March 1, 2012

ongoing expert reaction to Schmallenberg virus in the UK

To accompany a background brieifing the SMC ran on the emerging livestock disease caused by the Schmallenberg virus, comments from the speakers and a Fact Sheet on the virus were sent out.

Professor Peter Mertens, Research Leader, Vector-borne Viral Diseases Programme, Institute for Animal Health, said:

“Reported cases of deformed lambs and calves show that the Schmallenberg Virus spread very effectively across northern Europe in the summer of 2011; the most recent information indicates that it spread southwards, about halfway across France. This means that infections cover a wide area including a significant number of cattle holdings in central France.

“Tracking the number of affected farms in the Netherlands shows that the rate of new cases in sheep farms appears, for the first time, to be slowing down. This may be because the sheep giving birth now got pregnant as the winter approached, when insect numbers in Europe were decreasing and transmission would have slowed down.

“Another new result shows that on some of the infected farms in Europe, the rate of seroconversion (the proportion of animals infected) was very high – up to 100%. This isn’t necessarily good for the spread of a virus and it may be that the infection is ‘burning itself out’ on some farms. It is also looks possible that not every at-risk lamb will be deformed.

“In cattle, the number of infected farms is now increasing – probably because of their longer gestation period. It may rise further and approach the number of sheep farms affected, but that is still an unknown. We also don’t have any firm numbers for the proportion of at-risk calves that will have defects; it may or may not be the same as in sheep, however initial suggestions from Europe suggest that it may be lower than in sheep.

“Schmallenberg virus has some limited similarities to the Bluetongue Virus – another recent exotic, vector-borne virus that spread to northern Europe and then the UK. Bluetongue was effectively controlled with vaccines in the UK and Northern Europe. There are important differences but the emergence of Schmallenberg does show that present conditions in the UK are suited to this type of insect transmitted disease. We have the vector insects, we have susceptible animals and we have the right climate.

“We think the virus is transmitted by biting midges but it’s interesting that there’s a high density of local spread as well, which is different from what we observed in the initial stages of the Bluetongue outbreak in the UK in 2007. At the moment we just don’t know precisely how the virus spreads, including whether there is a route other than the presumed vector. That is the subject of some of the work we are doing in our labs at Pirbright to explore the possibility of transmission by biting midges and by mosquitoes. We are also looking to develop a high-throughput, immunological test for antibodies to the virus which will improve surveillance.”

Matthew Baylis, Professor of Veterinary Epidemiology, University of Liverpool, said:

“Eighty-three cattle and sheep farms in 14 counties of England are now known to have been infected with Schmallenberg virus. This is likely to be the tip of the iceberg, as detection currently relies on finding evidence of the actual virus in animals, which is difficult as viral infection is usually transitory.

“A better approach is to test for the antibodies that animals will have raised against the virus, and which then persist in the blood for months. However, no such serological test exists for Schmallenberg virus at present because the virus was only discovered, in Germany, a few months ago. When such a test is successfully developed, it is likely that much larger numbers of farms in Great Britain will be found to have been infected by the virus.

“Schmallenberg virus is transmitted by blood-sucking insects, and biting midges are the main suspects. In August of 2007 biting midges carrying a different virus, called bluetongue virus, were blown over to England from continental Europe, and in the outbreak that followed, about 150 cattle and sheep farms were infected, mostly in the south-east of England.

“A question regarding Schmallenberg virus is how it could already have spread over a large area of England if, as suggested by Defra, the most suitable times for infected midges to have been blown over to us from the continent was late October or early November. If the time of introduction was that late in the year, then Schmallenberg must somehow be capable of faster spread than bluetongue and the prospects for cattle and sheep farmers across the UK in 2012 would not be encouraging.

“An alternative, and perhaps more likely explanation, is that Schmallenberg has been with us for some time, but was not noticed; it may even have been present in the UK at or before the time that the virus was first discovered last August in Germany.”