Researchers have identified between eight and 10 genetic changes between the virus in horses and dogs, and they speculate that these changes affect the ability of the virus to attach to receptors on cells.

The illness first drew attention sweeping through kennels of racing greyhounds in several states and has also been found in household dogs in some states.

The flu kills between 5 percent and 8 percent of infected dogs, Dr. Cynda Crawford of the University of Florida College of Veterinary Medicine said at a briefing Monday. Some dogs get a cough, runny nose and fever while others show no symptoms at all.

The flu can cause symptoms similar to the common illness known as kennel cough, Crawford said. But kennel cough is usually caused by a bacteria while the new illness is caused by a virus.

Dr. Brad Fenwick, vice president for research at the College of Veterinary Medicine at Virginia Polytechnic Institute, said he thinks mortality from this flu is even less than estimated by Crawford. If infected dogs are treated, mortality can be much lower, Fenwick said in a telephone interview.

Easily Transferred Between Dogs

While the new virus is easily transferred between dogs, Crawford said people should not panic.

Owners of dogs that have a respiratory infection should keep the animal home for at least two weeks, she said.

Asked if dog owners should avoid kennels or other activities, Crawford said she plans to continue boarding her dogs when needed, walking them in areas with other dogs and entering them at dog shows.

Fenwick agreed, adding that people should not use vaccines intended for horses on their dogs because the safety of the vaccines hasn't been tested in canines.

As for transmission to people, Crawford noted that the strain of flu has been known in horses for more than 40 years and there have been no documented cases of it moving to people.

Nonetheless, Dr. Nina Marano of the Centers for Disease Control said the agency will monitor human exposure to the virus.

Virus Transmissions Across Species

Dr. Ruben Donis of the federal Centers for Disease Control in Atlanta called the transfer of a virus from horses to dogs "a very rare event of considerable scientific interest with regards to understanding influenza virus transmission across species."

Donis said the researchers have identified between eight and 10 genetic changes between the virus in horses and dogs, and they speculate that these changes affect the ability of the virus to attach to receptors on cells.

The findings of the research team led by Donis were published online by the journal Science.

He said the research began in Florida and has since traced the virus to dog tracks in other states and to pets in Florida and New York and possibly Massachusetts.

May Have Gone Largely Unnoticed

Fenwick, who was not part of the research team, said it is not clear whether the transfer occurred first from horses to the racing greyhounds or to house pets.

There are a lot more pet dogs around horses than racing dogs, he noted. The fact that it was first diagnosed in greyhounds could be because it spread to several animals in kennels where veterinarians were called in to treat the dogs. Since mortality is so low it may have gone largely unnoticed in pets, he suggested.

The published paper reported respiratory disease outbreaks in from June to August 2004 at dog tracks in Florida, Texas, Alabama, Arkansas, West Virginia and Kansas. From January to May 2005 outbreaks were reported at dog tracks in Florida, Texas, Arkansas, Arizona, West Virginia, Kansas, Iowa, Colorado, Rhode Island and Massachusetts.

The research was funded by the Division of Pari-Mutuel Wagering of the State of Florida.