The Schmallenberg virus that causes birth defects in sheep and cattle was carried to the UK by midges blown over from French and Belgian farms, say Oxford University scientists.
Schmallenberg was first seen in Germany in 2011 and spread rapidly across Europe through Culicoides midges, the same insects that carry the bluetongue virus. The researchers showed that Schmallenberg spreads more widely than is revealed by birth defects and is also highly dependent on wind direction.
The model suggests that the disease, which first hit UK farms in 2011, was introduced from across the Channel by infected midges from at least ten farms in France and Belgium. It also shows that around half of infected farms across Europe are 'dead-ends' that do not go on to spread the disease further.
These latest findings, published in Scientific Reports, could help farmers and policymakers to understand more about the spread of viruses such as Schmallenberg and plan how best to control them.
'We found that most birth defects in sheep were caused by Schmallenberg infections approximately five to six weeks after conception,' said Dr Luigi Sedda of Oxford University's Department of Zoology, lead author of the paper. 'The first infected sheep was reported on 14 December 2011, so that was probably caused by an infectious midge bite around the 11 August. The lag time between infection and detection makes it difficult to control the virus, particularly as it spreads so quickly.'
A Schmallenberg vaccine was made available earlier this year, but it is not currently clear how many farmers have chosen to vaccinate. There was concern at the time, especially among sheep farmers, that vaccine costs were too high for routine application to be affordable to protect sheep flocks.
To date, more than 8,000 farms across Europe have been affected by the Schmallenberg outbreak. Sheep and cattle that were infected last year should have developed immunity to the disease, but it is difficult to predict where these will be.
'One of the problems with diseases like Schmallenberg is the lack of a national strategy for reporting or control,' said Professor David Rogers of Oxford University's Department of Zoology, senior author of the paper. 'It is impossible to get up-to-date infection data with farm-by-farm detail. Farmers are understandably cautious about declaring infections, but better reporting across the country could benefit the whole industry. We could potentially predict the probability that a farm has been infected (and hence contains immune animals), allowing farmers to make more informed decisions about whether or not to spend money on vaccinations.
'In Germany and the Netherlands Schmallenberg is a so-called "notifiable disease" which means that all cases are reported, but in the UK (where the disease is not notifiable) we don't record it or routinely test for it. Previous tests in Belgium have shown that the disease is far more widespread than the reported cases, as animals that are not at the critical stage of pregnancy may carry the disease unnoticed. There are probably many "stepping stones" in the path of the disease that we don't see because four out of five farms may not have susceptible pregnant animals when the midges arrive.'
These missing links in the disease spread add to the difficulty in any attempts to control the disease at a local level. The scale of the spread makes midge eradication impractical, and the lag time in detection makes it difficult to adopt a reactive farm-by-farm approach.