All farmers need to be on the lookout for unusually high incidences of abortion or deformities as the Schmallenberg virus was detected in north Wales and western England.
Vets have said infected sheep were brought to the UK from the continent during the autumn breeding sales.
Recent weeks have seen positive tests for the virus in dairy herds, heifers and lambs. The disease can hit productivity, such as milk output, with abortions possible in pregnant animals and birth defects in lambs and calves.
Due to the mild autumn, it is thought that midge activity also persisted longer than usual.
Schmallenberg virus causes congenital malformations and stillbirths in cattle, sheep, goats, and possibly alpaca.
The virus is named after Schmallenberg, in North Rhine-Westphalia, Germany, from where the first definitive sample was derived.
The disease can cause a temporary dip in production, such as milk output, although abortions, stillbirths and birth defects can result when pregnant animals are affected.
Farmers' Union of Wales Senior Policy Officer Dr Hazel Wright said: "All farmers in Wales need to be on the lookout for any unusually high incidences of abortion or congenital abnormalities – deformed lambs, swollen heads, weak lambs etc.
"There is a strong need to continue to monitor SBV in Wales and to this end livestock farmers should be vigilant and report anything unusual to their private veterinary surgeon."
There is no known risk to human health from SBV but the advice for pregnant women remains to be cautious around farm animals and to follow strict hygiene procedures, the Union has stressed.
