Summary
A series of experiments was conducted over a period of four years and involved both young (2-4 years) and old bulls (5-15 years) that were both naturally and experimentally infected with bluetongue virus (BTV). Several different virus serotypes were studied. In the Northern Territory, young bulls were exposed to natural infection with BTV over three wet seasons. During this time, bulls were infected with BTV-1, BTV-3, BTV-16 and BTV-20. In New South Wales, semen samples were examined from a large group of bulls of mixed ages that were naturally infected with BTV-1. Experimental infections in both young and old bulls (5-8 animals per group) employed both ‘wild-type’ and laboratory-adapted viruses from serotypes 1 and 23. A total of 41 bulls were included in the studies of natural BTV infection and 52 bulls in experimental infections. There was no evidence of BTV in any of the semen samples collected from naturally infected bulls or experimentally infected young bulls. BTV was detected intermittently in semen from a number of old bulls infected with both laboratory-adapted BTV-1 and BTV-23. These detections occurred during or immediately after the period of detectable viraemia. Virus was also detected in a few semen samples from very old bulls infected with ‘wild-type’ BTV-23. These samples were collected during the period of viraemia and there was usually evidence of blood in the semen. Viraemia varied in duration between 17 and 38 days. Following immunosuppression, there was no evidence of resurgence of viraemia, or excretion of virus in semen, even in animals in which virus had been previously detected in semen. When the bulls were slaughtered, virus was not detected in any tissues.

Keywords
Australia, Bluetongue virus, Bull, Cattle, Laboratory-adapted virus, Semen, Viraemia, Virus excretion, Wild virus.