There is widespread concern that cases of the Schmallenberg virus (SBV) are being under-reported by farmers.
A European Food Safety Authority (EFSA) report, published in June, said that mild clinical signs in adult animals were easily missed, and as a result were going undetected.
The detection of SBV in the eight affected European countries has largely been dependent on the detection of malformed newborns or still births, followed by the confirmation of the virus using scientific techniques.
"However, estimates for the within-region force of the infection critically depend upon the level of under-ascertainment," the report read.
In northern Europe the proportion of sheep holdings with confirmed SBV was 4% a country and 7.6% a region.
But evidence from the Netherlands, France and Germany showed the number of animals infected on a single farm had been as high as 70-100%, the report revealed.
"It's very possible that on some of the farms where infection has occurred, the disease is likely to have gone unnoticed."
Professor Peter Mertens, Institute for Animal Health
This suggests disease incidence could also be higher in the UK.
Animal health expert Peter Mertens said: "It's very possible that on some of the farms where infection has occurred, the disease is likely to have gone unnoticed."
He said this was because mild clinical signs, such as fever, diarrhoea and a drop in milk yield, were more difficult to spot than birth defects.
But he said it was "very important" for farmers to report such cases to help scientists build a more precise picture of the disease and predict a further spread.
Meanwhile, DEFRA is conducting sampling in counties not previously affected by the Schmallenberg virus to discover how far the disease could have spread and is expected to publish the data on its website in due course.
More on this topic
See the latest news, information and videos on the Schmallenberg virus