BY: ANDREW SWALLOW
New evidence Bovine Viral Diarrhoea (BVD) is present in New Zealand sheep flocks has prompted vets to warn farmers attempting eradication in cattle to be “mindful” sheep can be infected too.
Tests on 270 ewe hoggets across 18 sheep and beef farms, 15 from each farm, found 17 animals with antibodies to pestiviruses, indicating probable* exposure to BVD. All came from two farms that were among nine selected for the study because they were known or highly suspected to have BVD in their cattle herds. The other nine farms were selected for being considered BVD-free.
“These findings highlight that farmers who are trying to eradicate BVD from their cattle should be mindful that the infection may also be circulating in sheep, and both populations should be considered a possible risk to each other for generating transient and persistent [BVD] infections,” concludes the New Zealand Veterinary Journal report by Massey University Associate Professor Carolyn Gates and colleagues.
Speaking to Country-Wide about the study, Gates warned BVD in sheep causes “many of the same problems with reproductive performance and poor growth rates” as in cattle. Consequently, it could be causing even greater losses than the $150 million/year currently estimated, making eradication from New Zealand even more cost-effective than economic models have predicted.
The disease’s presence in sheep shouldn’t make much difference to how hard it will be to eradicate either, as it generally doesn’t persist in sheep.
“Outbreaks in sheep flocks are expected to burn out much faster because the virus isn’t as well adapted to that species,” she explained.
“There is much more risk of cattle spreading BVD to sheep than the other way around.”
Like calves, lambs infected with BVD in the womb can become persistently infected (PI) and shed large amounts of the virus for life. However, unlike PI calves which often survive several years, most PI lambs die within weeks or at most a few months so don’t have as many opportunities to infect other animals.
Besides the loss of those PI lambs, ewes which become infected (transient infection) may have reduced fertility, abort, or produce lambs with birth defects. Older lambs that are transiently infected may have reduced growth rates and increased susceptibility to other diseases.
“We don’t really know how much damage BVD is causing the sheep industry right now because not many sheep-and-beef farmers are currently testing their beef cattle for BVD let alone their sheep. However, based on what we do know about how the disease works, controlling BVD in the cattle populations should be enough to eradicate it from sheep too.”
The sheep study was conducted as part of the three year Sustainable Farming Fund project, BVD Free New Zealand, which now concludes at the end of December, following a six month extension due to Covid-19. The associated website, www.bvdfree.org.nz was due to be relaunched as Country-Wide went to press.
*A complicating factor in testing for BVD in sheep is hairy-shaker disease, caused by the Border disease virus (BDV). BDV is also a pestivirus, like BVD, and antibody tests available in New Zealand at the time of the study could not distinguish between the two. However, for technical reasons, BVD was the most likely stimulus for the antibodies found the report authors believe.
