updated 9/26/2005 1:53:06 PM ET 2005-09-26T17:53:06

The mysterious respiratory disease that has swept greyhound racetracks across the country and also afflicted pet dogs is a type of flu — an influenza strain that jumped from horses to dogs, researchers reported Monday.

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Such a rapid jump into a new species is rare; the flu usually evolves into new strains more gradually.

But genetic tests of sick dogs found their disease almost identical to the H3N8 influenza strain that afflicts horses, scientists at the Centers for Disease Control and Prevention and University of Florida discovered.

Moreover, they found evidence of widespread infection in racedogs around the country and in pets of various breeds in Florida and New York.

Since this is a new virus for dogs, they are unlikely to harbor a natural immunity to it.

There are no reports of people sickened by the new canine flu, which is genetically different from human flu strains — and from the bird flu that has killed more than 60 people in Asia.

The results were published online Monday by the journal Science.

How dangerous is it?
This new dog illness made headlines earlier this year as greyhound racetracks closed to control outbreaks. Veterinarians struggled to tell if the illness was a new variant of kennel cough or an entirely new disease.

The CDC researchers counted outbreaks at 14 greyhound tracks in six states from June to August 2004, and at 20 tracks in 11 states between January and May 2005.

It’s not clear how dangerous the new canine flu is to dogs. Some die, others experience only a fever and cough, but a large number show no symptoms at all, the researchers report.

While most attention has focused on racing dogs, the researchers tested 70 dogs of various breeds with respiratory disease in Florida and New York pet shelters and veterinary clinics. Some 97 percent showed antibodies to the new canine flu strain.

Tests of blood stored by racetracks suggests the new flu strain began infecting dogs sometime between 1999 and 2003, well before the first outbreaks were recognized, the researchers conclude.

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