Canine influenza (CI), or dog flu, is a highly contagious infection caused by an influenza A virus. The causative canine influenza virus (CIV) strains have been classified as H3N8 and H3N2, based on the amino acid composition of the hemaglutinin (H) and neuraminidase (N) glycoproteins in the lipid outer layer of the capsid. These glycoproteins facilitate entry into and release from host cells, and are important targets for antibodies against the virus (generated as a result of infection or response to vaccination). The viruses are 80-120 nanometers (nm) in diameter, and consist of a core of eight separate pieces of single-strand ribonucleic acid (RNA) surrounded by a spiked arrangement of glycoproteins.
H3N2 canine influenza appeared limited to Korea, China and Thailand until March 2015, when anoutbreak that started in the Chicago area was determined to be due to an H3N2 strain.
The H3N8 canine influenza virus represents a very rare event in adaptive evolution; the entire genome of the H3N8 equine influenza virus was transferred to dogs, and the virus adapted to the canine species to emerge as a new canine-specific virus.
The canine H3N2 strain, on the other hand, emerged in Asia in 2006-2007 among dogs suffering from respiratory disease. This strain in Asia likely arose through the direct transfer of an avian influenza virus – possibly from among viruses circulating in live bird markets – to dogs. The new canine virus spread widely among dogs in South Korea and in several regions of China, and caused an outbreak of respiratory disease among dogs in Thailand in 2012. In 2015, a canine H3N2 that was genetically almost identical (99% identical) to the Asian strain was detected in the United States. Although rumors have circulated that the virus was introduced to the U.S. through dogs rescued and imported from Asia, there is no evidence to confirm these rumors.
Canine influenza is a reportable disease in some U.S. states.
In March 2016, the University of Wisconsin School of Veterinary Medicine and the Wisconsin Veterinary Diagnostic Laboratory announced that the H3N2 strain had infected a group of cats in the Midwest. Additionally, they reported that their findings suggested that the virus was replicating in cats and could spread from cat to cat.