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SBV, What you need to know.

22nd March 2013

The UK sheep industry is justifiably very concerned about the Schmallenberg virus (SBV). There are many sources of information available to sheep farmers on the internet and from various organisations but there is a great deal of confusion out there on how to apply this information to the way you manage your flock. Here we have attempted to summarise the most important information in a way that you can apply it.

SBV is a virus which causes only a mild fever in adult animals lasting for approximately 5 days after infection. The ewes generate a strong antibody immune response to the infection that is thought to last for at least several months. The virus is transmitted by midges, rather than directly from sheep to sheep. A sheep will only have virus in its blood stream for a few days after infection so there is only a short period when this animal will act as a reservoir of infection for midges to pick up and transmit the infection further.

The harmful effects of infection in the adult animals are related to this rise in body temperature causing a temporary drop in milk yield, reabsorption of very young foetuses (causing more returns and empty ewes) or a temporary drop in sperm quality in rams. However, the effect on growing foetuses during pregnancy (Highest risk 25-63 days of gestation) can be severe with a proportion of lambs having deformed lambs with fused limbs. Not all foetuses are affected and lambs conceived after the ewe has been infected are protected by her antibodies whilst they are in her uterus. The pattern of disease is not the same in every flock. It depends on previous exposure to the virus and on the number of midges available to transmit the infection. This explains why early lambing flocks in the south west and south east of England have been far more severely affected, with up to 80% losses, than February or March lambing flocks in the same area or flocks elsewhere in the country.

Ewes that have been infected will have antibodies to the virus in their blood that can be detected by a straight forward blood test. We do not know at this stage how long lasting this naturally acquired immunity will last. Ewes without circulating antibodies has probably not previously encountered the virus so if infected during pregnancy will potentially produce deformed lambs.

The virus has spread over much of the country during 2012 and will likely reach the far north and west as well as Ireland in 2013. This does not mean that every sheep has been infected and is therefore immune. From our investigations of fertility problems in flocks across the UK we have seen that many ewes have not been exposed even in and will be susceptible to infection next year. 

There is currently a vaccine undergoing testing which will allow flocks to be protected prior to the breeding season. We do not know if the vaccine will require annual boosting or if it will last for more than one year. Currently there are no proven effective means of preventing infection so advice until now has centred on delaying lambing as late as possible if your system has that flexibility and the targeted use of insecticides with some repellent activity during tupping and pregnancy although no product has a licence for the prevention of SBV transmission. I would strongly recommend producers consider vaccination of their flocks, especially in high risk situations such as early lambing flocks and discuss the most appropriate vaccination strategy for their flock with a vet when it becomes available as the potential losses and welfare implications of the major outbreaks we have seen are very severe.


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