Evaluation of Bluetongue Virus Excretion in the Germplasm of Cattle

Summary

Experiments were conducted to monitor the duration of viraemia and excretion of virus in semen in both naturally and experimentally infected bulls. In the Northern Territory, bulls were exposed to natural infection with bluetongue viruses over three wet seasons, 1991-1993. During these years bulls were infected with BLU 1, BLU 3, BLU 16 and BLU 20. Serological monitoring of sheep inoculated with semen samples collected during the observation period failed to indicate any contamination of semen with these viruses.

In NSW, frozen semen samples were examined from a group of mixed age bulls which were naturally infected with BLU 1. There was no evidence of bluetongue virus in any of the semen samples collected around the period of viraemia. Experiments were conducted to examine the effect of age of bulls, laboratory adaptation of virus and bluetongue virus serotype on the excretion of virus in semen. Groups of 5-8 young bulls (2-4 years) and mature bulls (5-15 years) were inoculated with either "wild" virus or laboratory adapted virus.

Two serotypes were used: BLU 1 in NSW and BLU 23 in the Northern Territory. When mature bulls were inoculated with laboratory adapted virus all bulls became viraemic and virus was detected in the semen of several bulls with both serotypes 1 and 23. When mature bulls were inoculated with "wild" virus, only serotype 23 infected bulls excreted virus with semen. This may have been the result of blood contamination of semen in the aged bulls used in this experiment. When young bulls were inoculated with "wild" virus or laboratory adapted virus, no evidence of virus contamination of semen was found with either serotype 1 or 23. To examine bulls for latent infection with bluetongue viruses, seropositive bulls were immunosuppressed, then blood and semen samples were checked for bluetongue viruses by sheep inoculation. Seropositive bulls were also slaughtered and tissues collected from the spleen and reproductive tract. These were then checked for the presence of virus. No evidence of latent infection was found.