Canine influenza H3N8 viruses originated in horses, spread to dogs, and can now spread between dogs. H3N8 equine influenza (horse flu) viruses have been known to exist in horses for more than 40 years. In 2004, cases of an unknown respiratory illness in dogs (initially greyhounds) were reported in the United States. It has now been detected in dogs across most of the United States.
Canine influenza H3N2 viruses originated in birds, spread to dogs, and can now spread between dogs. Transmission of H3N2 canine influenza viruses to cats from infected dogs has been reported also. Canine influenza A H3N2 viruses were first detected in dogs in South Korea in 2007, and have been reported in dogs in China, Thailand, and Canada. H3N2 canine influenza viruses were first detected in the United States in April 2015, and has now been found in more than 30 states.