Statistics from Altmetric.com
ATTENTION over the past three weeks has understandably focused on foot-and-mouth disease but, with defra apparently feeling confident that the outbreak in Surrey has been contained, it remains important to be alert for the possibility of another viral disease that threatens uk livestock — bluetongue. Since its unexpected appearance in northern Europe in 2006, when it affected more than 2000 sheep and cattle herds, mostly in the Netherlands, Belgium and Germany, bluetongue virus serotype 8 has overwintered successfully in Europe and is again making its presence felt in 2007. Already this year at least 125 holdings are reported to have been infected in the Netherlands, 180 in Belgium, 109 in Germany, eight in France and, most recently, one in Luxembourg. The weather systems coming in from the Atlantic that have dominated the British climate this year have made for a rotten summer but, in terms of keeping bluetongue out of the uk, could well have been a blessing in disguise. That said, the possibility remains that Culicoides midges carrying the virus could be blown over to the uk from mainland Europe on the wind (VR, March 31, 2007, vol 160, pp 422-426). Rain-weary Britons may long for an Indian summer. However, as the late summer/early autumn peak in the midge population approaches, a change in the prevailing wind direction, with warmer winds from the east, could increase the chances of the virus arriving.
Airborne drift of the infected midge vector across the Channel is not the only means by which the bluetongue virus could arrive in the uk. In July, the bva, together with the British Cattle Veterinary Association, the Sheep Veterinary Society and the Goat Veterinary Society, issued a joint statement urging livestock farmers to be vigilant for the disease and to consider carefully the risk of infection when importing cattle. They advised farmers that, if importation was the only option, steps should be taken to help reduce the risk of infection, including avoiding transit through restricted areas, or at least ensuring non-stop transit during daylight hours only and avoiding dawn and dusk when midges are most active. Imports of susceptible live animals from restricted areas of the eu are prohibited. Meanwhile, as a precautionary measure, defra is carrying out postimport tests on all susceptible species imported from continental eu member states.
It is important to be vigilant for the disease, not least because, as discussed in a paper in this week's Veterinary Record (see pp 253-261), it can be hard to spot. The paper, by researchers at the Institute for Animal Health, describes the clinical signs and pathology shown by British sheep and cattle experimentally infected with bluetongue virus serotype 8 derived from last year's outbreak in northern Europe. There is considerable variation in the susceptibility of different sheep and cattle breeds to different serotypes of bluetongue virus and the study, which involved four poll Dorset sheep and four Holstein-Friesian calves, aimed to assess the signs that could be expected if uk breeds were infected.
All four sheep showed clinical signs of disease, but there was considerable variation in the severity of those signs, despite identical doses of virus being used. Unusually for bluetongue, clinical signs have been observed in cattle in some of the outbreaks on the Continent. The calves in the iah study were clinically unaffected, despite becoming viraemic, although postmortem examination revealed a pathology that was more severe than suggested by the lack of clinical signs.
The authors highlight the difficulty in reliably detecting bluetongue by clinical signs alone and point out that, if the virus arrives in the uk, infected cattle could play an important role in its amplification and transmission. The lack of clinical signs, associated with high levels of viraemia, could mean that infected, viraemic cattle could remain undetected for some time while being an important source of infection for the local vector population of Culicoides midges. The authors also suggest that, as happened in continental Europe, the first clinical signs may be detected only once the infection is well established and has spread to second or third infection-cycle hosts. They conclude that, while laboratory tests are available for the rapid diagnosis of the disease at an early stage of an outbreak, the limiting factor in the early detection of bluetongue if it arrives in the uk will almost certainly be the recognition of the disease in the field.
defra has produced a leaflet on the recognition of bluetongue and this, along with other information on the disease, is available from its website at www.defra.gov.uk/animalh/diseases/notifiable/bluetongue/index.htm. An article on bluetongue in the June issue of the bva's journal In Practice provided a clinical update for practitioners (In Practice, 2007, vol 29, pp 314-318). The Institute for Animal Health's website (www.iah.bbsrc.ac.uk) also contains much useful information, including updates on the situation in Europe.
Bluetongue has severe animal health and welfare implications. It is a notifiable disease, both nationally and internationally. It does not affect humans, but control measures in non-endemic areas include restrictions on animal movements over a wide area surrounding an outbreak, which can have significant socioeconomic consequences. The difficulties inherent in field diagnosis, coupled with the fact that the disease can be diagnosed rapidly in the laboratory, make it all the more important that any suspected cases are reported to the local Animal Health Office promptly.