Schmallenberg virus: Climate 'raising UK disease risk'

A flock of sheep Schmallenberg affects sheep and cattle and is probably carried by midges

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Climate change is raising the risk of diseases such as Schmallenberg in the UK and northern Europe, say scientists.

Schmallenberg virus affects sheep and cattle, and is probably carried by midges. It was identified in Germany last year, and in the UK in January.

Until 1990, Europe's midge-borne viral diseases were found only in Spain and Portugal; but two have emerged within the last six years in northern Europe.

Experts say the path of Schmallenberg is currently impossible to predict.

Schmallenberg virus - named after the German town where it was first identified - causes fever and diarrhoea in adult animals, but they recover.

However, infection during a critical stage of pregnancy leads to lambs and calves being born with deformation of limbs, spine or brain. Many are stillborn.

Currently it has been found on 83 farms in the UK, mainly in the southeast.

Unpredictable future

The next few months will almost certainly see the birth of more affected lambs and calves resulting from infections their mothers picked up last year, as farms progressively further north go through the calving season.

But after that, it could "burn itself out" or become a regular threat - or anywhere in between, according to leading scientists speaking at a briefing in London.

"There are these two scenarios," said Matthew Baylis from the Institute of Infection and Global Health at Liverpool University.

"The key question is whether the virus will be picked up by the vector (midges) from the calves and lambs that will be born later in the spring, after the midge season starts."

If that happens, he said, more cows and sheep will be infected, with problems emerging next year when they give birth.

But the path is very hard to predict as so little is known about a virus that was only identified a few months ago.

"There is the possibility it will simply die out, but I think that would be too good to be true," said Peter Mertens from the Institute for Animal Health in Surrey.

"There's a lot of virus about, and I think it's quite likely it won't simply go away in one year.

"Is Culicoides (the midge) the only means of spread, or is there something else on a local level - fecal-oral spread, or aerosol (airborne) spread?

"We don't know."

Vaccination needed?

One part of the puzzle that scientists have put together is the influence of climate change on the risks of midge-borne viral diseases.

A higher temperature means an increase in the number of midges, and that they feed more often. It also allows the virus to develop faster.

Using weather and climate models as well as information on the biology of viruses and midges, Prof Baylis's research group showed that recent climatic change in northern Europe has significantly increased the risk of viral midge-borne diseases.

"Temperature changes in Europe which to most of us have felt relatively small have in our model led to a large increase in the risk of viral midge-borne diseases," he said.

The modelling results, he said, reflected what has actually happened across the continent.

"Culicoides infections were first detected in Europe in the 1920s, but only in Spain and Portugal and on the eastern borders, around Turkey," he said.

"Then in 1998 we saw cases in Italy. Bluetongue then emerged in northern Europe in 2006/7, and now we have Schmallenberg."

The modelling suggests other similar diseases should be expected in future, said Prof Mertens, adding: "The doors are open."

Schmallenberg appears to pose no threat to humans, according to monitoring by the Netherlands government health agency.

It appears that currently, little can be done to curb the spread of Schmallenberg if it does emerge as a persistent threat for farmers.

Animals could be sheltered inside during the critical period of their pregnancy. Prof Baylis's team has shown this reduces the number of midges, but does not offer complete protection.

Australian farmers, combating a similar virus, have developed regimes whereby female cows that have not previously been infected are made pregnant before the insect season begins.

When they do become infected later in the year, their calves do not suffer, and the cows subsequently become immune.

There is no realistic way to control midge numbers, said Prof Mertens.

Vaccine manufacturers, meanwhile, are working on a vaccine for Schmallenberg, which would take about 18 months to develop.

But given all the things we do not know about the virus, said Prof Mertens, it was not yet proven that vaccination was needed.

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