Colitis X in Horses

Historically, Colitis X was a term reserved for cases of severe acute colitis in horses in which the causative agent was unknown. Colitis X was characterized by sudden onset of severe, profuse diarrhea and the development of hypovolemic shock, often resulting in death.

Colitis X was, in most cases, likely caused either by bacterial agents such as Clostridia and Salmonella, or secondary to the overuse of non-steroidal anti-inflammatory medications (NSAIDs) like phenylbutazone. Modern research and diagnostic testing has led to improvements in diagnostic capabilities and in many cases the cause of colitis can now be identified. As a result, the term “Colitis X” has generally fallen out of use in recent years.

Colitis refers to disease of the large intestine, often resulting in colic and diarrhea, which is caused by a variety of infectious and noninfectious agents. Infectious agents such as Salmonella spp. Clostridium spp., Neorickettsia risticii and Coronavirus can cause acute, severe colitis, as can encysted cyanthostomes. Right Dorsal Colitis secondary to NSAID administration can also present as acute and severe. The onset of the disease is also often associated with dietary or appetite changes and elevated stress levels, usually as a result of travel, illness or surgery. The causative agent may at times be difficult to identify due to limitations in time and/or resources, however true idiopathic colitis is becoming less common.

“Colitis X” is therefore best used as an encompassing term for presentations in which the colon is pathologically affected but that have no currently known etiology or etiopathogenisis – which may still be best described as unexplained colitis.

Clinical Signs of Acute Colitis in Horses

In cases of peracute colitis, death may occur within three hours of the onset of clinical signs, sometimes before diarrhea has developed, and enterocolitis is only observed at necropsy. More commonly, the onset is acute, with rapid development of abdominal pain, explosive diarrhea, and dehydration, which can be severe if left untreated. The abdomen may appear bloated (with fluid and/or gas) and the gastrointestinal sounds can vary from hyper to hypomotile.

Other clinical signs of severe colitis are those attributed to the onset of hypovolemic shock and/or endotoxemia, including:

  • Fever, that may wax and wane or become subnormal
  • Tachypnea
  • Tachycardia
  • Marked depression
  • Prolonged skin tent
  • Cold extremities (especially ears and muzzle)
  • Dry mucous membranes with a prolonged capillary refill time
  • Congested or injected mucous membranes
  • Weakness, trembling, collapse

Blood work often reveals an elevated Packed Cell Volume (PCV), due to dehydration. The blood protein levels at presentation may be low or appear artificially normal, but once the horse is rehydrated the protein levels will usually decrease and better reflect the degree of hypoproteinemia being caused by intestinal loss. Leukopenia with a marked neutropenia and left shift is usually suggestive of infectious disease such as Salmonella or Clostridia, or the presence of a secondary septic process such as septicemia or bowel perforation. Blood biochemistry usually reveals hypoalbuminemia, with a secondary hypocalcaemia (approximately 50% of blood calcium is protein-bound, particularly to albumin). Electrolyte derangements such as hyponatremia and hypochloremia are common, as is an elevated lactate and metabolic acidosis.

Abdominal ultrasound is non-specific and may reveal edematous large colon, with excessive fluidy or gaseous content. Abdominal radiographs can be quite useful in foals to help rule out other causes of abdominal pain.

Post mortem findings often show edema and hemorrhagic colitis-typhlitis with thrombosis of the mucosal capillaries.

Treatment for Acute Colitis in Horses

Treatment options for acute colitis lacking a definitive cause may include:

  • IV fluid therapy to counteract severe dehydration.
  • IV electrolyte replacement.
  • Plasma or synthetic colloids to maintain plasma oncotic pressure in the presence of hypoproteinemia. Plasma products can also assist in treating endotoxemia.
  • Management of inflammation and endotoxemia using non-steroidal or steroidal anti-inflammatories, and polymixin B. Care must be taken using these medications in hypovolemic/dehydrated horses.
  • Antibiotic use should be considered carefully.

Possible complications include laminitis, coagulopathies and thrombosis, renal failure and pulmonary or hepatic abscesses. Close monitoring is required. The digital pulses should be checked regularly, and signs of lameness monitored closely. Prophylactic digital cryotherapy is strongly recommended. Mortality rates can be highly variable and prognosis with unexplained colitis is usually guarded.

References & Further Reading

Larsen, J. (1997) Acute colitis in adult horses. A review with emphasis on aetiology and pathogenesis. Vet Q 19 (2): 72–80

Smith, B. P. (2015) Large Animal Internal Medicine. 5th ed. Elsevier Inc. St. Louis, MO