By Jonathan Amos BBC News Online science staff, in Salford

There is no treatment for the infected animal

The bluetongue virus has long blighted Africa but in recent years has begun to spread northwards into Europe as the range of the biting insects has increased.

Veterinary experts told the British Association annual science festival that efforts were underway to develop new, more effective vaccines to protect the national flock should disease reach these shores.

Affected animals suffer extensive swelling and haemorrhaging in and around the mouth and nose. They also go lame and have difficulty eating properly.

Midge movement

Dr Philip Mellor, of the Institute for Animal Health, said that once the disease got into a flock up to 70% of the animals could die.

"It's horrendous," he told the festival. "Animals that have mouths full of blood don't eat very well; animals that have affected eyes don't see very well; animals that have damaged feet don't walk very well." There is no treatment.

The Culicoides ranges overlap

The bluetongue is transmitted by the Culicoides imicola midge, which has been steadily moving to higher latitudes.

"The virus exists around the world in a very broad band from about 40 degrees north to about 35 degrees south. But in the last five years we've had outbreaks in Europe which have been spreading up to 44 degrees north," Dr Mellor said.

"This is about 800 kilometres further north than ever before."

Plan in place

This was almost certainly caused by climate change, he said. The worry now is that midge species living even further north are picking up the virus and continuing the trend.

"The Culicoides ranges overlap and they are handing on the virus rather like a baton in a relay race."

Dr Mellor said the midges common to the UK - Culicoides obsoletus and C. pulcaris - were known to be capable of carrying the virus although neither as yet had been exposed to it.

The good news is that the vaccination programme in continental Europe and the presence of many animals that have survived infection mean there is currently widespread immunity in EU flocks.

This makes it difficult for bluetongue to make further, more northerly incursions.

But Dr Mellor warned that the turnover of sheep in the next five years would change this situation and the British authorities needed to be ready to deal with any threat.

"The British Government is sufficiently concerned that it has drawn up a contingency plan to eliminate bluetongue virus should it gain access to the UK."

He is leading a programme to develop new vaccines which should be ready within three years.