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Back to ANIMAL HEALTH - GENERAL Homepage

The Bluetongue virus threat
to UK livestock this summer

James Irvine

Teviot Scientific. Cultybraggan Farm, Comrie, Perthshire, Scotland

Filed 12 Apr 07
©www.land-care.org.uk

Bluetongue is a viral infection that mainly affects sheep, but also cattle, goats and deer. It does not affect man. The Bluetongue virus (BTV) is transmitted by midges (Culicoides). If the disease gets into a flock of sheep the morbidity and mortality rates can be high, depending on the type of the infecting virus. In highly susceptible sheep the mortality can be as high as 70% (1).

The disease in cattle may not be so severe, but again can be serious according to the type of BTV. There are 24 types of BTV. There is no treatment for Bluetongue Disease, but antibiotics can be used to counter bacterial secondary infections that may occur.

BTV has long been a problem globally, particularly in Africa and southern Europe, existing in a broad band from about 40 degrees north to about 35 degrees south. In recent years it has spread further north in Europe. The reason is thought to be related to climate change, allowing midges carrying the virus to survive in more northerly latitudes. In the last five years outbreaks have occurred, reaching 44 degrees north. That is some 800km further north than previously recorded.

Late in the summer of 2006 the disease was identified in the Netherlands, followed by Germany, Belgium, Luxembourg and parts of northern France.

 

Culicoides. obsoletus

Photograph kindly provided by
Simon Carpenter,
Head of Entomology and Mathematical Modelling,
Institute for Animal Health, Woking, Surrey, UK

(To enlarge photo Click Here)


As everyone in Scotland knows, midges are not confined to southern climes. What is particularly worrying is that the midge species in the affected areas, Culicoides imicola, can pass the virus on to other types of midge, such as the Culicoides obsoletus group and Culicoides pulicaris. Whether, if given the opportunity, the virus could also be passed on to the common biting midge of Scotland, Culicoides impuctatus, must be a matter of concern.

Certainly within some types of midges the virus can be passed on from one type to another, rather like passing a baton in a relay race.

Although Bluetongue Disease is not contagious, once the virus gets lodged in organic matter it can survive for prolonged periods.

Midges are spread by the wind, but are killed off by low temperatures. If the disease becomes active again this summer in northern continental Europe it is possible that midges bearing the virus could be blown across the Channel to Kent, which has a high population of sheep.

Midges infected with BTV could also be transported to areas far beyond their normal territory in aeroplanes, in lorries or on the person of travellers. They could also be carried on plants, or other materials that are being imported. A species of midge carrying the virus could then pass it on to another species of midge that is common in the hitherto unaffected geographical area.

The virus can be spread through the import of livestock from areas that are infected, or which have passed through such areas. It is illegal to import cattle or other susceptible livestock if they have originated from, or travelled through, a restricted BTV zone. Currently, BTV restricted zones include all of Belgium, the Netherlands and Luxembourg, as well as parts of Western Germany and Northern France (including the export centres of Calais and Dunkirk).

According to the National Beef Association (NBA) at least three recent instances have occurred where cattle had travelled through BTV restricted zones and subsequently entered the UK. The NBA documents an instance when 70 continental cattle, brought together in a single load, by various buyers in Scotland and England, in January 2007. The route the lorry took went through an area where the animals could have attracted biting midges carrying BTV. Fortunately, subsequent testing showed the cattle to be negative for BTV, but the risk was real.

Not only should the importers have known the rules but the border checks should have been more rigorous - a frequent and long standing complaint of the UK's livestock industry and of the EU's approach to the control of spread of diseases affecting livestock.

 

Culicoides impunctatus

An electron micrograph of
the prominent biting midge of Scotland

Photograph kindly provided by
Dr Alison Blackwell, Advanced Pest Solutions Ltd,
University of Edinburgh.


The type of BTV that is closest to the shore of the UK is currently V8. In southern Europe the types involved are V2 and 4. However, from what has been described above, there is the potential for the UK to be hit by a wider range of types of BTV, although V8 is the most likely.

There are fears that the BTV may have survived last winter in parts of northern Europe - possibly in both the host and the vector. With climate change, milder but wetter winters are predicted for the UK.

There is, therefore, a very real threat to the UK's livestock this summer from Bluetongue - a disease that hitherto has not been seen in Britain. With the current early and unusually warm Spring, the infected midge may arrive on UK shores earlier than the authorities may lead us to believe.

Symptoms and signs of Bluetongue

The most prominent signs of BTV8 infection in sheep as recorded in the recent outbreaks in north western Europe (2, 3, 4) are:

fever, salivation, erosions of the oral cavity, facial oedema, dysphagia and, apathy and tiredness, congestion, erythema, redness of oral mucous membranes and lameness.

In cattle BTV8 caused:

crusts/lesions of nasal mucous membrane, salivation, fever, conjunctivitis, dysphagia, serous nasal discharge, apathy and/or tiredness, hyperaemic/purple colouration, lesions of teats, lameness and coronitis.

This is in contrast with previous experience that BTV does not produce more than transient and mild, if any, clinical signs in cattle. The recent experience in northwest Europe with BTV8 has shown that a few cattle within a herd can show distinct clinical signs. This is borne out by the experience of Sabine Zentis who runs a pedigree herd of Longhorns in Germany, who states (5) that

"Cattle infected with V8 and showing clinical signs are really ill. If treated properly
they will survive, but beside the short term lesions of the mouth, tongue or teats, they suffer from laminitis for quite a time. The longterm effects of calves born to dams infected during pregnancy have not yet been investigated, nor a suspected reduction of fertility or milk yield after infection."

In sheep, BTV8 clinical signs and mortality are much more prominent than in cattle.

Differential diagnosis of Bluetongue

With such non-specific clinical signs in both sheep and cattle, it is essential that an accurate and rapid diagnosis can be made. This is where it is greatly disappointing that the lessons that should have been learned from FMD UK2001 apparently have not been learned.

According to the latest version of the UK BT Contingency Plan, blood samples taken from anywhere throughout the UK have to go to Pirbright, Surrey in the south of England for testing. While this may be fine for establishing the serotype of the first case, it is far from satisfactory when it comes to checking whether a beast on a farm is suffering from BTV or from some other condition. What is required is rapid on-farm tests that can be done within a few hours, not a few days. RT-PCR technology is available for that internationally, but for some reason the UK authorities do not wish to use it. There must surely have been abundant opportunity to "validate" such tests if there was a will to do so.

It should also be remembered that rapid on-farm slide tests were available during FMD UK2001 but were not used.

Modern RT-PCR testing should be able to rapidly test for a spectrum of diseases all in one go, on farm.

How can BTV be controlled?

As every Scot knows who has taken a summer holiday on the West Coast, it is very difficult to control midges. As mentioned above, they are carried on the wind and by other means. They can be very prevalent even in the absence of livestock or dung heaps.

By the time a case of BTV is diagnosed in farm livestock there are likely to have been many beasts that have been subclinically infected for a considerably period on that farm, and possibly other farms in he area. The vector midges will be all over the vicinity and beyond. It is the area that is infected, not just the particular farm where the disease was first found. There would, therefore, be little point in slaughtering the identified host in a rather forlorn attempt to reduce the viral load. The host beast is likely to survive and to develop immunity to future infection, at least to that particular BTV type.

It would appear patently obvious that the only way to control the spread of BTV is through an effective vaccination policy. Science is perfectly capable of producing BTV vaccines that are sufficiently effective to be useful in the field, but there is a clear lack of an effective integrated policy within Europe to achieve this essential objective (6, 7)

We are told that Merial may have a BTV8 vaccine by 2008 or 2009. But vaccine manufacturers will only produce vaccines if they are contracted and paid to do so. What the Chinese managed to do in relation to vaccine production for Avian Influenza was impressive in its timescale and effectiveness.

There is a European vaccine bank that allegedly holds some millions of doses of combined vaccine against BTV2&4, and which has been used for the past 2-3 years in Spain. It would be interesting to discover how effective it has been, and what complications may have arisen. The fact that the vaccination policy is allegedly continuing in southern Europe would suggest that there is confidence in its success.

But in the UK, DEFRA reiterates the same message that it had during the disastrous management of FMD UK2001:

"In the event of a decision to vaccinate against Bluetongue, supplies of vaccine would be ordered by the CVO from the European Bluetongue Vaccine Bank. In a worse case scenario around 7.4 million doses of vaccine might be required.

"At present the UK does not hold a marketing authorisation for use of such vaccines. Vaccination could could therefore only be used on a vaccinate to kill basis.If such a policy was adopted further details would be issued to holders of the contingency plan concerning the quantity of vaccines necessary"

This makes most depressing reading. Why is DEFRA stuck in the mindset of not getting a marketing licence because it has not asked for it? Does the vaccination programme in Spain rely on a vaccinate to kill basis? It would hardly make any sense.

Warnings are also made about the possible teratogenic effect that vaccines may have, and the risk of viruses adapting to vaccines that are only partially effective to create new virulent strains that could possibly escalate rather than help the situation. But are these warning based on experience in the field, or just on theoretical fears of academics? Surely, with the extensive experience with BTV vaccines around the world there should be some reasonably objective assessment of these risks.

How much funding has the UK government put into EU collaboration - or indeed wider international collaboration - directed at producing BTV vaccines that are relevant and promptly available? Or do they simply leave such matters to market forces, whilst proclaiming their devout interest in animal health and welfare?

There is deep suspicion that DEFRA may be taking the same , largely misguided, advice from the same group of vets (or indeed from those without any relevant veterinary experience) that advised them so poorly in FMD UK2001, and again with regard to the threat to the UK of Avian Influenza H5N1 (8).

Conclusion

The reality is that, through lack of applying science fast enough, the UK this summer is highly vulnerable to BTV arrriving on virgin territory - and we are virtually defenceless to do anything about it before it gets a grip.


©www.land-care.org.uk


References

1. DEFRA (2007). Technical review - Bluetongue
http://defra.gov.uk/animalh/diseases/notifiable/pdf/bluetongue_technical.PDF
(Accessed 10th April 07)

2. Institute of Animal Health (2007). Clinical signs of Bluetongue in sheep (pdf)
http://www.iah.bbsrc.ac.uk/virus/Reoviridae (Accessed April 07, 2007)

3. O.I.E. (2006). Bluetongue detected for the first time in northern Europe.
http://www.oie.int/eng/press/en_060823.htm (Accessed April 07, 2007)

4. European Food Safety Authority (2007). Report on the epidemiological analysis of the 2006 bluetongue virus serotype 8 epidemic in north-western Europe: provisional findings through 31January 2007.
http://efsa.europa.eu

5. Zentis, Sabine (2007). "Gluing fly swats to cattle's tails didn't do the trick"
http://www.warmwell.com - See April 10, 2007 entry.

6. Tigner, Brooks (2007). Europe's bio-threat readiness questioned.ISN Report Apr 11th, 2007
http://www.isn.ethz.ch (accessed April 12th, 2007)

7. European Commission: Scientific Committee on Animal Health and Welfare (2000).
Possible use of vaccination against Bluetongue in Europe.
http://ec.europa.eu/food/fs/sc/scah/out47_en.pdf

8. Irvine, James (2007). DEFRA statements on the role of vaccination in the control of virulent livestock viruses could make better informed people weep,
See SCIENCE Homepage, filed 12 Feb 07, www.land-care.org.uk Click Here to View


Finis