The threat of Bluetongue Disease - Peter Green, Honorary Veterinary Adviser to the British Deer Society
The current appearance of Bluetongue Disease in Suffolk has great significance for both the farming and wildlife populations of the UK. Bluetongue is a well recognised disease of ruminant animals in many parts of the world, but the UK has always remained free of the infection, although it has affected large areas of northern Europe since 2005.
The disease is caused by a virus and spread by biting midges. It is not infectious from animal to animal, but depends upon the insect vector to transmit the virus from one animal to another in much the same way as malaria is transmitted from person to person. A wide range of ruminants may be infected by the virus, but the response varies considerably from species to species. Of the domestic animals, sheep are the most seriously affected, with severe disease in adult sheep causing high mortality. Signs include swelling of the head, high fever, nasal discharge, lameness, pneumonia and interference with the blood supply to the tongue and lips, which turn blue or even black. In milder outbreaks, clinical signs may be less severe but abortion, stillbirth and weak, fading or deformed lambs occur. Cattle are usually either only very mildly affected, or are not affected at all - although the current disease in
Suffolk was first recognised in a cow; other cattle as well as sheep are now being tested. The type of virus in Suffolk has been identified as BTV8, which is the same strain as in the recent outbreaks near the English Channel and North Sea coasts in Europe. It probably arrived with a wind borne midge.
Some parts of the southern
USA have continual and persistent infection, as do some parts of Mediterranean southern Europe and many other sub-tropical agricultural economies are plagued by the disease. The virus belongs to a closely related group of viruses that includes the virus causing African Horse Sickness and a severe disease of deer called Epizootic Haemorrhagic Disease [EHD]; in fact, the deer virus and the sheep virus are so closely related that some scientists believe that they are simply variations of the same virus. Others believe that the viruses are distinct, but very similar. The deer disease [EHD] causes serious and fatal signs in white tailed deer in many parts of the USA and Canada. Outbreaks sometimes kill hundreds of animals, usually mostly white tails, but in severe incidents mule deer and pronghorn antelopes are also affected and may die. The clinical signs in deer are the same as the signs in sheep with Bluetongue. Rather perversely white tailed deer are also the most seriously affected by true Bluetongue virus, leading to great confusion in southern States of the USA where both diseases are endemic. Red deer, fallow deer, muntjac and roe deer are all potential hosts of the Bluetongue virus and in experimental inoculations with EHD all four species developed a period of infection, with multiplication and shedding of the virus, but none developed serious disease.
The vector of the disease in southern
Europe is a biting midge Culicoides imicola. This insect is not normally present in the UK and present climatic conditions mean that it is unlikely to become established. If temperatures fall below 12oC for periods of time the midge cannot survive and persistent periods of temperatures higher than 15oC are required for the virus to multiply within the insect. It is clear, however, that the midge has extended its range northwards across Europe in recent years. and the prolonged heat wave of July 2006 was an ideal breeding opportunity. This particular midge species would not survive for long in the UK, but it is possible that animals could have been bitten by the infected midges before they disappeared. Stalkers [especially those in the north] will be only too aware that we have several species of aggressive biting midge, some of which are other closely related Culicoides species and DEFRA has pointed out that these insects could potentially become vectors of the Bluetongue virus.
Almost all Eurasian deer species are members of the subfamily Cervinae, which contains species like the ed deer, sika, fallow, axis, the rusa and swamp deer. Wapiti, or American elk are also classified within this sub family and just like red and sika, can hybridise with European red deer. Most of the New World deer like the north American white tailed, the mule deer and the South American brocket deer, marsh deer, pampas deer and pudu are members of a different sub family of deer, the Odocoilinae. Bluetongue virus appears to be much more able to become established and to cause disease in the Odocoilinae deer [for instance the white tailed] than in the Cervinae species. The roe deer is a member of the Odocoilinae, not the Cervinae and is therefore much more closely related to American white tails than to British red or fallow. Although there has been a little work with EHD virus in roe, not a great deal is known about the pathogenesis of true Bluetongue in roe, which is generally a deer species of northern latitudes where Bluetongue has not traditionally occurred. If the disease becomes endemic in areas where roe deer and farm animals live closely together, the roe may become symptomless hosts of the virus or may suffer actual disease. This is speculation, but the national bodies with specific deer interests such as the British Deer Society and the Deer Initiative are closely monitoring the situation and will endeavour to keep web pages up dated.
For the time being, DEFRA is investigating the
Suffolk case by means of blood testing to determine whether the virus has spread. The UK Bluetongue Control Strategy is now being implemented; an outbreak will not be declared unless it is shown that the virus is circulating. Protection Zones and Surveillance Zones are much larger in the case of Bluetongue and the disease control is principally directed at elimination of the vector insects, although slaughter of infected livestock is a control option. The appearance of the disease at the end of the summer, rather than at the beginning, gives some hope that the infection will be limited and that the virus will not overwinter in the UK.
Deer stalkers and managers are advised to continue to be vigilant for signs of disease in deer, either those monitored in deer parks and farms or culled wild animals taken as part of routine management. The national restrictions on stalking because of Foot and Mouth Disease continue to be in force - shooting of deer is permitted and carcases for human consumption may be moved, but all non-edible parts of the carcase, including gralloch and heads may not be moved off the premises unless they are entering the officially licensed fallen stock collection schemes. Should deer with signs of disease be encountered, veterinary advice may be sought through the offices of the British Deer Society and The Deer Initiative and details of local DEFRA offices may be found in all BT telephone directories.
Peter Green Honorary Veterinary Advised
The British Deer Society
24th September 2007