How bluetongue virus exploded in the South West
Getty ImagesA virus which can be deadly to livestock successfully overwintered in parts of the UK before spreading rapidly, scientists believe.
Cases of the bluetongue virus, which affects sheep and cattle as well as other animals, continue to spread with the latest government figures showing confirmed cases on 684 farms across England, Wales and Scotland since the outbreak began in July.
Devon and Cornwall - which has almost two thirds of confirmed cases - is the epicentre of the outbreak.
Dr Simon Gubbins, an epidemiologist at the Pirbright Institute, which is testing for the virus, said the UK had "never seen anything like the South West cluster" but the strain was likely not reintroduced from Europe.
Spread mainly by midge bites, the virus was confirmed in a sheep in Cornwall in September 2025 in the first recorded case in the UK in 17 years.
This strain was introduced into Cornwall from midges blown in wind from northern France, said Gubbins.
With cases clustered in the south of France this year, Gubbins said the likely scenario was the virus "went from one season to the next" in parts of the UK.
Bluetongue, which causes a drop in milk production in dairy cows and lameness in sheep, can be transmitted to their offspring.
"The virus has essentially done that in the south-west of England, and then when the new crop of lambs and calves were born, it's come during a very warm period - and that is very favourable for bluetongue transmission," said Gubbins.
"In normal weather conditions we'd expect that to be one to two weeks during the height of summer - but for the past few months it's been a few days to a week.
"It's been much faster than we'd normally expect."
Gubbins, based in Woking, said Devon and Cornwall's high density of livestock also created favourable conditions, along with low rates of vaccination.
"There are lots of opportunities for spread between animals and between farms," he said.
Dr Ternenge Apaa, a veterinary scientist at the University of Reading, said environmental conditions "amplified disease spread once the virus reached areas with large sheep populations".
"Once the virus becomes established, local transmission can increase rapidly during the midge season," he said.
On 13 August, UK chief vet Dr Christine Middlemiss warned the outbreak would "get worse" after the region sweltered through its fifth heatwave of the year.
Middlemiss conceded there was a bottleneck in vaccine supplies but she urged farmers to continue inoculating livestock as the virus was "here to stay".
However, Gubbins was hopeful bluetongue could be eradicated in the UK despite being established in Europe.
"In the Netherlands, for their first season, they were having large numbers... but then they managed to control it through the use of vaccination and they haven't reported disease now for two years," he said.
"Vaccination is one of the best means of protection that we have had."
