, in some instances, of products such as meat and wool to a number of countries. An appreciation of the epidemiology of BTV in Australia and the complex relationships that exist between virus, vector and susceptible host subsequently led to a rationalisation of importation protocols. Some countries at present permit importation from all parts of Australia providing relevant testing is done. Other countries accept only livestock or germ plasm from vector-free regions of Australia and other such as the European Economic Community and Syria do not accept any Australian material.

Some measure of the complexity of the situation is seen in the fact that not all countries accept the same regions of Australia as vector-free. It is clear that an outbreak of bluetongue in Australian sheep would lead many countries to reassess the current arrangements with respect to importation of Australian livestock. This could have a major economic impact. For example, Middle Eastern countries which imported over 6 million sheep and goats in 1984/85 at present do not place any restrictions on the importation of Australian livestock.