BVD symposium report

By Keith Cutler, Endell Veterinary Group, Salisbury, Wilts

Vets often think they are doing well when they manage to persuade the majority of their clients to control BVD by vaccinating. However, Intervet’s BVD symposium, which I attended this summer, confirmed my view that vaccination is only truly effective when part of a five-step programme and it’s important to appreciate that vaccination alone is not necessarily enough.

Yes, it will reduce the severity of disease and prevent the creation of new PI calves, but it won’t ever eliminate the effects of the virus or fully control BVD in a herd. It’s just another tool. My view is that only vaccinating without also looking for, and culling, PIs (particularly in a breeding herd) is unwise; the gold standard remains that PIs are culled as soon as they are identified and the  rest of the herd is vaccinated.  Vets and researchers from the rest of Europe believe regular testing and monitoring are still needed to identify then cull any PI animals in a herd. This becomes particularly important in light of new French data (presented at the symposium) which showed that subclinical BVD, causing poor performance two years after initial infection, costs a 40-cow herd 0.78p a litre based on a 7500-litre average (see Tables 1 and 2). So imagine how big the losses must be in typical British herds which have more cows and higher yields.

Scandinavian countries have been able to eradicate BVD without vaccination by following a systematic approach, starting by clarifying herd status using bulk milk testing, followed by identifying and removing PI animals, then monitoring BVD-free status before finally adhering to biosecurity standards to prevent re-infection. However, Germany’s Prof Volker Moennig from the Department of Infectious Diseases at the School of Veterinary Medicine in Hannover, said that he didn’t believe BVD could be controlled without using vaccines. The Scandinavian results couldn’t be extrapolated to the rest of Europe with our higher cattle densities. But, he also pointed out that he didn’t know of any case where vaccination alone solved the problem.

It was the introduction of the first ELISA tests in the early 1990s that made low-cost screening and a systematic approach possible. But although he agreed that eliminating PI animals was the key to controlling BVD, Dr Jeff Laureyns, from Belgium’s Department of Veterinary Medicine, Ghent University, said without following strict biosecurity standards in a control programme, re-infection was highly likely. To him, biosecurity means dairy farms operating like a pig farm: providing washable clothing for farm visitors (he said people are a major source of spreading BVD as they travel between farms); quarantining new stock; and preventing nose to nose contact with animals on neighbouring farms. A vehicle wheel wash isn’t necessary, but it is important to stop livestock lorries coming onto the farm. Following such a protocol will help reduce the spread of other diseases such as Leptospirosis or Johne’s as well.

To me, good biosecurity starts with knowing a herd’s health status and not allowing contact with stock of unknown health status, whether they are bought in or belong to the neighbour across the fence. People are a risk, albeit a lesser one than an animal, and should always be clean and disinfected on farm. We vets must set a good example. I think, however, that farmers should supply clean wellies and overalls for all visitors including vets, the AI technician and foot trimmers. They should also realise that hiring and sharing equipment (such as muck spreaders or tractors) is another potential source of infection.

Dr Laureyns advised that monitoring should continue even when following a vaccination policy. His view is that it warns you before a lot of damage is done to livestock and when a herd shows positive results for serological tests, he believes vets must start looking for PI animals so they can be culled at the earliest opportunity. This must continue for one year after the last PI is removed from the herd – not just the nine months of gestation. Every new born calf must be tested and, each time another PI is found, the one-year testing period should be prolonged.

Belgian vets have found that when all stock are tested at one time in a control programme, there is less chance of administrative failures in identification and registration. (You don’t want to cull a BVD-negative animal, or leave a PI in the herd.) Other potential downfalls are mistaken calf or cow identity at pre-colostral testing, particularly when calves haven’t yet been ear tagged. Plus, farms which run youngstock on another unit, or manage several holdings, should test and monitor each herd or group of animals. Dr Laureyns also stressed the importance of strict quarantine and BVD testing for new animals brought on farm, particularly pregnant animals. These can be virus negative, but carry a PI foetus. He also reminded practitioners that bulk tank results are only representative of those cows delivering milk into the tank at that time. Dry cows, the stock bull and cows that have been treated with antibiotics (milk is therefore being withheld) must be blood tested.

One further complication he has found, is that farmers can hesitate to cull PI animals and they often stay far too long in the herd. The vet, therefore, has to be a skilled communicator to convince a farmer that his healthy, good looking PI must be culled immediately. Yet even after the last PI has disappeared, in dense cattle populations the chance of re-infection is high.

BVD virus is also adept at survival and can stay in a herd for more than 10 years. Because of this, Dr Christine Fourrichon, from the Veterinary School in Nantes cedex, assessed the effects of long-lasting infection using data from 7000 dairy herds in Brittany. She revealed the damage from underlying poor performance was up to 0.6kg/day of milk lost in the year following infection (see Table 1) and 11% of embryo loss after service. Cell counts were up to 25,000 cells/ml higher – an effect that lasts at least two years after initial infection. She said that even though we assume a large majority of animals have immunity, there was still a decrease in performance. In addition, Dr Fourrichon reported the average economic losses associated with subclinical BVD infection were comparable to those associated with mastitis.

This work from France backs up my perception: I’m convinced that the subclinical losses caused by endemic BVD are huge. My gut feeling is they are even higher than the French figures due to the bigger pool of cows and, therefore, more susceptible animals in our larger herds. The drop in herd performance in Table 2 is not only due to the direct effects of the virus, but also losses because other disease (such as mastitis and lameness) can gain a hold when BVD causes immune suppression. Our block calving herds are also highly at risk of creating more PI calves because all cows are pregnant at the same time. And the practice

e of buying in several sweeper bulls for the tail end of mating season is at the key risk stage for infection. A growing trend for contracting out heifer rearing, or sending dry cows off farm to expand the milking portion at home, increases the risks of contracting BVD away, or youngstock may return naive and succumb to endemic infection.

Remember: once a herd is infected, you get fertility problems, such as abortions, early embryonic deaths, or just poor conception rates. It’s a good idea, therefore, to discuss the implications with clients who are considering sending stock off-farm this winter. Then help them draw up a written health plan agreed between both parties and encourage them to vaccinate heifers before they get in-calf, probably at 6-8 months of age.  Giving the initial course, or booster dose, just before service – to both mature cows and heifers – means the animal’s immunity is highest during the early part of pregnancy, thus giving maximum protection to the developing foetus. It makes sense to vaccinate just before the time of maximum BVD challenge, so where the biggest threat is off-farm, give stock a booster before they leave.

If we look at the efforts farmers make to control mastitis losses, similar efforts should be made to control BVD. Unfortunately, survey data presented at the symposium by Prof Joe Brownlie's showed 65% of farmers questioned thought TB was the biggest threat to their business whereas, amongst vets  87% said it was BVD. With this kind of thinking, Prof Joe Brownlie said it was no wonder that farmers didn’t want to devote time and resources to tackling BVD. Without backing from industry or government for a national eradication programme, he is therefore looking to farm vets to lead BVD eradication. This means we vets need to step up our educational campaign about the long term and widespread damage that BVD does to a herd.

The direct approach is the best one, but I also think that communication is about realising how different people respond to different things. That’s where knowing your clients comes in: some respond to a one-to-one chat, others like to see the results of blood samples, or prefer to listen to an outside expert at a meeting. If you can’t get the message across one way, try another and realise that some farmers will never believe what you are telling them! Remember too, that some farmers listen to other farmers, rather than their vet.

Where vets can fall down is taking the ‘you must do this’ approach. Advice has got to be practical as well as effective – it stacks the odds in your favour as it produces benefits, so consider taking a step-by-step approach. Initially, this might mean vaccinating and blood testing only replacement heifers to detect and eliminate PIs before they enter the herd. This won’t be as quick and efficient at controlling BVD as whole herd testing and vaccinating, but it eases cashflow for the farmer and, because it’s a bit easier, it’s more likely to be done. A bulk milk antibody titre (or a blood sample from replacement heifers in a beef herd) gives you something to start talking about and bulk milk PCR tests are now sufficiently sensitive to detect one PI animal in a 300-cow herd. Farmers always look at the cost per cow of testing, but worked out in pence per litre (or kilogramme), it’s not that much – and the savings are potentially huge.

TABLE 1

Production effects of BVD infection in a dairy herd.

TABLE 2

AVERAGE EFFECT IN PERSISTENTLY POSITIVE HERDS

+ 1 abortion

+2% more AI returns

+ 27,000 cells/ml SCC

-8500 litres total herd yield

+1 more case retained placenta

+ 3 more cases mastitis

+ 5 more calf deaths

+ 8 more heifer treatments