H3N8/H3N2 Influenza Strains Affects Dogs, H3N2 Can Also Affect Cats

This form of influenza (called canine influenza) is a viral respiratory infection found primarily in dogs. There have been two identified strains of the virus in the US: H3N8 and H3N2. These viruses are considered to be endemic in the United States.


According to the Centers for Disease Control, the first evidence of H3N8 canine influenza in companion dogs was documented in spring 2005. It is believed to have jumped species from horses and was first identified in an outbreak of respiratory illness in racing dogs in Florida in 2004. H3N8 does not affect cats. Since the virus was identified in 2005, Oregon has had a few documented H3N8 cases.


In 2015, a canine influenza outbreak in the Midwest was determined to have been caused by the H3N2 subtype, which had been circulating in Asian dogs since 2007. H3N2 has been found in many US states since April 2015. Cases of H3N2 in dogs were confirmed in King County, WA in late 2015 and early 2016. To date, there have been no documented or confirmed cases of H3N2 flu in Oregon.

H3N2 in Other Species

Cats in a shelter in Indiana were confirmed to have been infected with the virus. However, the risk to most cats living outside of shelters is considered low, with the highest risk factor being infection in a household dog, so prevention of illness in dogs is key to the health of other animals in the household. There is some evidence that guinea pigs and ferrets can become infected.


Canine influenza is an airborne disease, much like kennel cough (Bordetella bronchiseptica). The virus can travel in droplets from a cough or sneeze and can be transmitted by contact with contaminated objects (for instance, a chew toy).

Practically speaking, if your dog stays at home and rarely contacts other dogs, its risk of contracting the virus is likely low. If your dog is boarded, goes to day care, or the dog park, it may be at a higher risk.

If your dog is coughing, it should not go to public places where it could contact other dogs until your veterinarian agrees it's safe for your dog to go out.


Symptoms include cough (the most common symptom), runny nose, and/or fever. Sick dogs may be lethargic. Dogs can become ill within 24 hours of exposure. Some dogs don’t show symptoms, but are able to infect other dogs for weeks. Most dogs diagnosed with canine influenza experience a mild form of the disease.

Dogs with canine influenza usually suffer from a persistent cough that may last for as long as three weeks and may experience a yellowish nasal discharge that can be treated effectively with antibiotics.

Dogs who experience a more serious case of canine influenza frequently have a high fever and exhibit increased respiratory rates and other indications of bacterial pneumonia. Antibiotics treat this form of the disease successfully in about 95% of the cases. The fatality rate is estimated to be between 5 and 8%, but with early diagnosis and appropriate treatment by a veterinarian, this rate can be much lower.

Call your veterinarian if your dog develops a cough, especially if it has already received the Bordetella vaccine. If your dog is coughing, do not take it out to locations where it may infect other dogs. Early intervention is key to limiting community outbreaks.

Be sure to tell your veterinarian if your dog has been boarded, sent to the groomer or involved in any social activities (dog park, day care, etc.) within the last month.

Remember, coughing can be indicative of a variety of significant diseases or conditions. Your veterinarian is best qualified to diagnose your dog.


After handling a sick dog, wash your hands before touching your dog. Avoid contact with dogs that appear sick.

Vaccines are available for both the H3N8 and H3N2 strains, the latter with conditional approval. Discuss your dog's specific risk factors with your veterinarian to determine whether vaccination against canine influenza is appropriate for your pet.

If your dog goes to day care or if you plan to board your dog, vaccination may be recommended. You should check in advance to see if the vaccination is required at these facilities. The Bordetella vaccine does not protect dogs against canine influenza.

Published: September 30, 2009    Updated: 2016-04-12 07:00:00

Sources: Information compiled from a variety of sources, including the AVMA, Oregon Public Health Veterinarian's office and the CDC.