Colitis-X is not actually a disease but a historic term used to describe undiagnosed causes of peracute, fatal enterocolitis in horses characterized by sudden onset of profuse, watery diarrhea and development of hypovolemic shock. Many affected horses have a history of stress. Differential diagnoses include peracute salmonellosis, clostridial enterocolitis, Aeromonas spp colitis, and endotoxemia. Salmonella spp and Clostridia difficile can be difficult to culture from fluid fecal material, and a diagnosis of salmonellosis or clostridial enterocolitis can easily be missed. Culture of GI tissue samples and mesenteric lymph nodes is recommended in addition to intestinal contents from necropsy cases. Negative cultures and toxin tests for clostridia do not necessarily rule out these conditions; therefore, thorough disinfection of the premises, hospital facilities, and trailers are recommended in all cases.

Clinically, there may be a short febrile period, but body temperature soon returns to normal or subnormal. Tachypnea, tachycardia, and marked depression are present. An explosive diarrhea develops, followed by extreme dehydration. Sometimes death occurs before diarrhea becomes evident, with severe enterocolitis evident at necropsy. Hypovolemic and endotoxic shock are manifest by poor capillary refill time, purplish mucous membranes, and cold extremities. Death may occur within 3 hr of onset of clinical signs. In less acute cases, death occurs within 24–48 hr. The mortality rate approaches 100%. At necropsy, edema and hemorrhage in the wall of the large colon and cecum are pronounced, and the intestinal contents are fluid and often blood-stained.

Typically, the PCV is >65% even shortly after the onset of clinical signs. The leukogram ranges from normal to neutropenia with a degenerative left shift. Metabolic acidosis and electrolyte disorders are also present.

Disease onset is often closely associated with stress, eg, surgery or transport. Signs are similar to those of other diarrheal diseases, including peracute salmonellosis, toxemia caused by Clostridium spp, Potomac horse fever, experimental endotoxic shock, and anaphylaxis. A similar condition may be seen after administration of lincomycin to horses. Colitis-X is the term reserved for those cases in which no definitive diagnosis can be made and the horse dies.

Treatment for colitis-X usually is not effective (by definition) but would be similar to that for salmonellosis (see Intestinal Diseases in Horses and Foals: Salmonellosis in Horses). Large volumes of IV fluids are needed to counter the severe dehydration, and electrolyte replacement is often necessary. Plasma or synthetic colloids are required to maintain plasma oncotic pressure if hypoproteinemia occurs secondary to protein-losing enteropathy. Flunixin meglumine may decrease inflammation, and polymyxin B can help bind endotoxin. Broad-spectrum antibiotics are indicated to treat bacteremia that often occurs secondary to bacterial translocation across the damaged GI tract.

Last full review/revision March 2012 by Allison J. Stewart, BVSC (Hons), MS, DACVIM-LA, DACVECC; John E. Madigan, DVM, MS