Control of bluetongue in an epizootic situation: Australian plans.

Epizootics of BT outside Africa have occurred in the Middle East, Indian subcontinent, Spain and Portugal, and the United States of America. The disease has only been eradicated from a major region once, this being Spain and Portugal in 1956-60 when it was achieved by quarantine, compulsory vaccination and slaughter of some infected animals. Because of the serious economic effect that BT would have on the Australian sheep industry the policy in this country is to attempt eradication, if at all feasible. The prospects of eradication will be greatly enhanced if the disease can be diagnosed quickly, when the outbreak is still localised. The laboratory diagnosis of BT involves inoculation of blood samples, collected from febrile animals on the suspect property, into groups of susceptible sheep in insect-proof quarters. The diagnosis would be confirmed at the reference laboratory in South Africa by serological and other identity tests. The diagnosis of an outbreak that originated in the northern cattle areas might be very difficult, because of the probable mild clinical nature of the disease in cattle. It is suggested that sentinel sheep flocks be maintained in these areas in strategic places near likely points of entry. Control measures, based on current knowledge of the epizootiology and pathogenesis of bluetongue; are discussed. The disease is transmitted by Culicoides that ingest infected blood from viraemic ruminants. Control is based on reducing the number of viraemic ruminants in the infected area, reducing the population density of Culicoides, and reducing the availability of susceptible ruminants. It is vital to prevent the movement of potentially infected ruminants to new localities. Contingent plans for the eradication of an epizootic of BT in Australia have been prepared. These call for the creation of two control zones. In the inner infected area, which extends for about five miles around known infected farms, there would be an intensive disinsection programme with aerial and ground spraying. Ruminants in the infected area would probably be slaughtered. The quarantine zone would extend for another 50 miles and in this area there would be prohibition of the movement of ruminants from farms. Regular clinical inspections and serological surveys would be carried out to detect secondary outbreaks. If control measures failed, it might be necessary to mount urgently a massive vaccination campaign to prevent disastrous losses to the Australian sheep industry. The difficulties in obtaining sufficient quantities of monovalent vaccine from overseas at short notice are discussed. Plans have been made to produce BT vaccine in Australia, in a specially designed high security laboratory, should the need arise.