The first true confirmed canine influenza virus (CIV) was the H3N8 canine flu that evloved from  H3N8 equine flu. That’s the virus that spread to and amongst dogs in various parts of North America. The general consensus has been that only this strain should be called CIV, since it’s been the only true dog-adapted influenza virus that’s developed the ability to stay and circulate in the dog population.

More recently, another canine flu virus has emerged in dogs, this time a type H3N2 in Asia. H3N2 is a common human flu type, but birds are the ultimate reservoirs of all flu viruses, and based on the genetic relationship of H3N2 from dogs and birds, it’s thought that this virus came to dogs from birds.

Anytime a new infectious disease is encountered, it’s important to figure out who/what it can infect. When H3N8 CIV emerged, it was shown that even though it came from horses, it was no longer adapted to readily infect horses. So, knowing a virus’ origin or typical infection trends can be useful but it doesn’t necessarily tell you the whole story.

Cats and ferrets are susceptible to many different types of influenza viruses, and are good species to look at when figuring out if a virus can spread to other domestic animals. A study in the recent edition of Influenza and Other Respiratory Viruses (Kim et al, May 2013) looked at transmission of H3N2 CIV between dogs, cats and ferrets.

In that study, researchers infected dogs with CIV and kept them in close proximity to cats and ferrets, but without direct contact. They also infected cats and ferrets to see whether they could transmit the virus to other cats or ferrets.

Here are some highlights from the study:

  • All directly infected animals developed some degree of illness, with cats and dogs typically developing sneezing, coughing, increased respiratory effort and nasal discharge, and ferrets only developing sneezing.
  • Cats could become infected by being in proximity to both infected dogs or infected cats.
  • Ferrets didn’t get infected when exposed to infected dogs.
  • Ferrets did not develop disease after exposure to an infected ferret but 2/3 developed antibodies against CIV, meaning the virus had been transmitted, but not able to cause disease.
  • Cats shed higher amounts of virus than ferrets.
  • Dogs stopped shedding the virus by day 8 after infection. That’s not surprising since influenza shedding is short-term with H3N8 CIV. It shows that use of good infection control measures, particularly isolation, can be a key component of canine flu control.


  • Dogs with H3N2 CIV are potential sources of infection for cats and ferrets.
  • Cats that are exposed to the virus can get sick and be sources of infection for other animals, presumably including dogs. Cats may be another truly susceptible host for this virus.
  • Ferrets seem pretty resistant to the virus. It probably takes fairly high level exposure for them to get infected and they are less likely to be of concern for subsequent transmission.

Interspecies transmission of flu viruses, and other viruses, is obviously an issue. Most of the attention is paid to the bird-pig-human cycle, for good reason. Birds are the reservoirs of all influenza virus variants, pigs are  susceptible to both human and bird flu viruses and can act as a "mixing vessel," and humans are the species we’re ultimately most concerned about. However, the potential for disease in pets and for pets to be reservoirs of influenza for people or other animals shouldn’t be neglected. I’ve frequently had discussions with colleagues in the medical and public health fields about the need for parallel companion animal surveillance when plans are made for emerging infectious disease surveillance and response (e.g. SARS, H1N1 flu, novel coronavirus). They typically respond with general enthusiasm, but interest and application aren’t the same, and actually getting plans in place to perform coordinated parallel surveillance hasn’t happened. Studies like this are just one more piece in the puzzle that indicate the need for broader surveillance and consideration of pets.