The number of sheep and cows infected with the Schmallenberg virus in south-west England is higher than previously thought, say experts.
Researchers at the Duchy College in Callington said vets had estimated that the virus affected nearly 60% of dairy herds and 42% of sheep.
However, a new survey of more than 3,000 cattle and 2,000 sheep found 80% of herds and 70% of sheep tested positive.
Schmallenberg virus, which emerged in the Netherlands and Germany in 2011, can lead to sheep and cattle having stillborn or deformed offspring.
Scientists believe the virus, which was first detected in the South West in March 2012, was carried across the Channel by biting midges.
DEFRA and the European Union funded the Rural Business School at Duchy College to carry out the research across counties in the South West last July.
The college said many animals could still be carrying the virus, but do not display the symptoms.