H3N8 is a subtype of the species Influenza A virus that is endemic in birds, horses and dogs. It is the main cause of equine influenza and is also known as equine influenza virus. In 2011, it was reported to have been found in seals. Cats have been experimentally infected with the virus, leading to clinical signs, shedding of the virus and infection of other cats. Introduction Equine influenza (EI) is a highly contagious respiratory disease of horses and related animals such as donkeys, mules and zebras (collectively known as equines). Equine influenza is caused by a type A influenza virus in the family Orthomyxoviridae (genus Influenzavirus). Transmission of the equine influenza virus (EIV) to humans has not occurred during outbreaks of the disease in horses. A lineage of H3N8 has been found to infect humans, with the first reported case of infection found in the Henan province of China in April 2022. History In 1963, the H3N8 (A/equine/2/Miami/63) subtype created an epidemic of equine influenza in Miami and subsequently spread throughout North and South America and Europe, creating massive outbreaks during 1964 and 1965. Since 1963, the H3N8 virus has drifted along a single lineage at a rate of 0.8 amino acid substitutions per year. Between 1978 and 1981, there were widespread epidemics of the A/equine/2 strain throughout the US and Europe despite the development of vaccines. Since the late 1980s, evolution of the H3N8 virus has diverged into two families: an "American-like" lineage and a "European-like" lineage. A 1997 study found H3N8 was responsible for over one quarter of the influenza infections in wild ducks. H3N8 has been suggested as a possible cause of the 1889–1890 pandemic in humans, and also another epidemic in 1898–1900. Before the identification of H3N8 as a possible cause of the 1889 pandemic, the H2N2 subtype was suggested. At this point, it is not possible to identify the virus for either the 1889 or 1900 outbreak with certainty. Route of transmission Equine influenza virus (H3N8) can be spread by a few different routes. The ultimate source of the virus is respiratory tract secretions. Coughing horses can release the virus into the air, where it can spread up to 30–50 metres. It can also be spread by direct contact between horses, or indirectly via a person's hands or clothing, or on inanimate objects (e.g. buckets, tack, twitches). However, the virus doesn't survive outside of