Canine influenza virus subtype H3N8 (H3N8 CIV) is a highly contagious respiratory pathogen. The virus causes acute onset of coughing, sneezing, and nasal discharge that persists for 2 weeks or longer in most dogs and progresses to pneumonia in some.
H3N8 CIV infection should be included in the differential diagnosis for dogs with acute respiratory infection, particularly those with a history of boarding, day care attendance, or adoption from a shelter or rescue group within a week of the onset of clinical illness.1 CIV cannot be ruled out in H3N8 CIV–vaccinated dogs with acute respiratory disease; although currently available vaccines can reduce virus shedding and decrease the severity and duration of clinical disease, they do not protect against infection.2
Because CIV infection resembles that caused by other viral and bacterial pathogens in the canine infectious respiratory disease complex, it cannot be diagnosed by clinical signs alone.1 CIV infection has two diagnostic windows based on the virus-shedding period and appearance of antibodies (Figure 1). Although infected dogs shed virus for about 7 days, peak virus shedding occurs during the first 2 to 4 days of infection before onset of clinical signs.3 Virus shedding declines rapidly during the first 5 days of clinical disease.3 Serum antibodies to H3N8 CIV are detectable after 7 days and increase during the first month after infection.3,4
Diagnostic test selection for H3N8 CIV infection is based on the diagnostic window that applies at the time of presentation. Virus detection tests can be used for dogs that have been ill for 5 or fewer days and are still in the virus-shedding window. Antibody detection tests are needed for dogs that have been ill for longer than 5 days and are outside the virus-shedding window.