Canine flu (dog flu): a respiratory viral illness in dogs caused by strains of the type A influenza viruses that can infect dogs. These viruses, known as canine influenza viruses, have never been shown to infect humans. Of the two identified canine influenza viruses, one is an H3N8 virus and the other is an H3N2 virus. These two viruses are now endemic (commonly found) in dogs in the US. The H3N8 and H3N2 dog flu viruses originated in horses and birds, respectively, and eventually spread to dogs. They can now be spread freely among dogs. Even though there has been no proof that canine influenza viruses can spread to humans, influenza viruses are able to change rapidly, and it is possible that at some point in the future, a dog flu virus could change and become able to infect people.
CDC. Key Facts about Canine Influenza (Dog Flu). Updated: May 29, 2018. <https://www.cdc.gov/flu/canineflu/keyfacts.htm>