Finding persistently BVD-infected (PI) cows can be a time-consuming business when it involves testing every single cow to find one or two PI cows.
But this has been made easier by Intervet‘s Dairy Check BVD Service, which is available through vets. This now includes a PCR test sensitive enough to detect a single PI animal in a herd of up to 300 cows.
PI animals are born infected and remain so for their whole life, until they eventually die from mucosal disease. Calf infection in the uterus occurs before the calf’s own immune system develops. As a result, the virus creates no immune response and it can never eliminate the virus, or put a stop to virus excretion. BVD can, therefore, replicate every day at rates many times higher than in an infected cow whose immune system tries to fight it.
A PI is infectious every day of the cow’s life and, while it remains, the herd will never be rid of BVD. Vaccination alone isn’t enough. Although it reduces disease severity and prevents creation of new PI calves, it won’t ever fully eliminate the virus’ effects. This is why it needs to form part of a five-stage control programme.
The first step is bulk milk antibody tests. Cows need to be exposed to infection to develop antibodies – and a PI is a mobile source of infection. When results indicate no antibodies, or a low result, there is no PI in the herd and no PCR test is required. When the antibody test is positive, however, the next step may be the PCR test.
This test can find evidence of a virus in a sample at low levels. The advantage is you can rapidly screen large numbers of cows, without having to test individuals. It can be used on bulk milk samples from up to 300 cows. When the result is negative, you know there is no PI in that sample.
The disadvantage is that it doesn’t differentiate between a PI and a cow which is excreting virus because it has acute infection. And not every cow in milk contributes to the bulk tank: For instance fresh calvers those whose milk is being withdrawn due to antibiotic treatment or high-SCC cows whose milk is being dumped.
In addition, a positive PCR result could mean one or more animals are infected. As there is no calculation or ratio to work out potential numbers, the milking herd needs to be sub-divided to narrow down further tests. This could involve milking all high yielders first then taking a sample, followed by milking all low yielders first the next day. Once the PI is identified, it should be immediately culled and the bulk PCR test done again to find out whether there are any more PIs in that group.
Because most British herds are endemically infected with BVD – about 95% of the national herd – we don’t tend to see huge abortion storms or scours. Yet the virus causes all sorts of losses: Abortion, infertility, and deformed calves. It also suppresses the animal’s immune system making it more susceptible to other diseases, from mastitis, lameness, and IBR to calf pneumonia, scour, even possibly TB.
Culling all PI animals in a herd, therefore, becomes particularly important in light of new French data which shows the cost of subclinical BVD. Despite assuming most animals have immunity, poor performance was still evident two years after initial infection. The average economic losses associated with this were comparable to those for mastitis. One set of figures showed the cost to be 0.78p a litre based on a 7500 litre average in a 40-cow herd.