Equine Influenza, which caused a devastating shutdown to the New South Wales racing industry in 2007, can be airborne over a reasonable distance and can be transmitted indirectly, including through humans.
In an update today the BHA said it was still "in the early stages of assessing the scale and severity of the outbreak" and said it would receive the results from further tests this evening.
The last time the National Hunt festival was cancelled was during the foot and mouth crisis in 2001.
Equine influenza, sometimes referred to as equine flu or horse flu, is a highly contagious respiratory infection.
"Of course we do not want a more extended period without racing, but the BHA must act in the interests of equine welfare, a William Hill spokesperson said". Tens of thousands of horses can be affected by one outbreak.
While the infected horses have not been racing this week, the McCain stable has had runners at Wolverhampton, Ayr and Ludlow.
'This is a standard contingency in the event of an infectious disease affecting our horses.
Stuart Williamson of Newmarket Equine Hospital is one of a large number of vets, all decked out in protective clothing, involved in taking nasal swabs from horses in the yards under temporary closure. BHA procedures state that a racecourse can not race for at least four days following a deep clean having taken place.
Horse racing across the United Kingdom has been banned today, Thursday, in a bid to stop the spread of a serious disease.
Paddy Power Betfair is also backing the BHA with its decision to suspend racing activity, saying it expects the normal schedule to return before the end of next week.
A spokesperson for Defra said: "The disease is not notifiable in the United Kingdom and there's no public health risk".
The 100 stables involved include those of the jumps season's two leading trainers, Paul Nicholls and Nicky Henderson, and nobody will be allowed to make entries for their horses until tests have been completed to the BHA's satisfaction.
He added, "Typically, Jockey Club Estates are bending over backwards to help us and are making it very feasible for the trainers involved to get their horses exercised while having no contact with any of the other strings".
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