Colitis is an acute, highly fatal disease associated with an inflammation of the horse's colon. Colitis is a serious disease for horses, due to the large size of their colon. Two of the major infectious agents involved with the development of colitis are Clostridium difficile and Salmonella. C. perfringens is also thought to play a role since it is frequently found in the large intestine of horses with colitis, however its specific involvement is unknown.

There are a number of different causes associated with the development of colitis, which include both infectious and noninfectious involvement. Noninfectious causes include parasites, the use of NSAIDs and antimicrobials, consumption of blister beetles in alfalfa hay, or consumption of various types of poisonous plants. Some plants frequently recognized as cause of colitis include acorn and young oak leaves, oleander, buttercups, nightshades, rhododendron and azaleas, pokeweed, and castor bean.