Intestinum crassum
The equine large intestine is a large-volume fermentation vat. The cecum is a comma-shaped blind sac roughly 1 meter long with a capacity of about 25–35 liters. It is the primary initial site of microbial fermentation. The large (ascending) colon is folded into a double-horseshoe shape with four main regions (right ventral, left ventral, left dorsal, right dorsal) connected by flexures. It has relatively little mesenteric attachment, which allows mobility but predisposes to displacement and volvulus. The small colon continues water absorption and forms fecal balls. The rectum stores feces. Microbial fermentation of fiber produces volatile fatty acids that supply a large portion of the horse’s energy. The large intestine also absorbs substantial amounts of water and electrolytes. Sudden diet changes can disrupt the microbial population and contribute to colic or laminitis.
Large intestinal disorders (impaction, displacement, volvulus, enteritis) are major causes of colic. Diagnosis uses rectal palpation, abdominal ultrasound, and abdominocentesis. Capacity estimates vary by source but the cecum is commonly cited around 25–35 L and the large colon larger. The pelvic flexure is a common site of impaction.
## Overview Large colon volvulus is a twisting of the large colon around the mesentery that connects its dorsal and ventral portions. The large colon is unusually susceptible to this problem because
## Overview Cecal impaction is accumulation of ingesta within the cecum that fails to empty normally. It is less common than large-colon impaction but carries a higher risk of rupture because the cec
## Overview Sand colic results from accumulation of sand or dirt in the large colon, especially the right dorsal colon, transverse colon, and pelvic flexure. It is common in horses kept on sandy soil
## Overview Right dorsal colitis is ulceration and inflammation of the right dorsal colon, strongly linked to nonsteroidal anti-inflammatory drug use, especially phenylbutazone. Signs include diarrhe
## Overview Colonic displacement is abnormal positioning of the large colon within the abdomen. The main forms are left dorsal displacement (nephrosplenic entrapment), right dorsal displacement, and
Rotation of the large colon along its long axis (>270°). A surgical emergency and one of the most common causes of fatal colic. The large colon is predisposed because it lacks a mesenteric attachment
## Overview Large colon impaction is obstruction of the large colon by firm, dehydrated feed material, most often at the pelvic flexure. It is one of the most common causes of colic. Horses show mild
## Overview Nephrosplenic entrapment, also called left dorsal displacement of the colon, occurs when the left large colon moves up and becomes trapped over the nephrosplenic ligament between the left
Make all feed changes gradually over 7–14 days. Provide consistent high-quality forage. Know the signs of colic (pawing, looking at the flank, rolling, reduced manure) and call the veterinarian promptly.
## Overview Large colon volvulus is a twisting of the large colon around the mesentery that connects its dorsal and ventral portions. The large colon is unusually susceptible to this problem because it is long, can hold a large volume of gas and fluid, and has only limited attachments to the body wall. Most of the colon is free to move within the abdomen. When the colon twists, the blood vessels and lymphatic vessels that run in the mesentery become compressed. Depending on the degree of rotation, this can cause simple obstruction of the intestinal lumen or, more seriously, complete strangulation of the blood supply. Large colon volvulus is different from large colon displacement. Displacements (such as left dorsal or right dorsal displacement) move the colon out of its normal position and can obstruct the flow of gas and digesta, but they do not typically cut off the blood supply. Many displacements can be managed with medical therapy or resolve on their own. A true volvulus that compromises blood flow is a strangulating lesion and is always a surgical emergency. ## Causes The exact trigger for large colon volvulus is not fully understood. It is thought to be related to abnormal gas accumulation that makes the colon buoyant and more mobile. Postpartum broodmares are overrepresented in many surgical populations. The increase in free abdominal space after foaling is a plausible contributing factor, but this mechanism has not been directly established. Height and a history of prior colic are supported risk factors. Associations with recent diet or pasture changes have been identified in case-control data, but these are not proven causes. The condition can occur in horses of any age, sex, or breed. ## Diagnosis Your veterinarian will base the initial assessment on the severity and rapidity of the clinical signs. Rectal examination, abdominal ultrasound, blood lactate, and peritoneal fluid analysis can support a suspicion of large colon volvulus. On rectal examination the large colon is often markedly enlarged and the mesentery between the dorsal and ventral portions may feel thickened or edematous. Ultrasound findings of a distended colon with thickened walls or abnormal vessel positioning are suggestive but not definitive. Peripheral lactate is better supported as a prognostic marker reflecting tissue perfusion than as a lesion-specific diagnostic test. Abdominocentesis does not reliably distinguish volvulus from other severe colic lesions. Definitive diagnosis and assessment of bowel viability are usually confirmed at surgery. Stabilization measures such as intravenous fluids, pain medication, and nasogastric decompression support the horse during transport and preparation for surgery, but they do not correct the twist itself. ## Treatment Definitive treatment is emergency surgery. After the horse is anesthetized, the surgeon makes a ventral midline incision, exteriorizes the colon, and derotates it to restore normal positioning and blood flow. The surgeon then evaluates the color, thickness, and motility of the bowel to determine how much tissue is still viable. If a large portion of the colon is necrotic, it may be resected and the healthy ends reconnected (anastomosis). In broodmares with a history of recurrence, some surgeons perform a colopexy (suturing the colon to the body wall) to reduce the chance of future twists. Medical management alone is not a treatment for strangulating large colon volvulus. Pain control and fluid therapy are used only to stabilize the patient for transport and anesthesia. ## Recovery and Prognosis Outcome depends primarily on how long the bowel has been twisted before surgery and on the extent of irreversible damage found at surgery. Some high-volume Thoroughbred broodmare populations report survival to discharge around 88% to 93%, while broader strangulating large colon volvulus cohorts report lower discharge survival around 71% to 77%, with long-term survival lower still. Horses that survive surgery may still face complications such as diarrhea, laminitis, or further colic. Recurrence of large colon volvulus has been reported in approximately 15 percent of cases after one episode, with substantially higher recurrence risk after repeated episodes. ## Prevention There is no proven way to eliminate the risk of large colon volvulus. Practical steps that may reduce risk include making all diet changes gradually and providing close monitoring of broodmares in the late-pregnancy and early-postpartum periods so that colic can be recognized quickly. Owners of horses that have already had one episode of large colon volvulus should discuss the higher recurrence risk and possible preventive surgical options with their veterinarian. Signs include: Sudden severe unrelenting abdominal pain that responds poorly to analgesics; rapid heart rate and signs of cardiovascular shock; heavy sweating; progressive abdominal distension; rapid deterioration over hours
## Overview Cecal impaction is accumulation of ingesta within the cecum that fails to empty normally. It is less common than large-colon impaction but carries a higher risk of rupture because the cecum has a relatively thin wall and subtle early signs. A recent illness or surgical procedure is the most common history; dehydration and tapeworm infection may also contribute. ## Causes Dehydration, reduced motility after surgery or systemic illness, high-tapeworm burdens at the ileocecal junction, and abrupt dietary changes are the main causes. Solid-filled and fluid-filled forms are recognized. ## Diagnosis Mild or intermittent colic, reduced manure output, and an enlarged, firm cecum on rectal palpation are characteristic. Horses may show surprisingly little pain until rupture is imminent. Ultrasound and bloodwork support the diagnosis. ## Treatment Aggressive intravenous fluid therapy, analgesics, and prokinetics are the mainstay for solid impactions. Fluid-filled (dysfunctional) impactions may require different motility management. Surgery is indicated when medical therapy fails or rupture is suspected. ## Recovery and Prognosis Prognosis is fair with early recognition and treatment but becomes poor once mural compromise or rupture occurs. Mortality rates of 25 to 57 percent have been reported in some series. ## Prevention Constant access to fresh water, gradual diet changes, strategic deworming for tapeworms, and careful monitoring of postoperative manure output reduce risk. Signs include: mild to moderate colic; reduced manure output; enlarged cecum on rectal examination; depression; elevated heart rate in advanced cases
## Overview Sand colic results from accumulation of sand or dirt in the large colon, especially the right dorsal colon, transverse colon, and pelvic flexure. It is common in horses kept on sandy soil. Signs include recurrent mild colic, diarrhea with a cow-pat consistency, and weight loss. Sand can be detected by fecal sedimentation or abdominal radiographs. Auscultation may reveal sand sounds. ## Causes Ingestion of sand while grazing or eating off the ground. ## Diagnosis Fecal sand test, radiographs showing sand, and clinical signs. ## Treatment Fluids, laxatives (such as psyllium or magnesium sulfate), and pain relief. Surgery for complete obstruction. Prevention is critical. ## Recovery and Prognosis Good for most medical cases. Recurrent if sand intake continues. ## Prevention Feed in tubs or racks, not on the ground. Psyllium supplementation in sandy areas. Signs include: recurrent mild colic; diarrhea; weight loss; sand on fecal sedimentation; sand sounds on auscultation
## Overview Right dorsal colitis is ulceration and inflammation of the right dorsal colon, strongly linked to nonsteroidal anti-inflammatory drug use, especially phenylbutazone. Signs include diarrhea, weight loss, low blood protein, ventral edema, and colic. Ultrasound may show thickening of the right dorsal colon. Stopping all nonsteroidal drugs is essential. ## Causes Nonsteroidal anti-inflammatory drug toxicity, most often phenylbutazone, but also flunixin or meloxicam. ## Diagnosis History of nonsteroidal use, hypoproteinemia, ultrasound, and exclusion of infectious diarrhea. ## Treatment Immediate discontinuation of all nonsteroidal drugs, dietary management (low-bulk feeds), sucralfate, and supportive care including plasma if protein is very low. Prognosis is guarded. ## Recovery and Prognosis Guarded. Early recognition improves the chance of recovery. ## Prevention Judicious use of nonsteroidal drugs, especially avoiding prolonged high doses. Signs include: diarrhea; weight loss; low protein; ventral edema; colic; history of nonsteroidal anti-inflammatory drug use
## Overview Colonic displacement is abnormal positioning of the large colon within the abdomen. The main forms are left dorsal displacement (nephrosplenic entrapment), right dorsal displacement, and large colon volvulus. Volvulus is a surgical emergency because it rapidly compromises blood supply. ## Causes Gas accumulation, changes in motility, and anatomic factors allow the mobile large colon to move out of position. Risk factors include recent transport, changes in exercise, and, for volvulus, being a broodmare. ## Diagnosis Colic signs of variable severity, reduced manure, and findings on rectal palpation or ultrasound. Volvulus produces rapid systemic deterioration. ## Treatment Left dorsal displacement may respond to medical management or rolling under anesthesia. Right dorsal displacement and volvulus usually require surgery. Aggressive fluid and supportive care are essential. ## Recovery and Prognosis Good for uncomplicated displacements treated promptly. Volvulus carries significant mortality even with surgery. ## Prevention Consistent feeding and exercise routines; prompt attention to early colic signs. Signs include: colic of variable severity; reduced manure output; abdominal distension; elevated heart rate; rapid deterioration in volvulus
Rotation of the large colon along its long axis (>270°). A surgical emergency and one of the most common causes of fatal colic. The large colon is predisposed because it lacks a mesenteric attachment along much of its length. Signs include: Severe, unrelenting abdominal pain; rapid cardiovascular deterioration (tachycardia >60 bpm, prolonged CRT); abdominal distension; nasogastric reflux; endotoxemia
## Overview Large colon impaction is obstruction of the large colon by firm, dehydrated feed material, most often at the pelvic flexure. It is one of the most common causes of colic. Horses show mild to moderate pain, reduced manure output, and a firm mass that can often be felt on rectal examination. ## Causes Dehydration, poor dental function, sudden feed changes, limited water access, or reduced exercise. ## Diagnosis Rectal palpation of a firm impaction, ultrasound, and clinical signs of mild colic with decreased fecal output. ## Treatment Intravenous or oral fluids, analgesics, and lubricants. Most cases resolve medically within one to several days. Surgery is rarely needed. ## Recovery and Prognosis Excellent with prompt medical treatment. Recurrence is possible if risk factors persist. ## Prevention Constant access to fresh water, regular dental care, gradual feed changes, and consistent exercise. Signs include: mild to moderate colic; decreased fecal output; firm mass palpable at the pelvic flexure; dehydration
## Overview Nephrosplenic entrapment, also called left dorsal displacement of the colon, occurs when the left large colon moves up and becomes trapped over the nephrosplenic ligament between the left kidney and spleen. It causes mild to moderate intermittent colic. The spleen is displaced medially, and gas-distended colon is seen on ultrasound. It accounts for a significant percentage of colic cases, especially in larger, deep-chested horses. ## Causes Gas distension allowing the colon to migrate into the nephrosplenic space. ## Diagnosis Rectal palpation and ultrasound showing the displaced colon and medially positioned spleen. ## Treatment Many cases resolve with medical management (phenylephrine, exercise, or rolling under anesthesia). Surgery is used when medical treatment fails. Prognosis is excellent. ## Recovery and Prognosis Excellent. Over 90 percent recover with appropriate treatment. ## Prevention Management practices that reduce gas colic. Some horses undergo prophylactic procedures if recurrent. Signs include: mild to moderate intermittent colic; reduced gut sounds on the left; spleen displaced medially; gas-distended colon on ultrasound
Large intestinal disorders (impaction, displacement, volvulus, enteritis) are major causes of colic. Diagnosis uses rectal palpation, abdominal ultrasound, and abdominocentesis. Capacity estimates vary by source but the cecum is commonly cited around 25–35 L and the large colon larger. The pelvic flexure is a common site of impaction.
Make all feed changes gradually over 7–14 days. Provide consistent high-quality forage. Know the signs of colic (pawing, looking at the flank, rolling, reduced manure) and call the veterinarian promptly.
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