East Anglia's livestock farmers have been warned of a growing threat of bluetongue virus as the weather warms during the coming months.

Dozens of cases of the potentially-fatal animal disease were found in Norfolk cattle and sheep during the winter, following the enforcement of a 10km temporary control zone (TCZ) centred near Cantley, where the county's first case was confirmed on December 8.

To the relief of livestock farmers, those restrictions on animal movements were eased in February, after the cold weather had lessened the risk of midges spreading the virus.

But Defra has confirmed that this "seasonal low vector period" has now ended, due to an increase in biting midge activity during the warmer spring weather.

A government statement says: "We are planning for a possible increase of bluetongue virus over the coming months as the weather warms and the risk of infected biting midges blowing over from northern Europe increases.

"Farms close to the coast in counties along the east coast of England from Norfolk to Kent and along the south coast from Kent to Devon are at highest risk of incursion. 

"Farmers should continue to monitor their animals frequently for clinical signs and make sure their animals and land are registered with APHA (Animal and Plant Health Agency) so we can locate animals in the event of an outbreak."

Bluetongue does not affect human health or food safety. The virus affects ruminants including cows, sheep and goats, and camelids such as llamas. It is usually transmitted by bites from midges, which are most active between April and November.

Animal health officials said bluetongue can spread when average temperatures are at least 12-15C, adding "this is when the virus replicates and biting midges have time to become infectious after ingesting a blood meal from an infected host".

But Defra says there is "still no evidence that bluetongue virus is currently circulating in biting midges in Great Britain".

Livestock keepers must report suspicions of the virus immediately to the APHA on 03000 200 301. For more advice on symptoms and disease restrictions, see www.gov.uk/guidance/bluetongue.