Characteristics Of Naturally-Occurring Bluetongue Viral Infections Of Cattle

For 15 years, herds of cattle have been used as sentinels to detect the presence of bluetongue virus (BLU) in the northern part of Australia's Northern Territory during the active season for the principal Culicoides vector species. Blood samples have been taken from 848 sentinel cows, steers and bulls weekly or twice weekly. The blood has been cultured for bluetongue and other viruses. Semen from exposed bulls has also been tested for virus throughout the monitoring period, and calves born from exposed cows have been examined pre-suckling by virus isolation and serological techniques. All eight BLU serotypes found in Australia have been represented in the sampling results, although there have been no predictable patterns in the serotypes which are active each year or in the number of sentinel cattle infected. Different BLU serotypes varied in the duration of the viraemia they caused, with most (99.5%) viraemic periods lasting from less than 2 weeks to 2 months. In these natural infections, possible BLU contamination of the semen was observed only once, and there was no evidence of BLU infection of 79 calves which were in utero when their mothers were naturally infected.