The Schmallenberg virus originated in Germany and is carried on the wind by midges.
Outbreaks have tended to coincide with midge seasons during hotter weather.
There is no evidence of any health risk to humans, but symptoms in livestock include:
- Causes mild symptoms in adult cattle such as fever and diarrhoea
- Reduces milk in dairy cows
- Animals that have been infected are immune
- Two-30% of infected sheep, cattle and goats give birth to deformed or stillborn offspring
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Early enquiries suggest the men may have taken the drug Blue Valium. Two of them remain in hospital with breathing problems.
More than 8,000 people are expected to attend over the three day event, which starts at 7pm tonight.
The local MP and leader of the Liberal Democrats says it was a "joyful" day for locals and former locals alike.