Flu from horses is racing among dogs.

A highly contagious influenza virus that has killed greyhounds and made other dogs ill may have first jumped to canines from a single infected horse, a genetic analysis suggests. The equine flu that sparked the epidemic appears to have evolved recently to spread readily among dogs.

The first signs of the virus' horse-to-dog leap emerged at a racetrack in Florida in January 2004. A respiratory illness there killed 8 greyhounds and sickened 14 others. Virologists identified the cause as an influenza A virus of type H3N8, which had been known to infect horses but not dogs or people.

That virus has since infected dozens of dogs that were housed together at racetracks or animal shelters in several states. A research team led by Ruben O. Donis of the Centers for Disease Control and Prevention in Atlanta has found the same virus in blood samples taken from dogs as far back as 2000, but not in dogs tested earlier.

When the scientists compared the genetic sequences of three viral specimens from the dogs, they found only minor variations. In one important gene, however, all three differed from the equine flu, the scientists report in an upcoming issue of Science.

Those data suggest that the canine flu comes from a common ancestral virus that mutated in that gene around the time it began spreading among dogs.--B.H.