The Schmallenberg virus (SBV) which arrived in Britain from Europe in January has now been detected on farms Essex, Kent, East and West Sussex, Surrey, Hampshire, the Isle of Wight, West Berkshire and Wiltshire.
Across the UK, animals with the virus have been found at 74 farms. It affects sheep, cattle and goats, but has mainly been found in sheep here.
In adult cattle, the disease causes fever, reduced milk yields, loss of appetite, loss of body condition and diarrhoea.
The virus causes stillbirths and and foetal abnormalities mainly affecting lambs, but also calves and kids. It has been called Schmallenberg after the town in Germany where it was first seen.
Kent and East Sussex are among the worst affected counties in the UK. Twelve farms have tested positive for the infection in Kent and 14 in East Sussex. One farm in West Sussex has reported infection in cattle. There is currently no vaccine.
Last month, our reporter Iain McBride visited Kent just after the disease was first detected in the UK.
Farmers are not yet required by the Government to automatically reports cases of SBV, but the Department for the Environment, Food and Rural Affairs (DEFRA) is asking farmers to be vigilant and report suspicious cases to the Animal Health and Veterinary Laboratories Agency for testing.
In acute cases, SBV causes fever, reduced milk yield, loss of appetite, loss of body condition and diarrhoea in adult cattle. The same symptoms have not yet been reported in sheep.
It also causes abortion and stillbirths, which affects mainly sheep, but also cattle and goats.
The National Farmers Union has called for Europe-wide action to develop a field test to diagnose the disease and a vaccine to protect livestock. Farmers have warned that the full scale of the outbreak may not become clear until next year.
A DEFRA spokesman said the worst level of infection so far reported affected 20 per cent of a herd. In Europe, infection rates were at about 40 per cent. DEFRA said infection rates in the UK were more commonly about 4 per cent.