Bluetongue virus: knocking at the door.

The big animal health story in the newspapers in the UK this summer has been the return, after 6 years, of foot and mouth disease. The outbreak was almost certainly caused by the escape of the virus from the virology research laboratory in Pirbright, Surrey. It seemed as if the outbreak had been contained quickly and the disease controlled, but initial claims that the outbreak was dead were subsequently shown to be premature as more farms became infected in September, emphasising the highly infectious nature of the disease. 

The National Farmers Union has claimed that the outbreak of foot and mouth has cost the British farmers tens of millions of pounds, but it seems unlikely that it will get to the serious levels of the epidemic in 2001. While this outbreak has been capturing the attention of the farmers, veterinarians and ministry officials working to control it, the spectre of another animal disease has been hovering just across the English Channel in the form of bluetongue disease. The discovery on 23rd September of cattle in Suffolk with bluetongue shows that the disease is no longer just knocking on the door of the UK, but has found a way in. An epidemic of bluetongue is currently gripping the Netherlands, Luxembourg, Belgium, and parts of France and Germany. The strain identified in the UK is the same serotype (serotype 8) as that in the European outbreak.

Veterinary authorities have been worried for some time that climate change could extend the range of the bluetongue vector from the tropical and sub-tropical regions of Africa and Asia to southern Europe. The threat that P.S. Mellor identified in his review article in 1996
(Culicoides, vectors, climate change, and disease risk. Veterinary
Bulletin
, 1996, vol. 66 no.4) was that climate change could bring the virus into contact with new vector species of midge that could transmit the disease and survive the northern winters, thus establishing the disease much further north. 

Bluetongue first appeared in Northern Europe 2006 and its resurgence in 2007 showed that it had survived the northern winter and was established in the local midge population. The 2007 outbreak has resulted in more cases and a wider range than in 2006. Sheep and deer are severely affected by the disease whereas cattle tend to act as reservoirs of infection.

Bluetongue is a haemorrhagic disease caused by an Orbivirus genus of the family Reorvirades. At present, 24 distinct serotypes have been identified by serum neutralization tests. The virus is transmitted by a small number of species of biting midges of the genus
Culicoides. These vectors prefer to feed on large animals such as cattle. The main transmission cycle is between the
Culicoides midge and cattle, with sheep or deer being infected when cattle are not present or the midge population is high. Thus, cattle can be used to detect the presence of the virus, and can be used as sentinel animals. Culicoides populations peak in the late summer and autumn and this is the time when bluetongue is most prevalent. When the disease becomes established it takes on a seasonal cycle with peaks in the autumn.

Only about 20 of the more than 1,400 Culicoides species worldwide are actual or possible vectors of bluetongue virus. Continued cycling of the virus among competent
Culicoides vectors and susceptible ruminants is critical to the viral ecology. In Europe, Middle East and Africa, the main vector is
C. imicola. In the USA, the principal biological vector is C. variipennis sonorensis, which is mainly distributed in southern and western regions (although there are recent reports of bluetongue causing the death of deer and antelope in Montana) . In Australia the principal vector is
C. brevitarsis.

The fear in recent years is that climate change would extend the range of the main European vector,
C. imicola, and spread the disease up through Europe. What seems to have happened in Northern Europe is that the virus has become established in other species of midge such as
C. obsoletus, and that these are transmitting the disease. Also in the Netherlands it appears that
C. dewulfi is also infected with the virus. C. obsoletus can over-winter in the Northern European climate, unlike
C. imicola, so that the disease can return each autumn when midge numbers increase. 

A number of serotypes (including 4, 1, 2, 15, and 16) of bluetongue have appeared in southern Europe, and their progression has been tracked from North Africa and the Middle East. However, the serotype 8 that is now established in northern Europe, and now in the UK, appears to be related to strains of the virus from sub-Saharan Africa. The question of how the virus came to northern Europe is not an easy one to answer with certainty. Movement of infected midges or, more likely, of infected livestock seems to be the most probable way. The rapid movement of livestock across the globe as part of the globalized economy provides a huge boost to virus mobility. 

Currently the Belgian authorities are applying to the European authorities for permission to vaccinate against the disease to prevent its resurgence next year. The disease is causing large numbers of deaths among sheep and economic hardship for farmers coping with the restrictions. As the UK is the largest producer of sheep in the European Union with more than 30% of total production, the spread of the disease here would be particularly damaging. There is, as any visitor to the highlands of Scotland will testify, no shortage of midges in the UK.

Bluetongue is a serious infectious disease like FMD but is transmitted in a different way, and is particularly difficult to control because of its transmission by midges and its silent presence in cattle, as reservoirs. Control of bluetongue in Europe will probably require strategic vaccination, along with large scale testing and slaughter. The experiences in Europe show that when the disease becomes established in the local midge population it is very difficult to control.

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