The Schmallenberg virus, which causes severe deformities in new-born lambs and calves, was first identified in Germany in 2011. Since then, it has spread to virtually all countries in Europe and has affected over 8,000 herds, causing considerable economic loss to farmers.
The virus is transmitted by Culicoides midges, which were also responsible for the spread of bluetongue virus in 2006. The disease itself is usually relatively mild, with only adult cattle exhibiting symptoms. However, the real problem occurs when cows and ewes are infected within a critical period during pregnancy, as this results in an increased risk of birth defects and loss of the offspring.
Initially, it was unclear whether the virus was spread by imported animals or by midges. But now a paper in Scientific Reports confirms that midges blown by prevailing winds almost certainly are responsible for transmitting the disease around Europe.
The Culicoides midges are so tiny (1-3 mm) that they can easily be propelled by the wind for hundreds of kilometres. This was verified by data from a model which clearly linked the expansion of the disease with the prevailing wind direction. This also substantiates the UK Meteorological Office’s warning that the risk of Schmallenberg infection from Europe by windborne midges was greater on days when the wind was blowing from the continent. Nonetheless, the authors point out that the probability of a midge finding another host to feed upon is constrained by the strength of the winds at dusk, the time of the day when midges are normally active.
The study also revealed that the virus spreads far more widely than is suggested by the number of birth defects, as animals that are not at the critical stage of pregnancy may also carry the disease. In Belgium, a survey found that all cattle older than 6 months had already been infected by Schmallenberg by the beginning of 2012. This suggests that there are probably many other infected farms in the pathway of the disease that were never identified because they did not have susceptible pregnant animals when the midges arrived.
Diseases like Schmallenberg that are spread by insect vectors rather than direct animal-to-animal contact are particularly tricky to control. In addition, the long delay between initial infection and detection (around 113 days before foetal abnormalities become apparent) makes it difficult to determine the true extent of the disease.
Yet there is room for optimism. Most animals that have previously been exposed will naturally develop a strong protective immunity against future infections. And a vaccine has recently become available to ensure further protection, although many farmers remain undecided about this option. It is hoped that the findings in this study will also provide greater insight on how best to control any future midge-borne diseases.
Reference: Sedda L, & Rogers DJ (2013). The influence of the wind in the Schmallenberg virus outbreak in Europe. Scientific Reports, 3 PMID: 24285292
Image: O’Gorman Photography