They also note something relatively unique about the new strain: Unlike other "species-jumpers" like avian flu, this virus appears to pass quite easily from dog to dog.
The virus poses no health threat to humans, but has proven fatal for some of the infected dogs. Infection has been reported in pet dogs in Florida, New York and possibly Massachusetts, experts at the Centers for Disease Control and Prevention (CDC) said in a press conference held Monday.
"The data indicates that the virus is being transmitted efficiently from dog to dog, and this indicates that the equine virus was transmitted to dogs and is now well established in the dog population," said CDC researcher Ruben Donis.
While dog owners need to be aware of possible risks to their pets, humans are not considered vulnerable, he added.
"[This strain of influenza virus] has been in horses for 40 years, and in all these years, we have never been able to document a single case of human infection with the virus," said Donis, who was also senior author of a paper released online Monday by the journal Science.
"This is not to say there isn't any risk," he added. "We are going to monitor all cases of human exposure, but at this point there is no reason to panic."
Still, recent troubling reports out of Southeast Asia of "bird flu" jumping from poultry to humans has peaked scientific interest in this equine-to-canine transfer, he said.
The new finding "concerns a very rare event of considerable scientific interest with regards to understanding influenza virus transmission across species barriers," Donis said, "and that's something that's on everyone's mind these days for a variety of reasons."
Both media and scientific attention have lately been focused on avian influenza and its ability or inability to transfer to humans and cause a pandemic.
Although the current developments do not involve bird flu, some of the basic mechanisms are the same no matter what the species. According to background information in the paper, flu viruses can jump to new species in one of two ways: the whole virus, essentially unaltered, can cross over, or a host can be infected with two different viruses which then combine to produce a new strain.
The bird-to-human cases of avian influenza in Southeast Asia are an example of the first mechanism, according to the experts.
This type of transmission is very rare and generally does not result in easy transmission in the new host species (i.e., to other humans).
The experts said that what's notable about the horse-to-dog transmission is that the virus apparently is easily spread from one dog to another.
At the press conference, scientists related the trail that led them to discovery of this "unprecedented" event.
It started with a January 2004 outbreak of respiratory disease among 22 racing greyhounds at a Florida racetrack. Most of the dogs developed a mild initial fever followed by a cough for up to 14 days; slightly more than one third of the dogs died after developing hemorrhaging in the lungs.
Genetic samples from the greyhounds indicated they were infected with an influenza virus almost identical to H3N8, which is normally found in horses.
More testing turned up evidence of the virus at 14 greyhound tracks in six states from June to August 2004. Additional evidence was found at 20 tracks in 11 states this year, and in pet dogs in Florida, New York and possibly Massachusetts.
"A dog that is currently residing in Massachusetts was infected, but questions as to where it picked that infection up, we don't know," said Ed Dubovi, of Cornell University's Animal Health Diagnostic Center in Ithaca, N.Y. Dubovi, who is also an author on the Science paper, was the first to identify the virus.
Although the virus is transmitting easily among dogs, not all dogs get sick and not all dogs who get sick actually die. The mortality rate has declined to between 5 percent and 8 percent, added Cynda Crawford, of the University of Florida College of Veterinary Medicine and another author on the paper. Those numbers could change, she warned.
The virus causes a syndrome similar to "kennel cough" in dogs and is spread much the same way as the flu spreads among humans.
"Canine influenza is really the new kid on the block for vets to consider in the differential diagnosis for kennel cough," Crawford said.
Veterinarians should take precautions in isolating dogs suspected to have canine influenza from other dogs, Crawford added. But, overall, she said, 80 percent of dogs stricken will have a mild form of the disease, and even dogs who are not overtly sick could be contagious.
However, she added, she would be continuing with plans to have her two Pekingese groomed next week and will continue to board her pet greyhounds when she needs to.
"I will go to dog parks. I will participate in other community activities with other dogs," she said. "However, I think that dog owners who have a dog with a respiratory infection or that has just recovered from a respiratory infection should probably keep the dog at home until a couple of weeks have elapsed for the dog to recover fully."
For Dubovi, this new development is a clarion call for more surveillance among animal and pet populations
"We have 50 million-plus dogs and 50 million-plus cats, and sometimes they become sentinels for other things we're not quite expecting," he said. "We need to be looking for unusual events, and we have to have the capability to be able to identify unusual events. We were fortunate enough this time to be at the right place at the right time. This may not be the last time we're going to see this."
More information
The CDC has more on healthy pets.
Crucial post-stroke tests of the heart and neck arteries are underused in all stroke patients, but especially among women, researchers report.
These tests alert doctors to the potential of blood clots forming in the heart while allowing them to check for the health of carotid arteries. They can help improve treatment of ischemic stroke patients and reduce their risk of a second stroke, according to experts at the University of Michigan.
Their study suggests that gaps in the use of these tests between men and women may explain why women tend to have worse long-term outcomes after stroke -- including a higher death rate -- compared with men.
Ischemic strokes are caused by blood clots traveling to the brain or by blockages in the carotid arteries that supply blood to the brain.
Reporting in the September issue of Neurology, researchers examined data on 381 ischemic stroke patients, including 220 women, treated at Texas community hospitals.
They found that women were 36 percent less likely than men to receive an echocardiogram of the heart, which can detect clot-producing conditions and other problems.
Women were also 43 percent less likely than men to have their carotid arteries examined, the study found.
"Diagnostic evaluations that should be done on every ischemic stroke patient still aren't being performed on a third to a half of patients, and they're less likely to be performed on women," study senior author Dr. Lewis Morgenstern, director of the U-M Cardiovascular Center's stroke program, said in a prepared statement.
"Intervention is needed to increase access to quality stroke care for all patients, but especially women," he said.
More information
The U.S. National Institute on Aging has more about stroke prevention and treatment.
For many teenagers, alcohol is the drug of choice. It's used and abused more than any other substance, according to the Texas Medical Center.
