Colic in horses is defined as abdominal pain, but it is a clinical symptom rather than a diagnosis. The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. What makes it tricky is that different causes can manifest with similar signs of distress in the animal. Recognizing and understanding these signs is pivotal, as timely action can spell the difference between a brief moment of discomfort and a life-threatening situation. The most common forms of colic are gastrointestinal in nature and are most often related to colonic disturbance. There are a variety of different causes of colic, some of which can prove fatal without surgical intervention. Colic surgery is usually an expensive procedure as it is major abdominal surgery, often with intensive aftercare. Among domesticated horses, colic is the leading cause of premature death. The incidence of colic in the general horse population has been estimated between 4 and 10 percent over the course of the average lifespan. Clinical signs of colic generally require veterinary evaluation and treatment. The conditions that cause colic can become life-threatening in a short period of time.
Pathophysiology Colic can be divided broadly into several categories: excessive gas accumulation in the intestine (gas colic) simple obstruction strangulating obstruction non-strangulating infarction inflammation of the gastrointestinal tract (enteritis, colitis) or the peritoneum (peritonitis) ulceration of the gastrointestinal mucosa
These categories can be further differentiated based on location of the lesion and underlying cause (See Types of colic).
Simple obstruction
This is characterised by a physical obstruction of the intestine, which can be due to impacted food material, stricture formation, or foreign bodies. The primary pathophysiological abnormality caused by this obstruction is related to the trapping of fluid within the intestine oral to the obstruction. This is due to the large amount of fluid produced in the upper gastrointestinal tract, and the fact that this is primarily re-absorbed in parts of the intestine downstream from the obstruction. The first problem with this degree of fluid loss from circulation is one of decreased plasma volume, leading to reduced cardiac output, and acid-base disturbances.
The intestine becomes distended due to the trapped fluid and gas production from bacteria. It is this distension, and subsequent activation of stretch receptors within the intestinal wall, that leads to the associated pain. With progressive distension of the intestinal wall, there is occlusion of blood vessels, firstly the less rigid veins, then arteries. This impairment of blood supply leads to hyperemia and congestion, and ultimately to ischaemic necrosis and cellular death. The poor blood supply also has effects on the vascular endothelium, leading to an increased permeability which first leaks plasma and eventually blood into the intestinal lumen. In the opposite fashion, gram-negative bacteria and endotoxins can enter the bloodstream, leading to further systemic effects.
Strangulating obstruction
Strangulating obstructions have all the same pathological features as a simple obstruction, but the blood supply is immediately affected. Both arteries and veins may be affected immediately, or progressively as in simple obstruction. By far the most common strangulating obstruction in horses is from a pedunculated lipoma. Other causes of strangulating obstruction are intussusceptions, torsion or volvulus, and displacement of intestine through a hole, such as a hernia, a mesenteric rent, or the epiploic foramen.
Non-strangulating infarction
In a non-strangulating infarction, blood supply to a section of intestine is occluded, without any obstruction to ingesta present within the intestinal lumen. The most common cause is infection with Strongylus vulgaris larvae, which primarily develop within the cranial mesenteric artery.
Inflammation or ulceration of the gastrointestinal tract
Inflammation along any portion of the GI tract can lead to colic. This leads to pain and possibly stasis of peristalsis (Ileus), which can cause excessive accumulation of fluid in the gastrointestinal tract. This is a functional rather than mechanical blockage of the intestine, but like the mechanical blockage seen with simple obstructions, it can have serious effects including severe dehydration. Inflammation of the bowel may lead to increased permeability and subsequent endotoxemia. The underlying cause of inflammation may be due to infection, toxin, or trauma, and may require special treatment in order to resolve the colic.
Ulceration of the mucosal surface occurs very commonly in the stomach (gastric ulceration), due to damage from stomach acid or alteration in protective mechanisms of the stomach, and is usually not life-threatening. The right dorsal colon may also develop ulceration, usually secondary to excessive NSAID use, which alters the homeostatic balance of prostaglandins that protect the mucosa.
Types This list of types of colic is not exhaustive but details some of the types which may be encountered.
Gas and spasmodic colic Gas colic, also known as tympanic colic, is the result of gas buildup within the horse's digestive tract due to excessive fermentation within the intestines or a decreased ability to move gas through it. It is usually the result of a change in diet, but can also occur due to low dietary roughage levels, parasites (22% of spasmodic colics are associated with tapeworms), and anthelminthic administration. This gas buildup causes distention and increases pressure in the intestines, causing pain. Additionally, it usually causes an increase in peristaltic waves, which can lead to painful spasms of the intestine, producing subsequent spasmodic colic. The clinical signs of these forms of colic are generally mild, transient, and respond well to spasmolytic medications, such as buscopan, and analgesics. Gas colics usually self-correct, but there is the risk of subsequent torsion (volvulus) or displacement of the bowel due to gas distention, which causes this affected piece of bowel to rise upward in the abdomen.
Abdominal distention may occasionally be seen in adult horses in the flank region, if the cecum or large colon is affected. Foals, however, may show signs of gas within the small intestines with severe abdominal distention.
Impaction Pelvic flexure impaction This is caused by an impaction of food material (water, grass, hay, grain) at a part of the large bowel known as the pelvic flexure of the left colon where the intestine takes a 180 degree turn and narrows. Impaction generally responds well to medical treatment, usually requiring a few days of fluids and laxatives such as mineral oil, but more severe cases may not recover without surgery. If left untreated, severe impaction colic can be fatal. The most common cause is when the horse is on box rest and/or consumes large volumes of concentrated feed, or the horse has dental disease and is unable to masticate properly. This condition could be diagnosed on rectal examination by a veterinarian. Impactions are often associated with the winter months because horses do not drink as much water and eat drier material (hay instead of grass), producing drier intestinal contents that are more likely to get stuck.
Ileal impaction and ileal hypertrophy The ileum is the last part of the small intestine that ends in the cecum. Ileal impaction can be caused by obstruction of ingesta. Coastal Bermuda hay is associated with impactions in this most distal segment of the small intestine,Current Therapy in Equine Medicine, 5th ed. Reid Hanson. Chapter 3.13 Ileal Impaction although it is difficult to separate this risk factor from geographic location, since the southeastern United States has a higher prevalence of ileal impaction and also has regional access to coastal Bermuda hay. Other causes can be obstruction by ascarids (Parascaris equorum), usually occurring at 3–5 months of age right after deworming, and tapeworms (Anoplocephala perfoliata), which have been associated with up to 81% of ileal impactions (See Ascarids). Horses show intermittent colic, with moderate to severe signs and with time, distended small intestinal loops on rectal. Although most ileal impactions will sometimes pass without intervention, those present for 8–12 hours will cause fluid to back up, leading to gastric reflux, which is seen in approximately 50% of horses that require surgical intervention. Diagnosis is usually made based on clinical signs, presence of reflux, rectal exam, and ultrasound. Often the impaction can not be felt on rectal due to distended small intestinal loops that block the examiner. Those impactions that are unresponsive to medical management, which includes IV fluids and removal of reflux, may be treated using a single injection into the ileum with 1 liter of carboxymethylcellulose, and then massaging the ileum. This allows the impaction to be treated without actually cutting into the ileum. Prognosis for survival is good.
Ileal hypertrophy occurs when the circular and longitudinal layers of the ileal intestinal wall hypertrophy, and can also occur with jejunal hypertrophy. The mucosa remains normal, so malabsorption is not expected to occur in this disease. Ileal hypertrophy may be idiopathic, with current theories for such cases including neural dysfunction within the intestinal wall secondary to parasite migration, and increased tone of the ileocecal valve which leads to hypertrophy of the ileum as it tries to push contents into the cecum. Hypertrophy may also occur secondary to obstruction, especially those that have had surgery for an obstruction that required an anastomosis. Hypertrophy gradually decreases the size of the lumen, resulting in intermittent colic, and in approximately 45% of cases includes weight loss of 1–6 month duration and anorexia. Although rectal examination may display a thickened ileal wall, usually the diagnosis is made at surgery, and an ileocecal or jejunocecal anastomosis is made to allow intestinal contents to bypass the affected area. If surgery and bypass is not performed, there is a risk of rupture, but prognosis is fair with surgical treatment.
Sand impaction This is most likely to occur in horses that graze sandy or heavily grazed pastures leaving only dirt to ingest. Foals, weanlings, and yearlings are most likely to ingest sand, and are therefore most commonly seen with sand colic. The term sand also encompasses dirt. The ingested sand or dirt most commonly accumulates in the pelvic flexure, but may also occur in the right dorsal colon and the cecum of the large intestines. The sand can cause colic signs similar to other impactions of the large colon, and often causes abdominal distention. As the sand or dirt irritates the lining of the bowel it can cause diarrhea. The weight and abrasion of the sand or dirt causes the bowel wall to become inflamed and can cause a reduction in colonic motility and, in severe cases, leads to peritonitis.