Researchers characterize new canine influenza virus first seen in horses

October 2005

An influenza virus that has jumped the species barrier from horses to dogs appears to be more widespread, but carries a lower mortality rate than originally thought.

The virus was first reported in 2004, when racing greyhounds in Florida began to suffer an acute canine respiratory disease. At a recent press conference, sponsored by the CDC, researchers said that the equine influenza virus is more widespread than just racing dogs, and appeared to be present in domestic pet dogs.

“And that’s important because initially the virus was identified in racing greyhounds, and there was some speculation that the virus was exclusively causing disease in greyhounds,” said Ruben Donis, PhD, from the CDC. “The data indicate 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,” he added.

Cynda Crawford, DVM, PhD, from the University of Florida College of Veterinary Medicine, investigated the original reports. She collected samples and sent them to Ed Dubovi, PhD, at the Cornell University Animal Health Diagnostics Center, who identified the virus as an influenza virus.

Researchers from the Influenza Branch of the CDC typed the virus as an influenza type A H3N8 strain that is related to equine virus. After a multipronged epidemiological investigation, the researchers found that the virus was occurring in different breeds in other states; so far, disease has been reported in the California, Florida, Massachusetts, New Jersey, New York, Oregon and Washington.

Less fatal

In the first reports in the beginning of the year, it was thought that the fatality rate from the disease was as high as 36%, but now that health officials have looked at a larger number of animals with the disease, Crawford said, the mortality rate is actually much lower, only about 5% to 8%.

“Although it’s a new pathogen in dogs and nearly all dogs are susceptible to infection based on our knowledge about the virus to date, about 80% of them will have a mild form of disease, just characterized by cough and maybe some nasal discharge that will resolve over time with appropriate therapy,” Crawford said.

“Only a minority of dogs … experience complications such as pneumonia, just like humans infected with influenza, where certain populations of humans are more prone to development of pneumonia. And it’s a small number of humans compared to everyone else.

“So that is the same with canine influenza virus. It’s a small population of dogs that will develop complications, most likely bacterial complications, and these dogs do need to have their treatment supervised by a veterinarian,” she said.

Some animals also have subclinical infection, she said.

Crawford said when it is symptomatic, the respiratory disease mimics kennel cough.

“So the canine influenza virus is really the new kid on the block for veterinarians to consider in their differential diagnoses for kennel cough. They should consider canine influenza if a dog presents to them with a cough. They may have a nasal discharge and a fever also,” Crawford said.

She said all kennel cough is contagious; therefore, these dogs should be handled with the same precautions any veterinarian would use when treating a patient that has a potential infectious disease that is contagious to other dogs.

“This type of precaution would involve the isolation of the dog with respiratory disease for protection of other dogs in their clinics, while this particular dog is undergoing diagnosis and treatment and potentially hospitalization.

“In addition, not all dogs will show a clinical syndrome. … we’re working on a more rapid means of identification,” Crawford said.

The CDC is working with many other institutions and scientists to “understand how this newly emerging pathogen is affecting canine health and to seek their recommendations for management with veterinary clinics, advice for veterinarians, advice for people who run shelters and, of course, for pet owners,” said Nina Marino, for the CDC.

“Right now, it certainly appears that canine influenza is restricted to the dog population, and we’ll defer to our veterinary colleagues’ expertise and for advice and recommendations for management in the dog population. And I wanted to reiterate what Dr. Donis said earlier in the telephone call; that H3N8 has never infected humans; and that we will certainly monitor human exposure and we’ll work closely with our key partners here to coordinate any messages to monitor this emerging pathogen,” she said.

“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,” Dubovi added.

Use common sense

Crawford said that owners should use common sense in managing their pets. If their dog has an obvious infection, restrict its contact with other animals. However, the owner should not stop normal activities if the dog is healthy.

“I am taking my two Pekingese to be groomed next week to a groomer that sees many dogs during the day. I will continue to board my pet greyhounds at boarding kennels when I need to. I will continue to show dogs at the American Kennel Club sanctioned shows,” she said. “So I, myself, will continue on with my routine activities with my own dogs. They will go to dog parks. They will participate in other community activities with other dogs.

“However, I think common sense is a good term to use in 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, number one, for that dog to recover fully, and number two, in case that dog has an infectious disease, whether it’s flu or not, that could be communicable to other dogs that it is around.”

The virus may be sensitive to human antivirals, and researchers are working on a new canine vaccine for the disease, she added.

So far, there is no evidence that the canine version of the equine virus has been transmitted back to horses, although that is being monitored, according to Dubovi, and researchers do not believe there is any threat to human health, at the moment. However, the researchers reminded reporters that animals have become sentinels for human diseases, and pointed out the story of West Nile virus, which was first found in birds.

“H3N8 equine influenza virus has been in horses for over 40 years. In all these years, we have never been able to document a single case of human infection with this virus. So that is something that I want everybody to take note of so to dispel major panic. That’s not to say that there isn’t any risk. We are going to monitor all cases of possible human exposure, but, this point, there is no reason to panic,” said Donis.

For more information:

• Crawford PC, Dubovi EJ, Castleman WL, et al. Transmission of equine influenza virus to dogs. In press.