Sep 27, 2005 (CIDRAP News) – A respiratory illness that recently emerged in racing greyhounds is caused by an influenza virus that jumped from horses to dogs, disease experts reported yesterday.
The illness identified in some dogs in Florida and New York City is caused by an H3N8 strain of influenza A, commonly found in horses, officials said in a news teleconference hosted by the Centers for Disease Control and Prevention (CDC). The illness in dogs is usually mild, and the virus poses no known danger to humans, officials said.
Dr. Ruben Donis of the CDC called the illness's emergence "a very rare event of considerable scientific importance with regard to understanding influenza virus transmission across species barriers." Science published an online article yesterday about the virus's emergence.
The illness first surfaced at a greyhound track in Florida in January 2004 and was investigated by Dr. Cynda Crawford of the University of Florida College of Veterinary Medicine in Gainesville and others. After the virus was identified as influenza, the CDC sequenced its genes and determined that it was an H3N8 strain, Donis said.
"This type of virus has been in horses for over 40 years, and in all these years we've never been able to document a single human infection with this virus," he said. "That's not to say there isn't any risk . . . but at this point there's no reason to panic."
He added that in laboratory tests the virus is sensitive to antiviral drugs.
Donis said CDC investigators found that the hemagglutinin (H) gene of the canine virus had from eight to ten amino-acid changes compared with the equine virus. Hemagglutinin is a protein on the surface of flu viruses that enables them to attach to and enter host cells.
"We speculate at this point, because we haven't finished the research, that these changes may affect the interaction of the virus with cellular receptors," Donis said. "In addition to that, there are changes in other genes which may affect other aspects of the interaction of the virus with the host, and this is exactly the most important part of the study from the perspective of understanding interspecies transmission."
The report comes as the medical world is on high alert over another species-jumping flu virus, the H5N1 avian virus, which has infected at least 115 humans in Asia but has not yet shown an ability to spread easily from person to person.
In contrast, "The data indicates that the [H3N8] virus is being transmitted efficiently from dog to dog, and this indicates that the equine virus . . . is well established in the dog population," Donis said.
Donis was asked if the movement of the virus into dogs increases the risk that it could infect humans and possibly mix with human flu viruses to spawn new viruses.
He replied, "Basically the number of amino acid changes is still very small, so this virus is still very, very closely related to the equine virus, and so given the, quote-unquote, safety track record of the equine virus in humans, I think it would be a stretch to really raise the level of alarm in any way."
But he added that the CDC doesn't know for sure and will be working with veterinarians to monitor the virus.
Describing the illness in dogs, Crawford said it resembles the syndrome called kennel cough. About 80% of dogs exposed to the virus will have a mild illness involving a cough and maybe a nasal discharge, she said.
However, a few dogs—like humans suffering from flu—develop complications like pneumonia, Crawford said. Data on the disease are very scanty so far, but in laboratory-confirmed cases, the mortality rate has been about 5% to 8%, she told reporters.
But she also said, "I want to emphasize that this is not the deadly virus that certain sources have played it up to be."
In an Associated Press (AP) report yesterday, another veterinary expert, Dr. Brad Fenwick of Virginia Polytechnic Institute, suggested that the mortality rate is lower than estimated by Crawford. Mortality can be much lower if dogs are treated, he told the AP.
Crawford said some dogs infected with the virus never show any signs of illness, but experts think they can still spread the virus to other dogs.
She said the disease has turned up in animal shelters, kennels, and veterinary clinics in southern Florida. Dr. Ed Dubovi of the Cornell University Animal Health Diagnostic Center said the virus has been found in New York City and in at least one dog in Massachusetts.
Crawford said researchers have been working on a vaccine for the virus for 18 months, but she didn't predict when a vaccine might be available.
As for advice on protecting dogs from the virus, Crawford said owners of healthy dogs don't need to change their usual activities. "As far as your average dog owner, I think they should continue to walk their dog on the street, do all the other dog-related activities, but just use common sense about taking your dog out in public if it has had, or has recently recovered from, a respiratory infection," she said. In that case, the dog should be kept at home for about 2 weeks, she said.
Dubovi, who first identified the canine virus as influenza, said its emeregence points up the "tremendous need" to watch for unusual pathogens in pets and other animals.
"I think that what the take-home message has to be is that we need to be looking for unusual events and we need to have the capability at our regional laboratories to be able to identify these unusual events," he said. "We were just fortunate enough to have been in the right place at the right time to have picked up this particular event."
Transcript of Sep 26 news conference
Crawford PC, Dubovi EJ, Castleman WL, et al. Transmission of equine influenza virus to dogs. Science 2005 Sep 26 (early online publication) [Abstract]