Holistic approach is needed to fight ‘hidden’ BVD

Bovine Viral Diarrhoea is one of the most costly and prolific diseases in cattle, with 80% of all herds having been exposed to infection.

But it is a hidden disease, with few immediately apparent symptoms, so it can sit in herds undetected for years, costing the industry £40-£50m a year. Even vaccinated cattle can carry and spread BVD, so farmers need to take more concerted action to protect their animals, says Dick Sibley from the West Ridge Veterinary Practice, Witheridge, Devon (pictured).

“BVD is becoming a trade issue for British farmers, because a lot of our EU competitors are eradicating it. Now Scotland is manoeuvring itself to prohibit trade on BVD status grounds, so it’s more important than ever for us to get on top of this disease.”

Up to 20% of the UK’s cattle have active infection at any one time, leading to infertility, pneumonia, immunosuppression and scouring. “But diarrhoea is one of the last symptoms to show up, so most farmers whose cattle have BVD don’t know it.”

The disease accounts for 15% of all abortions, plus calf deformities, but cows that contract it between 30 and 120 days into pregnancy will produce a live calf that will be persistently infected (PI). Without showing any symptoms, this animal will carry and spread the disease across the herd throughout its lifetime.

“The PI animal is the key to this problem,” says Mr Sibley. “In Switzerland they have been testing all their cattle and slaughtering all the PIs, with a plan to eradicate BVD in just three years. But in the UK we have so many animals that it is just not feasible.”

As a viral disease, there is no cure for BVD, and although there is an effective vaccine, it does not provide 100% protection. “There is a misconception that vaccinating solves the problem – but it doesn’t,” says Mr Sibley. “Some farmers have been vaccinating for 10 years and are still finding PIs in their herds.”

Almost half of the UK’s cattle farmers buy in bulls without certifying they are BVD-free, with one-third buying in pregnant females, often to compensate for those culled due to TB, also without checking their disease status. “One in 25 pregnant females will be carrying a PI calf,” he warns. Buying in calves or sharing machinery and labour are other sources of potential infection.

“Only 38% of farms have secure boundaries between them and their neighbours, and in the South West the average cattle farm has five neighbours. Biosecurity is therefore vitally important, but neither that nor vaccination alone provides the whole answer.”

Eradicating BVD

The first step to eradicating BVD is to identify the herd status by testing animals, through a combination of blood tests, milk tests and ear tag tests, says Mr Sibley.

“You have to tag your calves anyway, and the cost is only marginally more to get them tested, so you might as well get it done.” However, ongoing tests as the animals get older are also important.

Any infected animals should be isolated and slaughtered, with continued surveillance. strict biosecurity and a vaccine programme are essential to maintain a disease-free status. “Some 65% of south-west herds are at very high risk of becoming infected. They are going to get this disease, unless they do something about it.”

Vaccination

By testing stock before bringing them on to the farm, and vaccinating routinely, producers should be able to protect their herds.

However, in a recent study of south-west dairy farms, 55% of BVD vaccine was not used properly. One-third of farmers didn’t look at the data sheet, only 24% completed the primary vaccination course prior to service and 21% gave the wrong dose or via the wrong route. “There have been three different vaccines with three different ways to administer them, so it is a very difficult vaccine to use,” says Mr Sibley. “And even when used 100% correctly, no vaccine is effective 100% of the time.”

But the correct use of vaccine in an infected herd will help decrease clinical signs of the disease and lower the number of PIs produced, says Richard Booth at the Royal Veterinary College. It will also help prevent disease re-entering the herd, which is especially important in areas where cattle density is high and biosecurity improvements are difficult.

“Used in conjunction with the identification and culling of PI animals, vaccination can be part of the quickest route to eradication,” he says. “But just because a herd vaccinates it does not mean it is BVD-free – farmers need to be asking more questions than just ‘Do you vaccinate?’.”

In a scientific trial on 34 farms in Somerset, 20 herds were found to be infected with BVD, nine of which had been vaccinating against the virus. More than 60 PIs were identified, 61% of which were in herds that were unvaccinated prior to the trial. More than half of the PIs were under a year old, but 28% were more than two years old.

Through a combination of improved biosecurity, testing of imported stock and vaccinating, the number of farms infected with BVD dropped to just five by the end of 2009 (and some of those still infected had chosen not to adopt all the control measures available).

“The work on these farms has shown BVD eradication is a real possibility, even in cattle-dense areas where biosecurity is a major issue,” says Dr Booth. “Within the UK, Cattle Health Certification Standards exist to provide comprehensive testing regimens and guidance to attain BVD freedom and subsequent certification. But although isolated groups and individual farms are undertaking BVD control, there needs to be a greater level of national coordination to eradicate this disease.”

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