By Jim Garrett
Like humans, dogs are susceptible to influenza viruses. And, like the viruses that infect humans, those that infect dogs may comprise different strains. The symptoms may not differ, but one strain may resist immunization by vaccines effective for another.
Until 2015, canine flu in the United States was caused by the H3N8 virus, for which an effective vaccine exists. However, in that year, a new virus appeared, designated as H3N2.