Most clinical mastitis occurs over calving, so if you’ve had a good spring, you probably feel like you’re in the clear.
BVD (Bovine Viral Diarrhoea) is a viral disease of cattle. It is wide-spread in New Zealand; about half of dairy and beef herds are “actively-infected” with BVD at any given time1. Understanding more about the disease will help you develop a farm-specific control plan with your vet.
BVD causes pregnancy loss, diarrhoea, milk drop, and reduced growth rates. It also suppresses the immune system, making animals more susceptible to other diseases, such as pneumonia and salmonella. A herd with BVD infection over mating is estimated to cost affected dairy farmers $87 per cow2 and affected beef farmers approximately $50 per cow3. Research in New Zealand and overseas shows that it pays to control BVD; it is always more cost-effective to do something than to do nothing4. Moreover, a well-executed BVD control plan will help you achieve other farm goals, like improving animal welfare, reducing antibiotic usage, and improving reproductive performance.
BVD is spread by “persistently-infected” or “PI” cattle. PI cattle are those which were infected with the BVD virus before birth, before their immune system had developed sufficiently to differentiate between ‘self’ and ‘other.’ PIs secrete high levels of BVD virus in their blood, saliva, faeces and body tissues for their entire lives. When PIs, or their saliva or faeces, contact cattle which have never seen the virus before, these “naïve” animals can become transiently infected with BVD for 2-3 weeks.
In this way, PIs act as the source of BVD spread. While some PIs are poor-doers who self-cull, about half live to be at least 2 years old4, indistinguishable from their herd-mates, making BVD a hidden menace!
The key to BVD control is therefore to find and eliminate PIs from within your herd, then protect your herd from contact with outside PIs. This can be accomplished by: monitoring the herd, testing individual animals, improving biosecurity, and strategically vaccinating ‘at-risk’ cattle. The remainder of this article will outline monitoring and testing. Next month, we will address biosecurity and vaccination.
To determine your herd’s BVD status, monitor by regularly testing the level of BVD antibodies in pooled blood samples or a bulk milk sample. Herd antibody results are reported as “S:P” ratios. The higher the S:P ratio, the more likely that a PI is in contact with the herd or is in the herd5. Checking your S:P ratio 2-3 times per year will help you track changes in your BVD status.
If your S:P ratio is 0.75 or greater, or if it has increased significantly since your last test, find and eliminate PIs by virus testing the blood, ear notches or milk of suspect animals. It’s best to work with your vet to decide how to conduct this “PI hunt,” since every farm has unique BVD risk factors. Virus positive animals are either PIs or have just been infected with the virus for the first time and haven’t yet cleared the infection. If an animal is confirmed to be a PI, it should be culled immediately. PI hunts are cheapest and easiest if you have several years of BVD monitoring results, records of all recent animal movements onto the farm, and accurate dam-progeny relationship records.
Monitoring the herd and testing animals to find and cull PIs will eliminate BVD from your herd. Next month, we will outline the steps necessary to keep BVD out. In the meantime, for more information about BVD, talk with your vet, or visit www.TopFarmers.co.nz for BVD videos, fact-sheets and more.
• Amanda Kilby is a MSD animal health veterinarian