Canine Influenza


What is Canine Influenza?

Canine Influenza, or dog flu, is a highly contagious viral infection that can affect dogs and cats. Canine influenza is a Type A influenza virus and further identified on based on the composition of two specific proteins in the lipid outer layer of the capsid. Currently, there are two strains identified in the United States, H3N8 and H3N2. 

Canine H3N8 influenza was first identified in Florida in 2004 in racing greyhounds. This strain is thought to develop from an equine H3N8 strain that jumped from horses to dogs. H3N8 has been identified in most US states and in DC. 

Canine H3N2 influenza was first identified in the United States in 2015 following an outbreak of respiratory illness in the Chicago area. Prior to this, this strain was restricted to South Korea, Thailand, and China. It is thought this strain arose through a direct transfer of an avian influenza virus. As of May 2017, canine H3N2 influenza was diagnosed in dogs in Florida, Georgia, North Carolina, South Carolina, Texas, Kentucky, Tennessee, Missouri, Louisiana, and Illinois. 

There is no evidence either of these strains can infect humans. 

How is Canine Influenza Transmitted? 

Canine influenza is transmitted through droplets or aerosols containing respiratory secretions from coughing, barking, and sneezing. Dogs in close contact with infected dogs in places such as kennels, groomers, day care facilities, and shelters are at an increased risk of infection. 

It can also be spread indirectly through objects or people that have been in contact with an infected dog. It is important to disinfect and clean objects that may have been in contact with an infected dog and to also wash your hands and clothing to avoid spreading the virus. The virus can remain viable on surfaces for up to 48 hours, clothing for 24 hours, and on hands for 12 hours. 

H3N8 has an incubation period of 1-5 days, with clinical signs arising in most cases 2-3 days after exposure. Dogs infected by H3N2 may start to show respiratory signs 2-8 days after infection. 

Dogs are most contagious during the incubation period and shed the virus even though they are not showing clinical signs of illness. 


Affected dogs may have a soft, moist cough or a dry cough similar to kennel cough. Nasal and/or ocular discharge, sneezing, lethargy, and anorexia may also be observed. Many dogs develop purulent nasal discharge and fever (104-105F). 

Some dogs are more severely affected and exhibit clinical signs of pneumonia, such as high-grade fever (104-106F) and increased respiratory rate and effort. 

Cats infected with H3N2 display signs of upper respiratory disease, including nasal discharge, congestion, malaise, lip smacking, and excessive salivation. 


Canine influenza cannot be diagnosed solely by clinical signs because these also present with other canine respiratory illnesses. Tests are available for diagnosis and identify strains of canine influenza virus. These tests include: virus isolation, immunoassays to detect virus antigen, PCR, and serology or antibodies. 


Veterinary expertise is required to determine treatment options and the best course of treatment. Treatment is largely supportive. Most dogs recover within 2-3 weeks. Other diagnostics and treatments may be needed depending on the presence of secondary infections. These treatments may include: antimicrobials, NSAIDs to reduce fever and inflammation, or fluids. 

To prevent transmission of the virus, infected dogs and other dogs in the household should be isolated for 4 weeks. 


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