Two strains of the canine influenza virus (CIV) have been identified, H3N8 and H3N2. The H3N8 strain was first identified in the USA in 2004, and the H3N2 strain was identified only in Asia until 2015, when an outbreak occurred in the USA. Outbreaks are most common when dogs are in close contact, eg, kennels, shelters, dog parks.
CIV is spread via respiratory secretions, contaminated objects (eg, water bowls), and people moving between infected and uninfected dogs. The incubation period is usually 2–4 days from exposure to onset of clinical signs, when dogs are most contagious; ~20% of infected dogs remain asymptomatic but can still shed virus.
Most exposed dogs (80%) develop mild infection, with a cough that persists 1–3 wk and may be similar to the cough of canine infectious tracheobronchitis (see Tracheobronchitis in Small Animals). Other possible clinical signs include ocular and nasal discharge, sneezing, fever, lethargy, and anorexia. Some dogs become severely ill, with high fever (104º–106ºF), pneumonia, and secondary bacterial infection. The mortality rate is <10%.
There is no rapid test for specific diagnosis. Nasal or pharyngeal swabs from dogs ill for <3 days can be submitted for PCR testing. After 4 days of illness, PCR testing may result in false-negatives, because the time of maximal virus shedding has passed. Serum antibodies to CIV may be detected as early as 7 days after onset of clinical signs. The best method for confirmation of infection is serologic testing with acute and convalescent serum samples.
Treatment is largely supportive; most dogs recover in 2–3 wk. Additional treatment (eg, antimicrobials, NSAIDs) is warranted to combat secondary bacterial infection, pneumonia, and other complications.
H3N8 canine influenza vaccines are available, but whether they protect against the H3N2 strain is unknown. Two vaccines for H3N2 canine influenza are available under conditional license from the USDA.
Routine infection control practices and good hygiene within facilities are key to preventing spread. CIV can persist in the environment for 1–2 days but is readily killed by common disinfectants. There is no evidence of transmission of CIV from dogs to people.