Possible actions to reduce the impact of Schmallenberg virus in sheep flocks
Disease information
There is evidence of another peak of Schmallenberg virus (SBV), infections with circulation during the Autumn and Winter of 2023. Active infection has been detected in England during the autumn with acute cases detected in cattle. The virus has also been confirmed in congenital deformed lambs, stillborn from December 23 onwards.
The virus itself gives rise to only mild symptoms in cattle which are transient including fever, drop in milk yield and sometimes diarrhoea. In sheep few if any signs are exhibited. If ruminant animals should become infected when pregnant, it can lead to abortion or malformations in the foetus.
For the most current information: APHA guidance-schmallenberg-virus
APHA are encouraging sample submissions (free testing), particularly in light of the current BTV situation, to ensure that presumed SBV is definitely SBV and not a completely new disease, or a new presentation of BTV.
Schmallenberg virus is spread by Culicoides sp. midges of the obsoletus complex. It does not spread directly from animal to animal and spread is dependent on the number of midges, which typically peak in late summer/early autumn and drop sharply once frosts sets in. Some transmission does occur over winter when midge numbers are low, but it is very reduced. Vaccines were developed and were available from 2015 to 2017, but none are currently commercially available in the UK.
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.
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.
- If possible, avoid synchronising breeding ewes. Exposure to the virus at the sensitive stage of pregnancy may then have a bigger impact with more lambs effected
- 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 , it is not possible to be certain that other counties not yet identified as being infected are free of the virus.
- Consider vaccination if a vaccine becomes available
Other Sources of Information:
- NADIS – National Animal Disease Information Service
- https://ahdb.org.uk/knowledge-library/schmallenberg-virus-sbv
- Schmallenberg virus: an update, In Practice 2015, Paul Roger: https://doi.org/10.1136/inp.h64
- Schmallenberg disease in sheep or goats: Past, present and future, Veterinary Microbiology, 2015, Lievaart-Peterson et Al: http://dx.doi.org/10.1016/j.vetmic.2015.08.005