Horse Colic

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. 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 treatment by a veterinarian. The conditions that cause colic can become life-threatening in a short period of time.

Pathophysiology

Colic can be divided broadly into several categories:

1. excessive gas accumulation in the intestine (gas colic)
2. simple obstruction
3. strangulating obstruction
4. non-strangulating infarction
5. inflammation of the gastrointestinal tract (enteritis, colitis) or the peritoneum (peritonitis)
6. 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 a 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. Common 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, 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.

Diagnosis is usually made by history, environmental conditions, auscultation of the ventral abdomen, radiographs, ultrasound, or fecal examination (See Diagnosis). Historically, medical treatment of the problem is with laxatives such as liquid paraffin or oil and psyllium husk. More recently veterinarians treat cases with specific synbiotic (pro and prebiotic) and psyllium combinations. Psyllium is the most effective medical treatment. It works by binding to the sand to help remove it, although multiple treatments may be required. Mineral oil is mostly ineffective since it floats on the surface of the impaction, rather than penetrating it. Horses with sand or dirt impaction are predisposed to Salmonella infection and other GI bacteria, so antibiotics are often added to help prevent infection. Medical management usually resolves the colic, but if improvement doesn't occur within a few hours then surgery must be performed to flush the colon of any sand, which procedure that has a 60–65% survival rate. Horses that are not treated, or treated too late after the onset of clinical signs, are at risk of death.

Horses should not be fed directly on the ground in areas where sand, dirt and silt are prevalent, although small amounts of sand or dirt may still be ingested by grazing. Management to reduce sand intake and prophylactic treatments with sand removal products are recommended by most veterinarians. Such prophylaxis includes feeding a pelleted psyllium for one week every 4–5 weeks. Longer duration of treatment will result in gastrointestinal flora changes and the psyllium to be broken down and ineffective for sand clearance. Other methods include feeding the horse before turnout, and turning the horses out in the middle of the day so they are more likely to stand in the shade rather than graze.

Cecal impaction

Only 5% of large intestinal impactions at referral hospital involve the cecum. Primary cecal impactions usually consist of dry feed material, with the horse slowly developing clinical signs over several days. Secondary cecal impactions may occur post-surgery, orthopedic or otherwise, and the cecum does not function properly. Horses usually show clinical signs 3–5 days post general anesthesia, including decreased appetite, decreased manure production, and gas in the cecum which can be auscultated. The cecum quickly distends due to fluid and gas accumulation, often leading to rupture within 24–48 hours if not corrected. This impaction may be missed since decreased manure production can be attributed secondarily to surgery, and often rupture occurs before severe signs of pain. Horses are most at risk for this type of impaction if surgery is greater than 1 hour in length, or if inadequate analgesia is provided postoperatively.

Diagnosis is usually made by rectal palpation. Treatment includes fluid therapy and analgesics, but surgery is indicated if there is severe distention of the cecum or if medical therapy does not improve the situation. Surgery includes typhlotomy, and although cecal bypass has been performed in the past to prevent reoccurrence, a recent study suggests it is not necessary. Surgery has a good prognosis, although rupture can occur during surgical manipulation. The cause of cecal impactions are not known. Cecal impassion should be differentiated from large colon impaction via rectal, since cecal impaction has a high risk of rupture even before developing severe pain. Overall prognosis is 90%, regardless of medical or surgical treatment, but rupture does occur, often with no warning.

Gastric impaction

Gastric impactions are relatively rare, and occur when food is not cleared at the appropriate rate. It is most commonly associated with ingestion of foods that swell after eating or feeds that are coarse (bedding or poor quality roughage), poor dental care, poor mastication, inadequate drinking, ingestion of a foreign object, and alterations in the normal function of the stomach. Persimmons, which form a sticky gel in the stomach, and haylage, have both been associated with it, as has wheat, barley, mesquite beans, and beet pulp. Horses usually show signs of mild colic that is chronic, unresponsive to analgesics, and may include signs such as dysphagia, ptyalism, bruxism, fever, and lethargy, although severe colic signs may occur. Signs of shock may be seen if gastric rupture has occurred. Usually, the impaction must be quite large before it presents symptoms, and may be diagnosed via gastroscopy or ultrasound, although rectal examinations are unhelpful. Persimmon impaction is treated with infusions of Coca-Cola. Other gastric impactions are often resolves with enteral fluids. Quick treatment generally produces a favorable prognosis.

Small colon impaction

Small colon impactions represent a small number of colics in the horse, and are usually caused by obstruction from fecaliths, enteroliths, and meconium. Horses usually present with standard colic signs (pawing, flank watching, rolling) in 82% of horses, and occasionally with diarrhea (31%), anorexia (30%), straining (12%), and depression (11%), and rectal examination will reveal firm loops of small colon or actually palpable obstruction in the rectum. Impactions are most common in miniature horses, possibly because they do not masticate their feed as well, and during the fall and winter. Medical management includes the aggressive use of fluids, laxatives and lubricants, and enemas, as well as analgesics and anti-inflammatories. However, these impactions often require surgical intervention, and the surgeon will empty the colon either by enterotomy or by lubricants and massage. Surgical intervention usually results in longer recovery time at the hospital. Prognosis is very good, and horses treated with surgical treatment had a survival with return to athletic function rate of 91%, while 89% of the medically managed horses returned to previous use.

Large colon impaction

Large colon impactions typically occur at the pelvic flexure and right dorsal colon, two areas where the lumen of the intestine narrows. Large colon impactions are most frequently seen in horses that have recently had a sudden decrease in exercise, such as after a musculoskeletal injury. They are also associated in the practice of twice daily feeding of grain meals, which causes a short-lived but significant secretion of fluid into the lumen of the intestine, resulting in a 15% decrease in plasma volume (hypovolemia of the circulatory system) and the subsequent activation of the renin–angiotensin–aldosterone system. Aldosterone secretion activates absorption of fluid from the colon, decreasing the water content of the ingesta and increasing risk of impaction. Amitraz has also been associated with large colon impaction, due to alterations in motility and retention of intestinal contents, which causes further absorption of water and dehydration of ingesta. Other possible factors include poor dental care, course roughage, dehydration, and limited exercise.

Horses with a large colon impaction usually have mild signs that slowly get worse if the impaction does not resolve, and can produce severe signs. Diagnosis is often made by rectal palpation of the mass, although this is not always accurate since a portion of the colon is not palpable on rectal. Additional sections of intestines may be distended if there is fluid backup. Manure production decreases, and if passed, is usually firm, dry and mucus covered. Horses are treated with analgesics, fluid therapy, mineral oil, dactyl sodium sulfosuccinate (DSS), and/or epsom salts. Analgesics usually can control the abdominal discomfort, but may become less efficacious over time if the impaction does not resolve. Persistent impactions may require fluids administered both intravenously and orally via nasogastric tube, at a rate 2–4 times the maintenance for the animal. Feed is withheld. Horses that do not improve or become very painful, or those that have large amounts of gas distention, are recommended to undergo surgery to remove the impaction via enterotomy of the pelvic flexure. Approximately 95% of horses that undergo medical management, and 58% of surgical cases, survive.

Enteroliths and fecaliths

Enteroliths in horses are round 'stones' of mineral deposits, usually of ammonium magnesium phosphate (struvite) but sometimes of magnesium vivainite and some amounts of sodium, potassium, sulfur and calcium, which develop within the horse's gastrointestinal tract. They can form around a piece of ingested foreign material, such as a small nidus of wire or sand (similar to how an oyster forms a pearl). When they move from their original site they can obstruct the intestine, usually in the right dorsal and transverse colon, but rarely in the small colon. They may also cause mucosal irritation or pain when they move within the gastrointestinal tract. Enteroliths are not a common cause of colic, but are known to have a higher prevalence in states with a sandy soil or an abundance of alfalfa hay is fed, such as California, a state where 28% of surgical colics are due to enteroliths. Alfalfa hay is thought to increase the risk due to the high protein content in the hay, which would likely elevate ammonia nitrogen levels within the intestine. They may be more common in horses with diets high in magnesium, and are also seen more often in Arabians, Morgans, American Saddlebreds, miniature horses, and donkeys, and usually occur in horses older than four years of age. Horses with enteroliths typically have chronic, low-grade, recurring colic signs, which may lead to acute colic and distention of the large colon after occlusion of the lumen occurs. These horse may also have had a history of passing enteroliths in their manure. Level of pain is related to the degree of luminal occlusion. Abdominal radiographs can confirm the diagnosis, but smaller enteroliths may not be visible. In rare instances, enteroliths may be palpated on rectal examination, usually if they are present in the small colon. Once a horse is diagnosed with colic due to an enterolith, surgery is necessary to remove it, usually by pelvic flexure enterotomy and sometimes an additional right dorsal colon enterotomy, and fully resolve the signs of colic. Horses will usually present a round enterolith if it is the only one present, while multiple enteroliths will usually have flat sides, a clue to the surgeon to look for more stones. The main risk of surgery is rupture of the colon (15% of cases), and 92% of horses that are recovered survive to at least one year from their surgery date.

Fecaliths are hard formations of ingest that obstruct the GI tract, and may require surgery to resolve. These are most commonly seen in miniature horses, ponies, and foals.

Displacement

A displacement occurs when a portion of the large colon—usually the pelvic flexure—moves to an abnormal location. There are four main displacements described in equine medicine:

1. Left dorsal displacement (nephrosplenic entrapment): the pelvic flexure moves dorsally towards the nephrosplenic space. This space is found between the spleen, the left kidney, the nephrosplenic ligament (which runs between the spleen and kidney), and the body wall. In some cases, the bowel become entrapped over the nephrosplenic ligament. LDD accounts for 6-8% of all colics.
2. Right dorsal displacement: the colon moves between the cecum and body wall.
3. The pelvic flexure retroflexes towards the diaphragm
4. The colon develops a 180-degree volvulus, which may or may not occlude the vasculature of the organ.

The cause of displacement is not definitively known, but one explanation is that the bowel becomes abnormally distended with gas (from excessive fermentation of grain, a change in the microbiota secondary to antibiotic use, or a buildup of gas secondary to impaction) which results in a shift in the bowel to an abnormal position. Because much of the bowel is not anchored to the body wall, it is free to move out of position. Displacement is usually diagnosed using a combination of findings from the rectal exam and ultrasonography.

Many displacements (~96% of LDD, 64% of RDD) resolve with medical management that includes fluids (oral or intravenous) to rehydrate the horse and soften any impaction that may be present. Systemic analgesics, antispasmodics, and sedation are often used to keep the horse comfortable during this time. Horses with left dorsal displacement are sometimes treated with exercise and/or phenylephrine—a medication that causes contracture of the spleen and may allow the bowel to slip off the nephrosplenic ligament. At times anesthesia and a rolling procedure, in which the horse is placed in left lateral recumbency and rolled to right lateral recumbency while jostling, can also be used to try to shift the colon off of the nephrosplenic ligament. Displacements that do not respond to medical therapy require surgery, which generally has a very high success rate (80–95%).

Reoccurrence can occur with all types of displacements: 42% of horses with RDD, 46% of horses with retroflexion, 21% of those with volvulus, and 8% of those with LDD had reoccurrence of colic. LDD may be prevented by closing the nephrosplenic space with sutures, although this does not prevent other types of displacements from occurring in that same horse.

Torsion and volvulus

A volvulus is a twist along the axis of the mesentery, a torsion is a twist along the longitudinal axis of the intestine. Various parts of the horse's gastrointestinal tract may twist upon themselves. It is most likely to be either small intestine or part of the colon. Occlusion of the blood supply means that it is a painful condition causing rapid deterioration and requiring emergency surgery.

Volvulus of the large colon usually occurs where the mesentery attaches to the body wall, but may also occur at the diaphragmatic or sternal flexures, with rotations up to 720 degrees reported. It is most commonly seen in postpartum mares, usually presents with severe signs of colic that are refractory to analgesic administration, and horses often lie in dorsal recumbency. Abdominal distention is common due to strangulation and rapid engorgement of the intestine with gas, which then can lead to dyspnea as the growing bowel pushes against the diaphragm and prevents normal ventilation. Additionally, compression can place pressure on the caudal vena cava, leading to pooling of blood and hypovolemia. However, horses may not have a high heart rate, presumably due to increased vagal tone. Rectal palpation will demonstrate a severely gas distended colon, and the examiner may not be able to push beyond the brim of the pelvis due to the obstruction. The colon may be irreversibly damaged in as little as 3–4 hours from the initial time of the volvulus, so immediate surgical correction is required. The surgeon works to correct the volvulus and then removes any damaged colon. 95% of the colon may be resected, but often the volvulus damages more than this amount, requiring euthanasia. Plasma lactate levels can help predict survival rates, with an increased survival seen in horses with a lactate below 6.0 mmol/L. Prognosis is usually poor, with a survival rate of approximately 36% of horses with a 360 degree volvulus, and 74% of those with a 270 degree volvulus, and a reoccurrence rate of 5–50%. Complications post-surgery include hypoproteinemia, endotoxic shock, laminitis, and DIC.

Small intestinal volvulus is thought to be caused by a change in local peristalsis, or due to a lesion that the mesentery may twist around (such as an ascarid impaction), and usually involves the distal jejunum and ileum.w It is one of the most common causes of small intestinal obstruction in foals, possibly because of a sudden change to a bulkier foodstuff. Animals present with acute and severe signs of colic, and multiple distended loops of small intestine, usually seen radiographically in a foal. Small intestinal volvulus often occurs secondary to another disease process in adult horses, where small intestinal obstruction causes distention and then rotation around the root of the mesentery. Surgery is required to resect nonviable sections of bowel, and prognosis is correlated to the length of bowel involved, with animals with greater than 50% of small intestinal involvement having a grave prognosis.

Intussusception

Intussusception is a form of colic in which a piece of intestine "telescopes" within a portion of itself because a section is paralyzed, so the motile section pushes itself into the non-motile section. It most commonly occurs at the ileocecal junction and requires urgent surgery. It is almost always associated with parasitic infections, usually tapeworms, although small masses and foreign bodies may also be responsible, and is most common in young horses usually around 1 year of age. Ileocecal intussusception may be acute, involving longer (6–457 cm) segments of bowel, or chronic involving shorter sections (up to 10 cm in length). Horses with the acute form of colic usually have a duration of colic less than 24 hours long, while chronic cases have mild but intermittent colic. Horses with the chronic form tend to have better prognosis.

Rectal examination reveals a mass at the base of the cecum in 50% of cases. Ultrasound reveals a very characteristic "target" pattern on cross-section. Abdominocentesis results can vary, since the strangulated bowel is trapped within the healthy bowel, but there are usually signs of obstruction, including reflux and multiple loops of distended small intestine felt on rectal. Surgery is required for intussusception. Reduction of the area is usually ineffective due to swelling, so jejunojejunal intussusceptions are resected and ileocolic intussusceptions are resected as far distally as possible and a jejunocecal anatomosis is performed.

Entrapment

Epiploic foramen entrapment

On rare occasions, a piece of small intestine (or rarely colon) can become trapped through the epiploic foramen into the omental bursa. The blood supply to this piece of intestine is immediately occluded and surgery is the only available treatment. This type of colic has been associated with cribbers, possibly due to changes in abdominal pressure, and in older horses, possibly because the foramen enlarges as the right lobe of the liver atrophies with age, although it has been seen in horses as young as 4 months old. Horses usually present with colic signs referable to small intestinal obstruction. During surgery, the foramen can not be enlarged due to the risk of rupture of the vena cava or portal vein, which would result in fatal hemorrhage. Survival is 74–79%, and survival is consistently correlated with abdominocentesis findings prior to surgery.

Mesenteric rent entrapment

The mesentery is a thin sheet attached to the entire length of intestine, enclosing blood vessels, lymph nodes, and nerves. Occasionally, a small rent (hole) can form in the mesentery, through which a segment of bowel can occasionally enter. As in epiploic foramen entrapment, the bowel first enlarges, since arteries do not occlude as easily as veins, which causes edema (fluid buildup). As the bowel enlarges, it becomes less and less likely to be able to exit the site of entrapment. Colic signs are referable to those seen with a strangulating lesion, such as moderate to severe abdominal pain, endotoxemia, decrease gut sounds, distended small intestine on rectal, and nasogastric reflux. This problem requires surgical correction. Survival for mesenteric rent entrapment is usually lower than other small intestinal strangulating lesions, possibly due to hemorrhage, difficulty correcting the entrapment, and the length of intestine commonly involved, with <50% of cases surviving until discharge.

Inflammatory and ulcerative conditions

Proximal enteritis

Proximal enteritis, also known as anterior enteritis or duodenitis-proximal jejunitis (DPJ), is inflammation of the duodenum and upper jejunum. It is potentially caused by infectious organisms, such as Salmonella and Clostridial species, but other possible contributing factors include Fusarium infection or high concentrate diets. The inflammation of the intestine leads to large secretions of electrolytes and fluid into its lumen, and thus large amounts of gastric reflux, leading to dehydration and occasionally shock.

Signs include acute onset of moderate to severe pain, large volumes orange-brown and fetid gastric reflux, distended small intestine on rectal examination, fever, depression, increased heart rate and respiratory rate, prolonged CRT, and darkened mucous membranes. Pain level usually improves after gastric decompression. It is important to differentiate DPI from small intestinal obstruction, since obstruction may require surgical intervention. This can be difficult, and often requires a combination of clinical signs, results from the physical examination, laboratory data, and ultrasound to help suggest one diagnosis over the other, but a definitive diagnosis can only be made with surgery or on necropsy.

DPI usually is managed medically with nasogastric intubation every 1–2 hours to relieve gastric pressure secondary to reflux, and aggressive fluid support to maintain hydration and correct electrolyte imbalances. Horses are often withheld food for several days. Use of anti-inflammatory, anti-endotoxin, anti-microbial, and prokinetic drugs are common with this disease. Surgery may be needed to rule out obstruction or strangulation, and in cases that are long-standing to perform a resection and anastomosis of the diseased bowel. Survival rates for DPJ are 25–94%, and horses in the southeast United States appear to be more severely affected.

Colitis

Colitis is inflammation of the colon. Acute cases are medical emergencies as the horse rapidly loses fluid, protein, and electrolytes into the gut, leading to severe dehydration which can result in hypovolemic shock and death. Horses generally present with signs of colic before developing profuse, watery, fetid diarrhea.

Both infectious and non-infectious causes for colitis exist. In the adult horse, Salmonella, Clostridium difficile, and Neorickettsia risticii (the causative agent of Potomac Horse Fever) are common causes of colitis. Antibiotics, which may lead to an altered and unhealthy microbiota, sand, grain overload, and toxins such as arsenic and cantharidin can also lead to colitis. Unfortunately, only 20–30% of acute colitis cases are able to be definitively diagnosed. NSAIDs can cause slower-onset of colitis, usually in the right dorsal colon (see Right dorsal colitis).

Treatment involves administration of large volumes of intravenous fluids, which can become very costly. Antibiotics are often given if deemed appropriate based on the presumed underlying cause and the horse's CBC results. Therapy to help prevent endotoxemia and improve blood protein levels (plasma or synthetic colloid administration) may also be used if budgetary constraints allow. Other therapies include probiotics and anti-inflammatory medication. Horses that are not eating well may also require parenteral nutrition. Horses usually require 3–6 days of treatment before clinical signs improve.

Due to the risk of endotoxemia, laminitis is a potential complication for horses suffering from colitis, and may become the primary cause for euthanasia. Horses are also at increased risk of thrombophlebitis.

Gastric ulceration

Horses form ulcers in the stomach fairly commonly, a disease called equine gastric ulcer syndrome. Risk factors include confinement, infrequent feedings, a high proportion of concentrate feeds, such as grains, excessive non-steroidal anti-inflammatory drug use, and the stress of shipping and showing. Gastric ulceration has also been associated with the consumption of cantharidin beetles in alfalfa hay which are very caustic when chewed and ingested. Most ulcers are treatable with medications that inhibit the acid producing cells of the stomach. Antacids are less effective in horses than in humans, because horses produce stomach acid almost constantly, while humans produce acid mainly when eating. Dietary management is critical. Bleeding ulcers leading to stomach rupture are rare.

Right dorsal colitis

Long-term use of NSAIDs can lead to mucosal damage of the colon, secondary to decreased levels of homeostatic prostaglandins. Mucosal injury is usually limited to the right dorsal colon, but can be more generalized. Horses may display acute or chronic intermittent colic, peripheral edema secondary to protein losing enteropathy, decreased appetite, and diarrhea. Treatment involves decreasing the fiber levels of the horse's diet by reducing grass and hay, and placing the horse on an easily digestible pelleted feed until the colon can heal. Additionally, the horse may be given misoprostol, sucralfate, and psyllium to try to improve mucosal healing, as well as metronidazole to reduce inflammation of the colon.

Tumors

Strangulating pedunculated lipoma

Benign fatty tumors known as lipomas can form on the mesentery. As the tumor enlarges, it stretches the connective tissue into a stalk which can wrap around a segment of bowel, typically small intestine, cutting off its blood supply. The tumor forms a button that latches onto the stalk of the tumor, locking it on place, and requiring surgery for resolution. Surgery involves cutting the stalk of the tumor, untwisting the bowel, and removing bowel that is no longer viable. If the colic is identified and taken to surgery quickly, there is a reasonable rate of success of 50–78%. This type of colic is most commonly associated with ponies, and aged geldings, 10 years and older, probably because of fat distribution in this group of animals.

Other cancers

Cancers (neoplasia) other than lipoma are relatively rare causes of colic. Cases have been reported with intestinal cancers including intestinal lymphosarcoma, leiomyoma, and adenocarcinoma, stomach cancers such as squamous cell carcinoma, and splenic lymphosarcoma.

Gastric squamous cell carcinoma is most often found in the non-glandular region of the stomach of horses greater than 5 years of age, and horses often present with weight loss, anorexia, anemia, and ptyalism. Gastric carcinoma is usually diagnosed via gastroscopy, but may sometimes be felt on rectal if they have metastasized to the peritoneal cavity. Additionally, laparoscopy can also diagnose metastasized cancer, as can presence of neoplastic cells on abdominocentesis. Often the signs of intestinal neoplasia are non-specific, and include weight loss and colic, usually only if obstruction of the intestinal lumen occurs.

Ileus

Ileus is the lack of motility of the intestines, leading to a functional obstruction. It often occurs postoperatively following any type of abdominal surgery, and 10–50% of all cases of surgical colic will develop this complication, including 88% of horses with a strangulating obstructions and 41% of all colics with a large intestinal lesion. The exact cause is unknown, but is suspected to be due to inflammation of the intestine, possibly a result of manipulation by the surgeon, and increased sympathetic tone. It has a high fatality rate of 13–86%.

Ileus diagnosed based on several criteria:

1. Nasogastric reflux: 4 liters or greater in a single intubation, or greater than 2 liters of reflex over more than one intubation
2. A heart rate greater than 40 bpm
3. Signs of colic, which may vary from mild to severe
4. Distended small intestine, based on rectal or abdominal ultrasound findings. On ultrasound, ileus presents as more than 3 loops of distended small intestine, with a lack of peristaltic waves.

This form of colic is usually managed medically. Because there is no motility, intestinal contents back up into the stomach. Therefore, periodic decompression of the stomach though nasogastric intubation is essential to prevent rupture. Horses are monitored closely following abdominal surgery, and a sudden increase in heart rate indicates the need to check for nasogastric reflux, as it is an early indication of postoperative ileus. The horse is placed on intravenous fluids to maintain hydration and electrolyte balance and prevent hypovolemic shock, and rate of fluids is calculated based on daily maintenance requirement plus fluid lose via nasogastric reflux.

Motility is encouraged by the use of prokinetic drugs such as erythromycin, metoclopramide, bethanechol and lidocaine, as well as through vigorous walking, which has also been shown to have a beneficial effect on GI motility. Lidocaine is especially useful, as it not only encourages motility, but also has anti-inflammatory properties and may ameliorate some post-operative pain. Metoclopramide has been shown to reduce reflux and hospital stay, but does has excitatory effects on the central nervous system. Anti-inflammatory drugs are used to decrease inflammation of the GI tract, which is thought to be the underlying cause of the disease, as well as to help control any absorption of LPS in cases of endotoxemia since the substance decreases motility. However, care must be taken when giving these drugs, as NSAIDs have been shown to alter intestinal motility.

Large intestinal ileus is most commonly seen in horses following orthopedic surgery, but its risk is also increased in cases where post-operative pain is not well-controlled, after long surgeries, and possibly following ophthalmologic surgeries. It is characterized by decreased manure output (<3 piles per day), rather than nasogastric reflux, as well as decreased gut sounds, signs of colic, and the occasional impaction of the cecum or large colon. Cecal impactions can be fatal, so care must be taken to monitor the horse for large intestinal ileus after orthopedic surgery, primarily by watching for decreased manure production.

Decreased intestinal motility can also be the result of drugs such as Amitraz, which is used to kill ticks and mites. Xylazine, detomidine, and butorphanol also reduce motility, but will not cause colic if appropriately administered.

Parasites

Ascarids (roundworms)

Occasionally there can be an obstruction by large numbers of roundworms. This is most commonly seen in young horses as a result of a very heavy infestation of Parascaris equorum that can subsequently cause a blockage and rupture of the small intestine. Rarely, dead worms will be seen in reflux. Deworming heavily infected horses may cause a severe immune reaction to the dead worms, which can damage the intestinal wall and cause a fatal peritonitis. Veterinarians often treat horses with suspected heavy worm burdens with corticosteroids to reduce the inflammatory response to the dead worms. Blockages of the small intestine, particularly the ileum, can occur with Parascaris equorum and may well require colic surgery to remove them manually. Large roundworm infestations are often the result of a poor deworming program. Horses develop immunity to parascarids between 6 months age and one year and so this condition is rare in adult horses. Prognosis is fair unless the foal experiences hypovolemia and septic shock, with a survival rate of 33%.

Tapeworms

Tapeworms at the junction of the cecum have been implicated in causing colic. The most common species of tapeworm in the equine is Anoplocephala perfoliata. However, a 2008 study in Canada indicated that there is no connection between tapeworms and colic, contradicting studies performed in the UK.

Cyathostomes

Acute diarrhea can be caused by cyathostomes or "small Strongylus-type" worms that are encysted as larvae in the bowel wall, particularly if large numbers emerge simultaneously. The disease most frequently occurs in winter time. Pathological changes of the bowel reveal a typical "pepper and salt" color of the large intestines. Animals suffering from cyathostominosis usually have a poor deworming history. There is now a lot of resistance to fenbendazole in the UK.

Large strongyles

Large strongyle worms, most commonly Strongylus vulgaris, are implicated in colic secondary to non-strangulating infarction of the cranial mesenteric artery supplying the intestines, most likely due to vasospasm. Usually the distal small intestine and the large colon are affected, but any segment supplied by this artery can be compromised. This type of colic has become relatively rare with the advent of modern anthelminthics. Clinical signs vary based on the degree of vascular compromise and the length of intestine that is affected, and include acute and severe colic seen with other forms of strangulating obstruction, so diagnosis is usually made based on anthelminthic administration history although may be definitively diagnosed during surgical exploratoration. Treatment includes typical management of colic signs and endotoxemia, and the administration of aspirin to reduce the risk of thrombosis, but surgery is usually not helpful since lesions are often patchy and may be located in areas not easily resected.

Foal colic

Meconium impactions

Meconium, or the first feces produced by the foal, is a hard pelleted substance. It is normally passed within the first 24 hours of the foal's life, but may become impacted in the distal colon or rectum. Meconium impaction is most commonly is seen in foals 1–5 days of age, and is more common in miniature foals and in colts more than fillies (possibly because fillies have a wider pelvis). Foals will stop suckling, strain to defecate (presents as an arched back and lifted tail), and may start showing overt signs of colic such as rolling and getting up and down. In later stages, the abdomen will distend as it continues to fill with gas and feces. Meconium impactions are often diagnosed by clinical signs, but digital examination to feel for impacted meconium, radiographs, and ultrasound may also be used.

Treatment for meconium impaction typically involves the use of enemas, although persistent cases may require mineral oil or IV fluids. It is possible to tell that the meconium has passed when the foal begins to produce a softer, more yellow manure. Although meconium impactions rarely cause perforation, and are usually not life-threatening, foals are at risk of dehydration and may not get adequate levels of IgG due to decreased suckling and not enough ingestion of colostrum. Additionally, the foals will eventually bloat, and will require surgical intervention. Surgery in a foal can be especially risky due to immature immune system and low levels of ingested colostrum.

Lethal white syndrome

Lethal white syndrome, or ileocolonic aganglionosis, will result in meconium impaction since the foal does not have adequate nerve innervation to the large intestine, in essence, a nonfunctioning colon. Foals that are homozygous for the frame overo gene, often seen in Paint horse heritage, will develop the condition. They present with signs of colic within the first 12 hours after birth, and die within 48 hours due to constipation. This syndrome is not treatable.

Congenital abnormalities

Atresia coli and atresia ani can also present as meconium impaction. The foal is missing the lumen of its distal colon or anus, respectively, and usually show signs of colic within 12–24 hours. Atresia coli is usually diagnosed with barium contrast studies, in which foals are given barium, and then radiographed to see if and where the barium is trapped. Atresia ani is simply diagnosed with digital examination by a veterinarian. Both situations requires emergency surgery to prevent death, and often still has a poor prognosis for survival with surgical correction.

Infectious organisms

Clostridial enterocolitis due to infection by Clostridium perfringens is most commonly seen in foals under 3 months of age. Clostridial toxins damage the intestine, leading to dehydration and toxemia. Foals usually present with signs of colic, decreased nursing, abdominal distention, and diarrhea which may contain blood. Diagnosis is made with fecal culture, and while some foals do not require serious intervention, others need IV fluids, antibiotics, and aggressive treatment, and may still die. Other bacterial infections that may lead to enterocolitis include Salmonella, Klebsiella, Rhodococcus equi, and Bacteroides fragilis.

Parasitic infection, especially with threadworms (Strongyloides westeri