EN
Equine influenza is highly contagious and spreads rapidly among susceptible horses. The disease occurs globally and is caused by two main strains: H7N7 and H3N8. The H7N7 strain has not been isolated since the 1980s, and H3N8 circulates in equine population throughout most of the world. The H3N8 virus has diverged into two antigenically and genetically different evolutionary lineages since the 1986s: the American and European ones. Equine influenza exists in an endemic form in many countries. Transmission of the influenza virus from one host species to another is a crucial feature of its ecology and epidemiology. Two basic mechanisms of interspecies transmission are possible. One is the direct transfer of an essentially unaltered virus from one species to another. The second mechanism is a consequence of the segmented nature of the influenza genome and genetic reassortment.