Canine influenza (CIV), or dog flu, is a highly contagious viral infection affecting dogs and also cats. Influenza viruses belong to the family Othomyxoviridae. Canine Influenza is a Type A influenza virus and is further identified based on the composition of two specific proteins in the lipid outer layer of the capsid: hemagglutinin (HA) and neuraminidase (NA). At present, two strains of canine influenza virus have been identified in the United States: H3N8 and H3N2.
Influenza viruses are able to quickly change and give rise to new strains that can infect different species. Both strains of canine influenza identified in the U.S. can be traced to influenza strains known to infect species other than dogs. At some point, these viruses acquired the ability to infect dogs and be transmitted from dog to dog.