BVD in young stock

Most of you are aware of the importance of persistently infected (PI) animals in the spread of BVD in dairy herds. PI’s are persistent shedders of the virus and spread it in faeces, urine, blood, semen, milk, uterine discharges and nasal secretions. I like to think of PIs as being surrounded by a cloud of viruses!

We know from bulk milk testing that 80% of herds (and 60% of cows) have been exposed to BVD at some stage. However, when we look at our farm’s bulk milk antigen (virus) levels, we rarely find the presence of PI’s in the herd and when we monitor bulk milk antibody levels we rarely see significant rises during or between seasons. So how can this be?

The simple answer is that most PIs (70-80%) die before they hit 2 years of age and enter the herd. So, does this mean if we don’t see changes in our bulk milk samples we don’t need to worry?

Whilst BVD does wreak havoc with repro and production if a PI heifer/ cow or bull makes it into the herd, in many respects, BVD is a more significant issue with young stock. In many countries where BVD has been eradicated one of the main things vets and farmers notice is the improvement in young stock health.

We know that the presence of a BVD PI in a mob of young stock can impact diseases such as scours, respiratory disease, pink eye etc. However, the other major effect is on growth rates, with global reports of reductions in growth rates of 20-30% (think 130 to 190 grams lower daily weight gains!). We have had several good case studies we have seen first-hand in the practice to back these figures up.

BVD can be a sly old fox in terms of how it enters herds. The clever fox doesn’t bust into the chicken run and kill all the chooks in one go - that’s a good way to get shot! The clever fox sneaks in and out and takes one bird now and again. In the same way, as the farmer more easily deals with the dumb fox, we have got on top of blood testing bulls and making sure we identify and remove PI milking heifers/ cows pretty smartly. However, we still have BVD getting through the farm gate!

Here are some common ways that BVD can get into your youngstock mobs with no signs of anything wrong on your bulk milk tests:

Unvaccinated heifers at run-off blocks coming back with PI calves inside them. If any of these calves are retained as keepers (intentionally or accidentally), a PI could enter your calf mobs. Vaccinating heifers if you are doing synchrony is especially important.
If a few cows get exposed to BVD over the fence from neighbouring stock (beefies are particularly high risk as stock are regularly bought and sold and often come from herds with poor BVD control/ monitoring), they can give birth to PIs at the next calving. Because only a small number of cows are infected, your bulk milk antibody level doesn’t change.
When cows walk past your calf paddocks during the mating period, this is a really good way to multiply up BVD in the herd if you have one or two PI calves in your calf mob. Again, a small number of animals being exposed won’t change your bulk antibody levels but it can certainly lead to more PIs being born next calving time.

There has been a fair bit of economic modeling of the costs of BVD, and one clear thing is that the more you spend on BVD control, the less the disease will cost you in the long run.

Prevention of the birth of PIs and/or early removal of PIs are 2 of the key tenets of BVD control. Unless you are vaccinating your herd/ heifers, the most robust prevention comes from testing calves before they leave the calf shed. The earlier you check your calve’s BVD status, the lower the risk of spread.

Our technicians offer a very competitively priced BVD blood testing service at debud time, which is a great way to check you have all your calves screened clear before they leave the shed!

Our goal at Franklin Vets is for every farmer to have their own BVD control plan. This needs to be based on informed decision-making, which means having a good understanding of how the disease works and what your farm’s risk factors are, which means sitting down with your vet and working through it. If you don’t have a robust plan, give your Franklin Vets vet a call and book a time to sit down and chat it through!

Dr Jason Fayers – Farm Vet & Regional Manager, Pukekohe