A new, highly contagious and sometimes deadly canine flu is spreading in kennels and dog tracks around the country, veterinarians said Wednesday.

The virus — which scientists say mutated from an influenza strain that affects horses — has killed racing greyhounds in seven states and has been found in shelters and pet shops in many places, though the extent of its spread is unknown.

Dr. Cynda Crawford, an immunologist at the University of Florida’s College of Veterinary Medicine who is studying the virus, said it spreads most easily where dogs are housed together, but it can also be passed on the street, in dog runs or even by a human transferring it from one dog to another. Kennel workers have carried the virus home with them, she said.

News of the illness has some dog care givers in Seattle on alert.

A puli was brought to the Capitol Hill Animal Clinic last month exhibiting symptoms of the flu, said veterinary assistant Anna Arena. The dog owners told clinic staff members that it had come from a boarding facility or day care center where other animals were infected, and the puli was given antibiotics, she said. (Editor’s Note: The original version of this story misstated Arena’s job title.)

The clinic also treated a young lab with similar symptoms — such as sneezing and runny nose — but didn’t know if that dog had the same flu, Arena said.

Central Bark, a dog day care center in Seattle, gave out notices to its customers about two weeks ago to watch out for a flu, said dog wrangler Amber Anderson. The center hasn’t had any problems with the flu, she said, and it asked dog owners to keep their pets home if they got sick.

Dogs may be picking up the illness from local dog parks, she said.

A few dogs at Wags2Whiskers became sick recently with a cough, said owner Jean Peters. She didn’t know what was wrong with the animals, but for about the past 2 1/2 weeks, the Belltown dog day care has taken its other clients on leash walks instead of visiting the local dog park, where she thinks they picked up the cough.

She was surprised to hear of a new flu.

“That’s really scary if it’s killed some dogs,” she said.

How many dogs die from the virus is unclear, but scientists said the fatality rate is more than 1 percent and could be as high as 10 percent among puppies and older dogs.

Immunologist Crawford first began investigating greyhound deaths at a racetrack in Jacksonville, Fla., in January 2004, where eight of the 24 greyhounds who contracted the virus died.

“This is a newly emerging pathogen, and we have very little information to make predictions about it,” she said.

She added that because dogs have no natural immunity to the virus, virtually every animal exposed will be infected. About 80 percent of dogs that are infected will develop some symptoms, Crawford said. She said the symptoms are often mistaken for “kennel cough,” a common canine illness that is caused by the bordetella bronchiseptica bacterium.

Both diseases can cause coughing and gagging for up to three weeks, but dogs with canine flu may have fevers as high as 106 degrees and runny noses. A few will develop pneumonia, which is sometimes fatal. Antibiotics and fluid cut the pneumonia fatality rate, she said. The virus is an H3N8 flu closely related to an equine flu strain. It is not related to typical human flus or to the H5N1 avian flu that has killed about 100 people in Asia.

Experts said there are no known cases of the canine flu infecting humans. But with the approach of the human flu season and fears about bird flu in Asia, there is much confusion among some dog owners who have heard of the disease.

Crawford said she was fielding calls from kennels and veterinarians across the country worried that they were having outbreaks.

“The hysteria out there is unbelievable, and the misinformation is incredible,” said Dr. Ann Hohenhaus, chief of medicine at the Animal Medical Center in New York.

Dr. Ruben Donis, chief of molecular genetics for the influenza branch of the Centers for Disease Control and Prevention, said that there is currently no vaccine for the canine flu. But he said one would be relatively easy to develop because a vaccine that prevents the related horse flu exists.

P-I reporter Christine Frey contributed to this report from The New York Times.