Chestnut Ridge, N.Y. ? Lots of dog lovers are worried these days. Experts say canine influenza virus, or dog flu, is spreading steadily through the nation’s dogs, with no vaccine available to curb it. Perhaps 5 percent of its victims are dying.

Researchers recently found to their surprise that the virus had crossed over from horses to dogs, striking greyhounds at racetracks in 11 states. Now it has been found in more than 100 pets, with cases documented in California, Florida, Massachusetts, New Jersey, New York, North Carolina, Ohio, Oregon and Washington state.

Cynda Crawford, a veterinary immunologist at the University of Florida, said researchers are getting positive readings on 30 percent to 40 percent of the blood and tissue samples sent in by veterinarians who think they might be treating a dog with influenza. The symptoms include a cough, low-grade fever and a runny nose.

Exactly how many dogs have died is unclear. Crawford said many of the animals were young and otherwise healthy.

Many pet owners and veterinarians have been fooled because some of the symptoms mimic a common, less dangerous bacterial infection known as kennel cough.

As with human influenza, dog flu is most easily contracted in gathering places – kennels, dog shows, animal shelters, even dog runs in parks.