Diarrhea is a common symptom in the horse that can be caused by a number of different factors. Inflammation of the large bowel, called colitis, is a common but serious disease of horses that results in diarrhea, fever, depression, and/or colic. There are many causes of acute colitis in horses which includes:

• Clostridiosis
• Potomac Horse Fever
• Phenylbutazone toxicity
• Blister Beetle Poisoning
• Plant Poisonings (less common)
• Chemical Poisonings (less common)
• Frequently the inflammation occurs with no known cause.

Clostridium spp. are well suited to persist in the environment because of the production of spores that are resistant to environmental extremes and many antiseptics. They persist a long time in the environment. Horses contract clostridiosis through the oral route. It is also hypothesized that the organism may be in the gi tract in low numbers and some stressor allows it to proliferate. Similar to salmonellosis, clostridial diarrhea (enterocolitis) most often develops in susceptible horses with risk factors that include

• antimicrobial administration
• concurrent gastrointestinal disease
• foals and the immunincompetant

Clostridiosis secondary to antibiotics is mostly associated with particularly antibiotics, including trimethoprim-sulfa (Tribrissen or tmp-smz tablets), penicillin, and erythromycin. It has also occurred in foals whose mares were administered erythromycin. It is assumed the concentration of antibiotic in the milk caused the disease. It can develop into a widespread problem affecting several animals in a hospital or an equine facility.

Clostridium difficile, and to a lesser extent C. perfringens, should be considered one of the likely causes of colitis in horses that have a history of antimicrobial administration. Some have commented that clostridial colitis diarrhea has a foul "spoiled fish" odor.