Keeping up to date with the science on bluetongue - Veterinary Practice
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InFocus

Keeping up to date with the science on bluetongue

Vetinary Practice reports on an educational day on “a significant risk” for the UK

“I LOOK forward to a bluetongue-free future.” That was how Ian Anderson closed a symposium on the disease, entitled Putting science into practice, held in Manchester last month.

Mr Anderson, marketing manager in the ruminant business unit of Intervet/Schering-Plough Animal Health, chaired the event which brought together speakers from the UK, France, Belgium, Holland and the USA to provide sheep and cattle vets in Britain with the latest information on bluetongue.

First speaker was Gareth Hateley, a member of the BTV working group who leads for the BCVA on the disease. He stressed the importance of keeping up to date with the science and being fully informed.

Pointing out that bluetongue was traditionally a tropical or sub-tropical disease which had been first described in South Africa in the 19th century, he said that BTV8 had suddenly parachuted in – and it could spread at up to 16km a week.

Vaccination campaigns across the EU – varying from compulsory in Germany, Belgium and Scotland to voluntary in England and Wales – had been extremely successful, Mr Hateley said, with no clinical cases of BTV8. But in France about 30,000 farms had been affected.

The BCVA, and others, regard Great Britain as one unit but Scotland had decided to “go it alone”. Great Britain is at significant risk for both BTV1 and BTV8, he said.

Listing the wide range of clinical signs to look out for – from fever and lethargy to being off food, having swollen lips, hypersalivation, lameness, etc. – he said the signs in cattle were often mild. The message about BTV8, he continued, is to vaccinate. “The threat is real and on-going and we have professional responsibilities – especially to report cases.”

Re-invention

Professor James Maclachlan from the University of California has worked with bluetongue for more than 30 years. “It keeps finding a way to re-invent itself,” he told the symposium. In North America serotypes 1, 3, 5, 6, 9, 12, 14, 19, 22 and 24 had recently emerged.

It is an historic disease but an extremely serious one, with pulmonary oedema sometimes a late and fatal manifestation. Twenty years ago no one would have thought the infection would reach the UK and Scandinavia. The best way to handle it is to prevent it, he declared.

The origin of serotype 8 in northern Europe was unknown but it could be due to translocation rather than climate change. It was “something very different” from the serotype found elsewhere.

“Serotype 8 is one of the most fascinating aspects of bluetongue history,” the professor continued. It is virulent to cattle and other domestic and wild ungulates and it has a profound impact on reproduction that other viruses don’t.

Epidemiology

Dr Chris Oura from the IAH at Pirbright discussed the epidemiology of bluetongue in Europe. He noted that bluetongue was basically 24 different diseases, all genetically different. There had been 12 different introductions of bluetongue into Europe since 1998 and BTV8 had appeared in 2006.

“BTV8 is particularly bad – and we’ve got it!” In Belgium between 11% and 15% of sheep had died in an outbreak – and the UK has 35 million sheep potentially at risk.

The disease is spread by the culicoides midge on wind, vehicles and planes. The virus does not replicate if the temperature is below 10ºC but the warmer the weather the faster the spread.

It also spreads more quickly over water than land. It can be spread by the movement of infected viraemic animals; it can also be spread by the transplacental route and possibly orally. It is not yet certain how BTV8 over-winters.

“We know that transplacental transmission of BTV8 occurs in the UK and northern Europe but we have yet to confirm that this is the major means of overwintering. It is quite a unique virus – which is a shame,” Dr Oura said.

Other points he made included:

■ vaccination for one year will not stop the virus – vaccination needs to continue as the disease may take three to five years to eradicate, although it could be here to stay;

■ be careful with animals imported from zones where BTV serotypes are circulating – vaccinate and leave the animals for 60 days before moving them.

“The problem with bluetongue is that you can’t restrict the midge,” he said and he finished with a warning that “we are in danger of becoming complacent”.

Economic impact

Details of studies by the Institut de l’Elevage in France on the economic impact of bluetongue were presented by Dr H. Petit. The studies were undertaken last year following 15,566 outbreaks of BTV8 infection in France in 2007.

He said that the global impact on dairy cattle production had been relatively low but there was a significant impact on beef cattle.

The average mortality in ewes had been nearly 8% but survivors often remained weak for a very long time. Even after all symptoms had disappeared, about 40% of rams were infertile and ewes suffered a drop in milk production.

All farms hit heavily by the disease suffered severe financial losses. (More information is on www.instelevage.asso.fr).

Transplacental transmission

Dr Linda van Wuijckhuise spoke of the work undertaken at GD, a private animal health service in Holland originally set up to help sort out the TB problem there. One of its tasks now is to aim for early detection of new and emerging diseases.

Bluetongue was discovered in Holland in August 2006 and in early studies no proof of transplacental transmission was found.

In October 2007 up to 2% of newborn calves developed cornea oedema – blue eyes; from July to December 2007 there was a huge rise in aborted calves; and from January to April 2008 a number of “dummy calves” (with little brain tissue) were born. After extensive research, Dr van Wuijckhuise and her colleagues concluded that transplacental transmission does occur with BTV8 but that it is prevented by vaccination.

Semen quality

Professor Natalie Kirschvink from the University of Liege in Belgium was unable to attend and her paper on the impact of BTV8 on ram’s semen quality was read by Dr Birgit Makoschey from Pfizer at Boxmeer in Holland.

The findings were that BTV8 infection induces a severe but transitory decrease in semen quality in rams and it can take between 60 and 140 days for recovery after the development of clinical signs.

Semen quality testing should be recommended because clinical signs of BTV8 infection can be poorly detectable and libido reappears earlier than fertility.

Vaccines

In her own paper, on BTV vaccines, Dr Makoschey described the BTV8 vaccination campaign last year in the Netherlands, calling it the biggest ever field trial of a vaccine.

The Dutch government ordered seven million doses of Bovilis BTV8 for a voluntary vaccination programme, with the vaccine supplied free to vets and the vaccination costs partly subsidised. Adverse reactions numbered just one case per 40,000 doses.

The vaccine proved to be safe in sensitive categories of target animals, including young lambs, calves, pregnant ewes and heifers. A three-week challenge study showed that vaccination did not interfere with protection conferred by maternal antibodies and vaccination in the face of MDA strongly reduced viraemia after challenge. Sheep were protected from fever and from BTV8 viraemia for at least six months after vaccination.

For the future Dr Makoschey hoped it would be possible to differentiate between vaccinated and infected animals and there would be vaccines developed to give broad protection against a number of BTV serotypes. She added that vaccination was a powerful tool for BTV control and eradication.

Vectors

“Until recently there have been very few vector-borne diseases in the UK,” said Dr Simon Carpenter, an entomologist in the vector-borne disease programme at the IAH at Pirbright.

It had been difficult to predict the arrival of BTV infection into the UK up until 2006: there had been no history of BTV incursion into northern Europe; potential transmission was not linked to a single culicoides species – and there had been plenty of other more pressing issues to be concerned about.

The outbreak in August 2006 in the Netherlands had led to a change in attitudes. There were problems, however, in assessing vector competence as the tests in use were hugely sensitive and “virtually everything that feeds on a viraemic animal tests positive”.

Dr Carpenter discussed various techniques for controlling midges, from repellents to habitat modification, but noted that no product provided 100% protection at any time point.

He hoped there would be more integration with decision makers, “many of whom have little scientific expertise”, especially if there was a willingness to learn from each other.

Questions

The first question asked in a lengthy discussion period was: “How realistic is eradication or should we concentrate on control?”

Dr Oura said that for eradication a concerted vaccination campaign with a high level of coverage would be needed; if a high level was not achieved it would not go away. He added later that there was little chance of a compulsory vaccination campaign across the UK.

Professor Machlachlan said that with only one serotype we could be optimistic that with high level coverage it would not persist – but it was always much easier to look backwards than forwards. “If you are going to prevent it you must have a totally immune population.”

Dr Oura said there were marked differences in susceptibility with different breeds of sheep; Dr van Wuijckhuise noted that there was a market reduction in prevalence in cows kept indoors.

Ian Anderson asked if delegates thought there would be an outbreak of bluetongue in Britain this year. Only a small number thought it was likely. A slightly larger number believed there would be no outbreak because the current rate of vaccination was adequate.

“If there is no outbreak this year and vaccination drops off, we will be at constant risk,” Mr Anderson said. One delegate, Chris Lewis, said it was essential to educate farmers that there was a constant risk.

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