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WE would like to report serological evidence of Schmallenberg virus (SBV) infection in a dairy herd in south-west Scotland in 2012.
The herd is managed as two separate groups: group P, consisting of 190 cows that had access to pasture grazing between May and September 2012; and group R, consisting of 60 cows that were housed throughout 2012 and were never at pasture. Both groups shared the same building from the end of September 2012, but were managed and milked separately.
As part of routine herd health surveillance, we tested bulk milk for antibodies against SBV on the two separate groups in December 2012. The sample from group P was positive for antibodies against SBV (97 per cent positivity, values >40 per cent considered positive) and the sample from group R was negative for antibodies (1 per cent positivity). Within the same month, we then blood sampled all the cows in the herd and tested them for the presence of antibodies against SBV. Eight cows were positive for antibody; all were from group P (4.2 per cent group seroprevalence). The seropositive cows were home-bred and had not been off the farm. Purchased animals were present in the herd, but all had originated from herds in Scotland and all were negative for antibodies against SBV. No clinical disease had been recognised and, to date, no congenitally affected calf has been born.
We collected a further bulk milk sample from each group of cattle in January 2013. Group P was again positive for antibodies against SBV, although the amount of antibody appeared to have declined over time (53 per cent positivity). Group R remained negative for antibodies.
The serological results and herd history are consistent with a limited exposure to SBV in group P during 2012, with no evidence of exposure in permanently housed cattle on the same holding.
Low herd seroprevalence has been reported in housed cattle in SBV-affected areas of the UK (Tarlinton and Daly 2013) and this outbreak conforms to this pattern. In contrast, reports from pasture-reared herds in northern Europe describe high within-herd seroprevalence following introduction of infection (Elbers and others 2012). Recently, serological evidence of SBV infection in dairy herds has been reported in Cumbria (Laing and others 2013) and it would appear that north England and southern Scotland currently represent the edge of the infected area. We speculate that low within-herd seroprevalence is likely to be found initially at the edge of the infected area, as in this case; however, there may be other factors involved. Lower ambient temperatures relative to much of mainland Europe are certainly a feature of this area and could be expected to reduce the transmission rate. The predominant species of midge may be different to that in the areas of high within-herd seroprevalence and both factors could reduce the spread of infection within a herd below that experienced in Belgium and The Netherlands. Nevertheless, once we enter the vector season this year, there may be sufficient virus transmission to lead to clinical disease in this herd.
Our experience with this herd indicates that a positive bulk milk result for antibodies against SBV should not be taken to indicate a high level of exposure within the herd and that many naive animals may still be present.
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