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.

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.