Possible actions to reduce the impact of Schmallenberg virus in sheep flocks
There is evidence that Schmallenberg virus (SBV) has over wintered and is again circulating. Active infection has now been detected in the north and north-west of England.
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Sheep in the second month of pregnancy are susceptible to the effects of the virus leading to the birth of deformed offspring. Reducing exposure of sheep to infected midges in the susceptible period may reduce the impact of the infection in a flock. However, in the absence of a vaccine, and the incomplete knowledge of the epidemiology of the virus, formulation of effective preventative strategies is difficult. Vaccines are being developed, although it is not certain when these will be available. Options include:
- Delaying tupping until midge activity is reduced. This may be applicable in some flocks depending on the management system and provided that consideration is given to any negative impact later lambing may have on farm profitability.
- Delaying breeding from ewe lambs until 2013. Employing this strategy would again depend on the management system and consideration of any negative impact this may have on farm profitability. Immunity may have then developed by the 2013 tupping season either by exposure to infected midges or through the use of vaccine which may then be available.
- Use of products which repel or control biting insects in early pregnancy. The likely benefit of these products is doubtful particularly as midges are widespread and appear to be particularly effective in transmitting the virus. Other measures such as housing ewes, and removing muck heaps to deny breeding habitats from the vicinity of housed sheep may help to reduce midge exposure. If you are advising farmers on the prevention of SBV by using insecticides please ensure that your clients are reminded to observe withdrawal periods prior to presenting any animals to the food chain. In addition, if recently treated animals are sold at markets, that any treatment information is passed on to the perspective buyer
- Serology may be used to assess previous exposure and therefore the likely susceptibility of a flock. The number of samples selected should be statistically based in order to provide meaningful interpretation, and further advice may be obtained from your local surveillance centre.
- Purchased stock may introduce infection into a farm, but as the period of viraemia is short the risk is likely to be low. Although infection has been confirmed in several areas of England and Wales, it is not possible to be certain that other counties not yet identified as being infected are free of the virus.