A highly contagious disease of the upper respiratory tract of horses and other members of the family Equidae, characterized by inflammation of the pharynx and abscess formation in lymph nodes. This disease occurs in horses of all ages throughout the world. The causative agent is Streptococcus equi, a clonal pathogen apparently derived from an ancestral strain of S. zooepidemicus. It is an obligate parasite of horses, donkeys and mules.