Canine influenza virus (CIV), or influenza A subtype H3N8, was transmitted from horses to dogs and adapted to enable transmission between dogs, causing influenza-like signs. The incubation period for CIV is 2­–4 days; antibodies to the H3 viral protein are detectable ~7 days postinfection. CIV has been identified in 39 states.

From 2005 through June 2009, serum samples were collected from pet and shelter dogs with influenza-like signs to estimate the seroprevalence of H3N8 CIV and determine intrinsic and exposure factors associated with seropositivity. Serum samples were analyzed from 1,268 dogs in 42 states; overall seroprevalence was 49%. Of seropositive dogs, 59% were from Colorado (overall highest seroprevalence), New York (second highest), and Florida (third highest). Year, geographic region, and exposure setting were associated with seropositivity. The study was limited by an unequal representation of states, variability of serum samples submitted each year, and unequal representation of pet and shelter dogs. The number of seronegative dogs with influenza-like signs was not compared, and study dogs were not randomly selected. Thus, results cannot be extrapolated for all dogs in the United States; however, data do suggest continued need for H3N8 surveillance and development of protocols to reduce risk for CIV transmission in communal canine housing.

In 2005, our hospital saw 23 pet dogs seropositive for H3N8 CIV with severe, secondary suppurative pneumonia. However, the overall incidence of pneumonia or death is extremely low. This study reminds us of our duty to provide proper preventive medicine, hygiene, ventilation, and isolation in group boarding, grooming, or shelter facilities, and that continued surveillance of H3N8 CIV seroprevalence is important, as is watching for emergent pathogens in a rapidly evolving part of nature.—Heather Troyer, DVM, DABVP, CVA

Prevalence of and exposure factors for seropositivity to H3N8 canine influenza virus in dogs with influenza-like illness in the United States. Anderson TC, Crawford PC, Dubovi EJ, et al. JAVMA 242:209-216, 2013.