Farmers are being urged to get their livestock vaccinated against bluetongue.
The virus has recently been confirmed in a case in East Yorkshire following outbreaks in other parts of the country.
Clare West, the National Farmers’ Union’s regional livestock board chairwoman, said: “Vaccination is the only proven defence that we have against bluetongue.”
Defra said it was monitoring the situation closely. Bluetongue does not affect humans or food safety.
The virus mainly affects sheep, cattle and goats, and it is spread by biting midges. It creates high fevers and mouth and feet ulcers, and it can be fatal.
West said the NFU was “rightly concerned about livestock and those that own them”.
She added that the hot, dry weather had created favourable conditions for midges.
“We’ve had a massive viral load and fast replication of the midges. So what could have just ticked away quite quietly has, quite frankly, exploded.
“Every farmer’s primary focus is the welfare of their animals so anything at all that is impacting welfare of animals is going to be a huge concern to us.”