Canine influenza (CI), or dog flu, is a highly contagious respiratory infection of dogs that is caused by an influenza A virus. In the U.S., canine influenza has been caused by two influenza strains.
The first strain reported in the United States, beginning in 2004, was an H3N8 influenza A virus. This strain is closely related to the virus that causes equine influenza, and it is thought that the equine influenza virus mutated to produce the canine strain. In 2015, an outbreak that started in Chicago was caused by a separate canine influenza virus, H3N2. The strain causing the 2015 outbreak was almost genetically identical to an H3N2 strain previously reported only in Asia – specifically, Korea, China and Thailand. In Asia, this H3N2 strain is believed to have resulted from the direct transfer of an avian influenza virus, possibly from among viruses circulating in live bird markets, to dogs.
Since March 2015, more than 2,000 dogs have been confirmed positive for H3N2 canine influenza across the U.S. Two clinical syndromes have been seen in dogs infected with the canine influenza virus, mild form of the disease and a more severe form that is accompanied by pneumonia.
Dogs suffering with the mild form of canine influenza develop a soft, moist cough that persists for 10 to 30 days. They may also be lethargic and have reduced appetite and a fever. Sneezing and discharge from the eyes and/or nose may also be observed. Some dogs have a dry cough similar to the traditional “kennel cough” caused by Bordetella bronchiseptica/parainfluenza virus complex. Dogs with the mild form of influenza may also have a thick nasal discharge, which is usually caused by a secondary bacterial infection. Dogs with the severe form of canine influenza develop high fevers (104ºF to 106ºF) and have clinical signs of pneumonia, such as increased respiratory rates and effort. Pneumonia may be due to a secondary bacterial infection.
Because this is still an emerging disease and dogs in the U.S. have not been exposed to it before, almost all dogs, regardless of breed or age, lack immunity to it and are susceptible to infection if exposed to the active virus. Virtually all dogs exposed to the virus become infected, and nearly 80% show clinical signs of disease, though most exhibit the mild form described above. However, the risk of any dog being exposed to the canine influenza virus depends on that dog’s lifestyle.
Dogs that are frequently or regularly exposed to other dogs – for example at boarding or day care facilities, dog parks, grooming salons, or social events with other dogs present – are at greater risk of coming into contact with the virus. Also, as with other infectious diseases, extra precautions may be needed with puppies, elderly or pregnant dogs, and dogs that are immunocompromised. Dog owners should talk with their own veterinarian to assess their dog’s risk. Fatal cases of pneumonia resulting from infection with canine influenza virus have been reported in dogs, but the fatality rate is low (less than 10%). Most dogs recover in 2-3 weeks.
The first recognized outbreak of canine influenza in the world is believed to have occurred in racing greyhounds in January 2004 at a track in Florida. From June to August of 2004, outbreaks of respiratory disease were reported at 14 tracks in 6 states (Alabama, Arkansas, Florida, Kansas, Texas, and West Virginia). Between January and May of 2005, outbreaks occurred at 20 tracks in 11 states (Arizona, Arkansas, Colorado, Florida, Iowa, Kansas, Massachusetts, Rhode Island, Texas, West Virginia, and Wisconsin). The canine influenza virus has been reported in at least 40 states and Washington, DC.
There is no evidence of transmission of canine influenza virus from dogs to people. There is no evidence of transmission of H3N8 canine influenza from dogs to horses, cats, ferrets, or other animal species. The H3N2 strain, however, has been reported in Asia to infect cats, and there’s also some evidence that guinea pigs and ferrets can become infected. As with any disease caused by a virus, treatment is largely supportive.
Good animal care practices and nutrition assist dogs in mounting an effective immune response. The course of treatment depends on the pet’s condition, including the presence or absence of a secondary bacterial infection, pneumonia, dehydration, or other medical issues (e.g., pregnancy, pre-existing respiratory disease, compromised immune system, etc.). Antibiotics may be prescribed to fight secondary infections as well as a non-steroidal anti-inflammatory (to reduce fever, swelling and pain). Dehydrated pets may need fluid therapy to restore and maintain hydration. Hospitalization may also be necessary for more severe cases.
The first canine vaccine for H3N8 canine influenza was approved in 2009, and there are several H3N8 canine influenza vaccines available. It is not known whether the H3N8 vaccine will offer any protection against the H3N2 strain. Canine influenza vaccines are considered “lifestyle” vaccines, meaning the decision to vaccinate is based on a dog’s risk of exposure. Dog owners should consult their veterinarian to determine whether vaccination is needed.