The spread of Bluetongue 8 across some parts of Europe has been fast and furious this autumn with two other serotypes 1 and 6 now present. Post import testing in Britain has found eight farms with imported livestock that tested positive for Bluetongue 8 but there have been no ‘home grown’ cases.
In November the first case of Bluetongue Serotype 1 was detected on a farm in Lancashire. The five cattle originated from South West France and have been culled. Chief Veterinary Officer Nigel Gibbens said: “This is the first case of BTV1 infected animals being imported into the UK. Taking into account the epidemiological evidence and the consideration of risk represented to British livestock, I have taken the decision to cull these animals. This incident shows how important it is for farmers to consider potential disease risks when buying stock. Buyers need to consider how best to protect their own businesses and those of their neighbours and make sure they are clear about the stock they are intending to buy.”
DEFRA conducted 453 investigations in 2008 but have found no evidence that the virus is circulating in this country.
It is thought that the high uptake of the vaccine in the East and the South may have protected the rest of England from the disease by acting as a buffer. The figures show that the approximate uptake percentage in the South East was 100%, the East of England 94%, the South West 83%. It is a different story in the rest of the country with the East Midlands 71%, West Midlands 53%, Yorkshire and the Humber 42%, the North West 27% and the North East 26% – figures that are well below what is required to adequately protect the country’s livestock from the disease.
Vaccination in England and Wales is voluntary and sales data suggests that enough vaccine has been purchased to vaccinate 60% of the susceptible livestock population in England and 25% of the susceptible livestock population in Wales. Scotland has compulsory vaccination which began on November 3.
In France there were 24,488 cases of Bluetongue Serotype 8 and 4,469 cases of Bluetongue Serotype 1 up to November 28, 2008. It was also detected in Sweden and Hungary in September. The Dutch have reported 55 BTV8 infected premises during 2008 and BTV1 in four animals imported from France. They also reported the first case of BTV6 in Europe on October 24. Tests have confirmed the presence of the virus in four cows on four premises. The Swiss authorities have identified an entirely new bluetongue-like virus in goats that was originally called Toggenburg Orbivirus but has now been classified as a new serotype bringing the number of bluetongue serotypes known to 25.
One puzzle is how the bluetongue virus manages to ‘overwinter’ when both midge activity and virus replication cease at cool temperatures, interrupting transmission, but then reappear after several months, far longer than the typical lifespan of the midge or the normal period of infection in livestock.
Anthony Wilson, Karin Darpel and Philip Scott Mellor of the Institute of Animal Health at Pirbright published a paper in August 2008 titled ‘Where Does Bluetongue Sleep in the Winter?’ One theory is that the adult midge Culicoides, which normally survives for 10-20 days, can live for much longer in mild winter conditions. In the laboratory this was found to be the case with individuals living for three months at 10 degrees C. The winter of 2006-2007 was the mildest on record and adult Culicoides were caught throughout that period so the possibility exists that some infected adults survived long enough to bridge the gap between transmission seasons. It was also believed that adult midges were reluctant to enter buildings but recent studies have shown that small numbers may move indoors when temperatures drop.
The other route is through overwintering in the ruminant population whether by chronic or latent infection of some individuals, transmission across the placenta from mother to foetus or during sexual intercourse.
The authors write; ”The recent spread of bluetongue into areas such as Northern Europe, which are both highly susceptible to the economic effects of the disease and cold enough that classical transmission is interrupted for a significant part of the year, has stimulated field, laboratory and analytical research into the mechanisms by which bluetongue may overwinter in temperate climes. The next steps for bluetongue research must be first, to establish these newly discovered transmission routes beyond reasonable doubt and second, to determine their importance for the transmission of BTV in the field.”
It is clear that livestock is not necessarily safe from bluetongue infection in the winter and the message is clear Vaccinate.