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Last Updated: Friday, 28 September 2007, 17:07 GMT 18:07 UK
The rise of bluetongue disease
The virus
The virus is of the same type as northern Europe suffers

Bluetongue is a mysterious disease, with no suitable vaccine, but there are still reasons for optimism for British farmers, despite the confirmation of an outbreak in Suffolk.

If bluetongue were to take full hold in Britain it would change the landscape.

Anywhere which has hills dotted with sheep would be devastated. The strain of the disease found in animals in Suffolk has come from northern Europe.

There where it has struck, flocks of sheep have seen 30-35% of their numbers dying.

Strain is type 8
Same as northern Europe
Kills 30-35% of infected sheep
Kills 1% of infected cows
Culicoides obsoletus or pulicaris midges responsible
One midge bite enough for infection
Transmission to midges much harder

The likely route for its arrival is in a cloud of midges blown by warm winds across from Germany, the Low Countries or north-eastern France. One bite from an infected midge is enough to transfer the disease to an animal.

No-one knows how bluetongue came to be in northern Europe, says Professor Peter Mertens of the Institute of Animal Health at Pirbright in Surrey.

In southern Europe as temperatures have risen, the disease has spread from Turkey through Bulgaria and into Greece and the Balkans, as well as from North Africa to Italy, and from Morocco to Spain and Portugal.

Devastating potential

But there has to be a fourth route of transmission, Prof Mertens says. The strain found in Suffolk, and in northern Europe, is type 8, not the same as that affecting southern Europe.

There are also hundreds of miles of uninfected territory between the northern and southern outbreaks. The type 8 strain has originated in Africa, but how it travelled north is a mystery.

Now its shadow hangs over the UK, with some of the densest populations of sheep and cattle in the world.

And because the virus is new to Britain, an outbreak could be devastating. The herds are "na´ve", unexposed to the virus and with none of the antibodies needed to fight it, and the most susceptible animals have not been selected out as they are in regions where it is endemic.

Hopefully we are going to get some really nasty cold weather, a really bad winter with lots of snow would be perfect
Prof Peter Mertens

But there is cause for optimism, Prof Mertens suggests.

"The only thing saving us from bluetongue is our climate. There is hope. If we start having frost, it will kill off the majority of adult midges. A few good frosts will really bring the midge season to an end. When that happens it's the end of transmission."

Beneath 15C the virus cannot replicate.

"That is why the warm temperatures in northern Europe were so important - they allowed it to get well established."

And the disease is now established in the Low Countries and Germany and increasingly in France. After outbreaks last summer, the disease has returned with greater intensity this year.

And in that return lies the concern for the UK now there is a confirmed outbreak.

Despite the seasonal death of midges, the disease somehow returns the next year, a process known as "overwintering".

"We don't yet know how it does this, hides away somewhere when the conditions are right, when the midges come back so does the virus," says Prof Mertens.

'Killed' vaccine

There is no vaccine usable in Britain currently on the market. In South Africa where the disease has been endemic, a "live" vaccine has been used, but when a similar technique was used in Italy it prompted at least one outbreak.

Such a vaccine would also kill British sheep. A "killed" vaccine is being developed by Meriel, who have facilities at Pirbright, and is predicted to be ready by the new year.

An alternative approach was used in Greece where infected animals were killed and insecticide was used widely to control the midge populations. After four years the country was clear of the disease.

The likely culprits, a midge from the Culicoides obsoletus or pulicaris groups, could be attacked with targeted attacks on dungheaps and other breeding areas, Prof Mertens said.

But ultimately the best defence remains the weather.

"Hopefully we are going to get some really nasty cold weather, a really bad winter with lots of snow would be perfect."

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