Welsh farmers are being urged to be vigilant after the Schmallenberg virus was detected in animals in the country for the first time.
The virus, which causes severe deformities in lambs and calves, was detected in three cows and one calf in Ceredigion.
The Farmers' Union of Wales (FUW) described the development as "very concerning".
The history of the three animals suggested they were infected by SBV while on the holding, up to a year ago.
FUW animal health and welfare committee chairman Catherine Nakielny 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 and weak lambs.
"They should report anything unusual to their vet, especially given the recent developments."
The presence of SBV in Wales is not unexpected. The Animal Health Veterinary Laboratories Agency (AHVLA), the Welsh government and the Wales Animal Health and Welfare Strategy Steering Group have kept SBV under close scrutiny since its arrival in Britain late in 2011.
As of July this year, there were 275 UK farms reporting positive for Schmallenberg. Of these, 53 were in cattle, 219 in sheep and three in both.
"There is a strong need to continue to monitor SBV in Wales and to this end livestock farmers should be vigilant and report suspicions to their private veterinary surgeon," said Dr Nakielny.
The Schmallenberg virus is not currently a notifiable disease in the UK, but test samples taken from suspect animals are still being paid for by the government for the time being,
It remains possible that midges could continue to spread SBV in Britain through the autumn and into the winter, but the FUW hopes the poor weather that has blighted the industry over the summer has reduced the risks of transmission.
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.
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