5 Causes of Colitis in Horses

Colitis is inflammation of the large or small colon that can lead to diarrhea, proliferation of harmful bacteria, and even death in severe cases. This dangerous condition can stem from a variety of causes—many of which veterinarians can’t pinpoint until long after they’ve received and begun treating the equine patient, due to the length of time it takes to get test results. However, here are five of the most common culprits:

  • Infectious agents. This includes pathogens such as Salmonella, Clostridium difficile, Clostridium perfringens, coronavirus, and Neorickettsia risticii (the causative agent of Potomac horse fever);
  • Parasitism, particularly small strongyles, also called cyathostomins. As part of the normal life cycle of small strongyles, the third stage larvae (L3) migrate into the intestinal walls of the cecum and colon. Once they mature (which takes months to years), the fourth stage larvae (L4) migrate back into the intestinal lumen as luminal L4s before they reach the final and sexually mature stage (L5). Horses can harbor hundreds of thousands of encysted larvae. If the L4s emerge en masse, inflammation of the cecum and colon occurs, which can cause diarrhea.
  • Antibiotic-associated diarrhea. Any medication, particularly antibiotics, has the potential to disturb the intestinal microbiome (the ecosystem of microbes living in the horse’s gut) and cause diarrhea. The antimicrobials ceftiofur sodium and trimethoprim sulfadiazine are among the most common culprits, but they are also the most commonly prescribed antibiotics in equine practice, so the association likely reflects the frequency of use. In one study researchers found that administering healthy horses these antibiotics for 25 days altered their fecal microbiota (Costa et al. MCV Vet Res 2015).
  • Non-steroidal anti-inflammatory drug (NSAID)-associated diarrhea. Some horses develop diarrhea caused by ulceration (destruction) of the inner lining of the large colon following administration of NSAIDs such as phenylbutazone (Bute) and flunixin meglumine (Banamine). The right dorsal colon appears to be particularly prone to ulceration, though the reason remains unclear. Veterinarians also don’t know why certain horses have this response to NSAIDs while others do not. Dehydration, however, might increase the risk.
  • Indiscriminate (random) causes. These include sand impactions, dietary imbalances, food allergies, neoplasia (cancer), inflammatory bowel disease, toxin ingestion, among others.

Some colitis cases are idiopathic, meaning the veterinarian never determines the inciting cause. Regardless the cause, if your horse has persistent or significant diarrhea, contact your vet right away so he or she can initiate potentially life-saving treatment.