British sheep farmers are bracing themselves for a nail-biting lambing season this spring, as the full impact of the mysterious Schmallenberg virus on lamb birth rates is set to become clear.

The virus - named after the German town in which it was discovered last year - affects sheep and cows, and causes most damage in pregnant animals, leading to stillbirths, malformations and brain damage. Non-pregnant animals that contract it can get diarrhoea and suffer from reduced milk yield, but typically recover.

As of this week, 40 cases of Schmallenberg had been reported in the UK, on 29 farms in Norfolk, Suffolk, Essex, Kent, Hertfordshire, East Sussex and West Sussex, according to the Animal Health and Veterinary Laboratories Agency. Thirty-nine of those were in sheep. There have also been cases in Germany, the Netherlands and Belgium.

Pete Garbutt, chief livestock adviser at the NFU, said it was difficult to gauge how many further cases there would be in the UK until the lambing season kicked off later this month.

There was no suggestion that Schmallenberg presented any danger to human life, Garbutt added, stressing that lambs that were stillborn or malformed as a result of Schmallenberg would not enter the food chain anyway. But farmers might suffer significant economic damage if the virus caused high lamb mortality, Garbutt said.

Schmallenberg is a windborne virus, but scientists do not yet know what, if any, role animal-to-animal transmission plays in spreading it. As the virus was only discovered late last year, there is no vaccine. “That is a big concern for many farmers at the moment,” Garbutt said.

At present, farmers do not have to notify cases to the authorities, but the NFU is calling on its members to report suspected cases to their vets.