The disease, which has spread from the Netherlands and Germany, causes abortions and birth deformities in farm animal.
Schmallenberg disease mostly affects sheep, but also cattle and goats. There is no treatment or vaccine.
A virus that causes miscarriages and birth deformities in farm animals, though it is not known to affect humans, has been confirmed at four sheep farms in Norfolk, Suffolk and East Sussex.
The Schmallenberg virus is believed to be carried by midges. It surfaced in the Netherlands and Germany in August 2011, and since then on hundreds of farms there and in Belgium. The microbe is difficult to detect in adult animals, and is apparent only when they gestate. There is no known treatment or vaccine.
The Animal Health and Veterinary Laboratories Agency in Weybridge has done laboratory tests confirming Schmallenberg virus is in the UK. It said in a statement: "Although there are still some uncertainties, the risk to human health from the disease is likely to be very low."
According to the Department for Environment, Food and Rural Affairs, it is likely the disease has been here since the autumn, carried via insects blown across the channel. Defra anticipates more cases will appear and has asked for "enhanced surveillance" from vets and farmers.
The disease has now been confirmed on farms in the UK, as well as Germany, the Netherlands and Belgium.
Animals that have the virus and become pregnant have either miscarriages, stillbirths or offspring with deformities such as a twisted neck, brain abnormalities or contracted limbs. There are mild symptoms of the virus in other adult animals - a few days of fever, low milk yield, loss of appetite, and sometimes diarrhoea - but these cannot be easily distinguished from other common illnesses. It mostly affects sheep but can also enter cattle or goats.
Russia banned imports of sheep and goat meat as well as live animals from the Netherlands on 18 January, joining Mexico. The Dutch agriculture ministry said China had asked for more information.
The UK's agency said the true extent of Schmallenberg is not known. The Netherlands has been hardest hit, with at least 240 farms affected, while in Belgium it has been detected in 126 farms. In Germany, where the virus was first discovered, 22 farms are affected.
The disease is not "notifiable", meaning farmers have no legal duty to report it, and there are no control measures imposed on infected farms or restrictions on buying and selling animals. "This may change based on forthcoming information over the coming weeks, but at present it is not anticipated," said Defra. The carrier is thought to be midges, which do not emerge until May, allowing vets time to gather more information.
A Defra spokesman said it was "very much early days" in determining the course of further action;
The National Farmers Union said the discovery of the virus in Britain would be a worry for all livestock keepers. "This is a new virus and there is still much that we don't know about it," said NFU animal welfare adviser Catherine McLaughlin. "Farmers can help by reporting any unusual symptoms, or abortions, to their private vet. If they are considering importing ruminants from the affected parts of continental Europe, the NFU would strongly recommend that they discuss their plans with their vet first to reduce the risk of buying in the virus."
According to an initial risk assessment carried out by public health authorities in the Netherlands and a follow-up risk assessment by the European Centre for Disease Prevention and Control, although there are still some uncertainties, the risk to human health from SBV is likely to be very low. Nevertheless, farmers and veterinary surgeons are advised to take sensible hygiene precautions when working with livestock.
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