A strain of flu virus that jumped from horses to dogs has caused sometimes deadly respiratory infections at dog tracks and kennels in at least 11 states and among some household pets as well, health officials reported yesterday.

While cautioning dog owners to keep their pets away from other dogs if they have a respiratory infection, to try to minimize further spread of the virus, experts said most dogs recover from the infection.

The discovery comes as public health officials are increasingly alarmed about a dangerous strain of flu that has been spreading among birds in Asia and occasionally infecting humans, but there is no evidence that the dog virus can spread to humans.

"We are going to monitor all cases of human exposure, but at this point there is no reason to panic," said Ruben Donis of the federal Centers for Disease Control and Prevention in Atlanta. Donis noted that it has been known for about 40 years that the virus causes the flu in horses, with no reports of its infecting humans. Tests also indicate it is sensitive to antiviral drugs.

The CDC held a news briefing to discuss the virus to coincide with the release of a paper by the journal Science documenting the identification of the virus.

The discovery of the virus began with an outbreak of a mysterious respiratory infection among greyhounds at a racetrack in Florida in 2004. Veterinarian Cynda Crawford of the University of Florida in Gainesville collected samples from the sick dogs and sent them to Edward Dubovi at Cornell University in Ithaca, N.Y., who detected evidence that the animals were infected with influenza.

Further analysis by CDC scientists determined that the dogs were infected with a strain of influenza virus nearly identical to one previously seen only in horses.

Testing indicated the virus probably was responsible for a series of mysterious outbreaks of respiratory disease at six racetracks in Florida, Texas, Alabama, Arkansas, West Virginia and Kansas in 2004, and then at 20 tracks in Florida, Texas, Arkansas, Arizona, West Virginia, Kansas, Iowa, Colorado, Rhode Island and Massachusetts earlier this year, officials said.

Since then, Crawford and Dubovi have been testing samples from veterinarians from across the country, and have confirmed the virus in pet dogs in Florida, probably the New York City area, and possibly Massachusetts, the researchers said.

"The data indicate the virus is being transmitted efficiently from dog to dog," Donis said. "It is now well established in the dog population."

Dubovi predicted the number of states with documented infections will increase rapidly because veterinarians are just now becoming aware of the new virus and are starting to send samples for testing.

"That map should expand . . . by the end of next week," Dubovi said.

Because the virus is new to dogs, most would have little immunity against it. The virus presumably spreads among dogs in the same way the human virus spreads among people -- primarily by an infected animal sneezing or coughing on another.

Crawford noted that the infection can mimic the symptoms of "kennel cough," a usually benign bacterial infection: coughing, runny nose and a fever. Although the mortality rate from the new flu virus remains unclear, so far it appears to kill 5 to 8 percent of infected dogs.

"About 80 percent will have a mild form of disease characterized by cough and nasal discharge that will resolve over time," Crawford said. "Only a minority . . . of dogs experience complications such as pneumonia."

Most animals recover with no treatment. Others require the same kind of treatment people need: plenty of fluids and rest. In severe cases, dogs may require intravenous fluids and antibiotics to fight secondary infections.

Tests on blood stored by racetracks indicates the new flu began infecting dogs between 1999 and 2003. Genetic studies indicate a few genetic changes in the horse virus enabled the microbe to begin infecting dogs.

Researchers already have begun trying to develop a vaccine for the virus, Crawford said.