Investigations into the epidemiological and clinical effects of bovine viral diarrhoea virus infection in sheep and other non-bovine species in Australia

Abstract

Bovine viral diarrhoea is a disease of cattle known to cause severe reproductive dysfunction and immunosuppression in infected animals. It has also been reported that the BVD virus (BVDV) is able to cross species barriers infecting non-bovine species such as sheep, goats, deer and alpacas. The ability of BVDV to infect species other than cattle is of great concern when developing control or eradication programs for BVDV, particularly in areas where cattle regularly come into contact with other susceptible species. As such, the main objective of this thesis was to investigate the epidemiological and clinical effects associated with BVDV infections in sheep and other commonly farmed Australian livestock species. Firstly, the clinical and reproductive outcomes following acute infections of pregnant ewes was studied. Twenty two ewes were experimentally infected with BVDV-1c between 59 to 69 days of gestation and the clinical and reproductive outcomes of these ewes were compared to those of a control group of 11 BVDV naïve and uninfected ewes. In the naïve ewe flock a lambing rate of 172.7% was recorded, lambs were born clinically normal and had a high survival rate. In comparison, a lambing rate of just 31.8% was reported for the infected ewe flock, as well as poor lamb survival and lambs presenting with a wide range of pathological lesions; including anasarca, hydranencephaly and skeletal deformities. A viable, neonatal, BVDV-1c persistently infected (PI) lamb was also born during this study. Three experimental trials then investigated the ability of a neonatal BVDV-1c PI lamb to infect susceptible sheep and cattle. Results indicated that there was a low rate of transmission to susceptible ewes and lambs (n=9) co-paddocked with the PI lamb. The remaining two trials showed no transmission to susceptible steers; housed either adjacent to (n=5) or exposed directly to (n=5) the PI lamb. Furthermore, the transmissibility of BVDV-1c from sheep undergoing acute BVDV-1c infections was studied by co-paddocking five experimentally infected wethers with five susceptible wethers. Results confirmed acute infection in the experimentally infected wethers however the naïve sheep did not seroconvert. In addition, the clinical and haematological effects associated with acute BVDV-1c infections in alpacas were investigated. Results indicated that transmission of BVDV can occur naturally from PI cattle to alpacas housed together. However, there were no apparent signs of infection in any of the alpacas. Lastly, the presence Pestivirus-specific antibodies was determined for 875 South Australian sheep and 245 Northern Territory water buffalo at a prevalence of 0.34% and 4.5%, respectively. This body of work has indicated that acute BVDV-1c infections of pregnant ewes can lead to severe reproductive losses and the birth of persistently BVDV-1c infected lambs. However, results also indicate that sheep are unlikely to play a pivotal role in the spread and persistence of the virus in Australia. Further work needs to be undertaken to fully understand the role other non-bovine species have in the spread and persistence of the virus in Australia.