A new canine influenza has sickened dogs at greyhound racing tracks, boarding kennels, and animal shelters in as many as 11 states, killing some animals, causing respiratory infections in others, and striking fear in the hearts of pet owners nationwide. From January to May 2005, reports of outbreaks of a new, respiratory disease primarily in racing dogs came into the Centers for Disease Control and Prevention from Florida, Texas, Arkansas, Arizona, West Virginia, Kansas, Iowa, Colorado, Rhode Island, and Massachusetts. Not all of these are confirmed cases of canine influenza. However, since this spring, scientists say they've confirmed cases of dog flu in pets in Florida, New York, and Massachusetts, and calls are coming in from veterinarians, shelters, and kennels elsewhere.

First identified in 22 racing greyhounds at a Florida racetrack in January 2004, the dog flu is caused by an influenza A virus known as H3N8, previously seen only in horses. Scientists believe that the virus made this unprecedented jump from horses to dogs by a process called "reassortment," where two virus strains simultaneously infect one animal and a new, hybrid strain emerges. Since dogs appear to have no resistance to this new hybrid strain, "Nearly all dogs are susceptible," explains Cynda Crawford, a professor at the University of Florida's School of Veterinary Medicine and one of the scientists who identified the new virus.

About 80 percent of the dogs infected with the virus will suffer only a mild form of the disease, says Crawford, a short-term cough and respiratory illness. But in some dogs, the flu results in complications, such as pneumonia, that can lead to hemorrhage in the respiratory tract and death.

"Despite the rumors on the Internet and elsewhere, this disease is not as deadly as some have made it seem," says Crawford. "We don't have all the answers yet, but we are looking at a mortality rate on the order of 5 to 8 percent of dogs that become infected."

The dog flu mimics kennel cough, an illness animals sometimes pick up when they're boarded in kennels with other dogs. But kennel cough is usually caused by a bacteria called Bordetella bronchiseptica and can be prevented with a vaccination. "It's important for dog owners to keep up with their bordetella vaccinations so if their dog becomes ill, they know it's not caused by that particular bacteria."

Canine influenza is different from kennel cough in that fever often accompanies a dog's cough, as does a runny nose and sneezing–which is apparently how the virus is passed from dog to dog, just as influenza is passed among humans. Humans do not seem to be susceptible to the H3N8 virus. It has been common in horses for some 40 years, but has never been transmitted to humans. However, whether or not a dog lover's close proximity to their pet might increase the risk of human infection is unclear. "The bottom line is we don't know," says Ruben O. Donis, a researcher at the CDC.

Work is underway to develop a vaccine against canine influenza. But in the meantime, veterinarians urge dog owners to use common sense.

"Dog owners who have dogs with a respiratory infection should keep their dogs at home," says Florida's Crawford. "As far as the average dog owner, they should continue to walk their dog on the street."