Influenza is a viral disease that has long been known to affect birds and some mammals, but was only recently recognized in dogs. Each influenza virus is maintained in one or more related host species; however, host specificity is not absolute. A virus may occasionally infect other animals, or on rare occasions, become adapted to a new species. No influenza viruses were known to circulate in dogs until 2004-2006, when a virus caused outbreaks of severe and often fatal respiratory disease among racing greyhounds in the U.S. This virus was acquired from horses, and probably entered greyhound populations several years before these outbreaks. Although it has spread to other dogs since this time, the illness in these animals has been more typical of influenza. The most common syndrome is a relatively mild upper respiratory disease with a persistent cough. Pneumonia is possible, generally as the result of secondary infection with bacteria or mycoplasma, but uncommon. At present, infections tend to be seen mainly in animal shelters, kennels, dog day care facilities, or other sites where groups of susceptible dogs are in close contact. This virus does not seem to have spread widely in other pets, and it has not yet been reported outside North America.
A second canine influenza virus was recognized in 2007, when a different virus caused an outbreak of severe respiratory disease in South Korea. This virus seems to have been acquired from birds, and may have entered canine populations around 2005. It was subsequently reported in China and Thailand, and can affect cats as well as dogs. Many reported clinical cases from Asia have been severe, but antibodies have been found in significant numbers of healthy dogs and cats there, suggesting that some animals have milder illnesses. This virus entered North America in 2015. While large numbers of cases have been reported among dogs in the U.S., there have been few deaths, as of November 2015, and most cases appear to be mild.
Other influenza viruses can also affect dogs, without persisting in canine populations. Equine influenza viruses have caused a few small outbreaks, and there are occasionally reports of infections or clinical cases caused by viruses adapted to birds or humans.
Iowa State University
Iowa State University Center for Food Security and Public Health, "Canine Influenza" (2016). Center for Food Security and Public Health Technical Factsheets. 30.