Colonic Impaction

Colonic impaction is one of the most common causes of colic in horses, characterized by the accumulation and compaction of ingesta within the large colon that creates a firm obstruction preventing normal passage of material. The equine large colon, measuring approximately 10 to 12 feet in length with multiple flexures and diameter changes, has several anatomical regions predisposed to impaction formation. The pelvic flexure, where the large colon narrows significantly and makes a 180-degree turn, represents the most frequent impaction site, though material can also become impacted at the right dorsal colon, transverse colon, or small colon. Understanding this condition's frequency and treatability helps horse owners respond appropriately when signs develop.

Colonic impaction accounts for approximately 10 to 20 percent of all colic episodes in horses, making it among the most commonly diagnosed causes of equine abdominal pain. This condition affects horses of all ages, breeds, and management systems, though certain factors increase susceptibility. Senior horses with dental problems that prevent proper feed processing face elevated risk. Horses with inadequate water intake, whether from insufficient availability, unpalatable water, or cold temperatures discouraging drinking, commonly develop impactions. Changes in management including stall confinement, dietary alterations, and reduced exercise create periods of increased vulnerability.

The impact of colonic impaction on equine health ranges from mild discomfort resolving with simple treatment to life-threatening obstruction requiring emergency surgery. Most impactions respond well to medical management with intravenous fluids and enteral laxatives, resolving over 24 to 72 hours without surgical intervention. However, large or firm impactions, those failing to respond to aggressive medical therapy, and cases complicated by secondary issues may require surgical correction. Unresolved impactions can lead to intestinal distension, compromised blood supply to the colon wall, and potential rupture with fatal peritonitis. Early recognition and treatment significantly improve outcomes.

With appropriate treatment, the vast majority of horses with colonic impaction recover completely and return to normal function without lasting effects. Survival rates for medically managed impactions exceed 95 percent, while surgical cases have survival rates of 70 to 90 percent depending on severity and complications. Recurrence occurs in some horses, particularly those with underlying predisposing factors that cannot be completely eliminated. Understanding the causes, recognizing symptoms, and implementing preventive management strategies helps reduce both initial impaction risk and likelihood of repeat episodes in susceptible individuals.

The primary causes of colonic impaction center on factors that reduce water content in colonic ingesta or slow the transit of material through the large intestine. The horse's large colon relies on adequate hydration to maintain ingesta in a consistency that moves easily through its various segments. When ingesta becomes excessively dry or motility decreases, material accumulates at natural narrowing points, particularly the pelvic flexure where diameter decreases substantially. Dehydration from any cause, whether inadequate water intake, excessive sweating, or pathological fluid losses, reduces the water available for intestinal secretions. Decreased gut motility from various causes allows material to remain in the colon longer, permitting additional water absorption that further dries and compacts contents.

While colonic impaction itself is not a genetic condition, certain heritable factors may influence individual susceptibility. Poor dental conformation that develops in some breed lines can prevent adequate feed processing, allowing coarse material to reach the large intestine. Some horses may inherit tendencies toward reduced gut motility or altered thirst responses. Body size affects both the amount of feed processed and the physical dimensions of the gastrointestinal tract. However, management and environmental factors typically outweigh genetic influences in impaction development, and any horse of any breeding can develop this condition under appropriate circumstances.

Environmental and management factors play dominant roles in colonic impaction development. Inadequate water availability or unpalatable water sources reduce intake and contribute to dehydration. Cold weather represents a significant risk period, as horses often drink less when water temperatures drop. Dietary factors including poor-quality hay, coarse or overly mature forage, inadequate fiber, and sudden feed changes contribute to impaction risk. Stall confinement without adequate exercise reduces gut motility, while excessive bedding ingestion in horses eating straw or shavings provides difficult-to-digest material. Transport stress, hospitalization, and changes in routine create periods of elevated susceptibility.

Risk factors for colonic impaction encompass individual patient characteristics and management elements. Senior horses face increased risk from multiple factors including dental deterioration, reduced water intake, decreased activity, and potentially diminished gut motility. Horses with preexisting dental disease cannot properly process feed regardless of age. Recent changes in hay source or type create adaptation periods when impaction risk increases. Reduced exercise, whether from stall rest for medical reasons or simply inadequate turnout, slows intestinal transit. Certain medications, particularly nonsteroidal anti-inflammatory drugs, may affect gut motility and mucosal health. Geographic regions with sandy soil pose additional risk for sand-mixed impactions.

The pathophysiology of colonic impaction follows a progressive pattern of accumulation and compaction. Initially, ingesta begins to accumulate at narrowing points in the colon, particularly the pelvic flexure. As transit slows, additional water is absorbed from the material, making it increasingly dry and difficult to move. The growing mass obstructs further passage of material, creating a blockage that distends the colon proximal to the impaction. Continued accumulation increases colonic distension, which triggers pain responses and can compress blood vessels in the intestinal wall. Severe or prolonged distension may compromise mucosal integrity and blood supply. If untreated, progressive distension can lead to rupture of the colon with fatal spillage of intestinal contents into the abdominal cavity.

Early warning signs of colonic impaction often develop gradually over hours to days before obvious colic becomes apparent. Horses may demonstrate subtle decreases in appetite, eating more slowly or leaving portions of meals uneaten. Reduced water intake, if noticed, represents both a potential cause and early indicator. Changes in manure production provide important early clues, with fecal balls becoming smaller, drier, harder, and less frequent. Mild restlessness or subtle changes in behavior that attentive owners might recognize as abnormal can precede more obvious discomfort. Some horses demonstrate reduced gut sounds that can be detected by an observant owner placing an ear to the flank.

Common symptoms of established colonic impaction include mild to moderate colic signs that typically wax and wane over time. Affected horses paw intermittently at the ground, look at their flanks, and may kick at their abdomen. Lying down more than normal, rolling, and stretching as if to urinate indicate abdominal discomfort. Unlike severe surgical colic, impaction pain often allows periods of relative comfort between episodes. Appetite is usually reduced, with horses showing decreased interest in grain and hay. Fecal production decreases markedly, with passage of only small amounts of dry, hard material or cessation of defecation entirely. Some horses may posture to defecate repeatedly without producing manure.

Behavioral changes in horses with colonic impaction reflect ongoing discomfort and altered gut function. Affected horses often appear dull and less interactive than normal. They may spend excessive time lying down or adopt unusual standing postures. Reduced willingness to move or work becomes apparent, with horses resisting exercise. Some horses display a characteristic stretching posture, extending the hindquarters backward. Depression and social withdrawal from herd mates may be observed. Flank watching, where the horse repeatedly turns to look at its side, indicates abdominal awareness. Water intake often decreases even when water is readily available, potentially worsening the underlying problem.

Physical signs detected on veterinary examination help confirm the diagnosis and assess severity. Heart rate is typically mildly elevated, often 40 to 50 beats per minute, though severe cases may show higher rates. Gut sounds are usually decreased in frequency and intensity throughout the abdomen. Mucous membrane color and capillary refill time remain normal in uncomplicated cases but may deteriorate if significant pain or systemic compromise develops. Rectal examination reveals the impaction as a firm, doughy mass within the large colon, most commonly at the pelvic flexure. The impaction may be describable in terms of its size, consistency, and the degree of colonic distension proximal to it.

Symptom progression in colonic impaction varies based on severity and response to treatment. Mild impactions may produce only subtle, intermittent signs before resolving with simple treatment or even spontaneously with increased water intake. Moderate impactions cause more persistent discomfort requiring active management over 24 to 72 hours. Severe impactions produce more constant pain, marked decrease in gut sounds, and potentially progressive deterioration if treatment fails. Impactions complicated by significant colonic distension may show increasing pain intensity and cardiovascular changes. Successful treatment produces gradual improvement with return of appetite, gut sounds, and eventually fecal production.

Emergency symptoms requiring immediate intensive veterinary care include severe unrelenting pain despite analgesics, rapid heart rate exceeding 60 beats per minute, toxic-appearing mucous membranes, and signs of cardiovascular compromise. Progressive abdominal distension visible externally suggests serious colonic distension with risk of rupture. Sudden cessation of pain after a period of severe discomfort may indicate rupture with release of pressure, a catastrophic event. Any horse with impaction failing to improve after 48 to 72 hours of aggressive medical management warrants surgical evaluation. Nasogastric reflux developing in an impaction case suggests secondary small intestinal involvement requiring urgent reassessment.

Physical examination provides essential information for diagnosing colonic impaction and determining appropriate treatment. The veterinarian assesses vital parameters including heart rate, respiratory rate, temperature, mucous membrane color, and capillary refill time to evaluate systemic status. Thorough auscultation of all abdominal quadrants documents gut sound character and frequency, with decreased sounds being characteristic. External observation may reveal mild abdominal distension or flank changes in severe cases. Assessment of pain level and response to analgesics helps gauge severity. The overall clinical picture guides treatment intensity and helps predict whether medical or surgical management will be required.

Diagnostic tests essential for confirming colonic impaction include rectal examination, nasogastric intubation, and often basic bloodwork. Rectal examination represents the most definitive diagnostic tool, allowing direct palpation of the impacted colon. The examining veterinarian describes the impaction's location, size, and consistency, typically finding a firm, doughy mass at the pelvic flexure or other common sites. The degree of proximal colonic distension provides prognostic information. Nasogastric intubation confirms that reflux is not present, ruling out small intestinal involvement. Blood work including packed cell volume and total protein assesses hydration status, while electrolytes evaluate mineral balance.

Advanced diagnostics are sometimes employed in colonic impaction cases, particularly those not responding to initial treatment or when complications are suspected. Transabdominal ultrasound can visualize colonic distension, wall thickness, and presence of peritoneal fluid. Serial ultrasound examinations may track impaction size and colonic diameter changes during treatment. Abdominal radiography, when available, can identify sand accumulations contributing to impaction or visualize gas patterns suggesting intestinal dysfunction. Abdominocentesis, collection of peritoneal fluid, may be performed if intestinal compromise is suspected, with analysis revealing changes suggesting mucosal damage. Blood lactate levels provide information about tissue perfusion.

Differential diagnosis for colonic impaction includes other causes of colic in horses, particularly those producing similar mild to moderate intermittent signs. Gas colic from excessive fermentation may present similarly but typically resolves more quickly without the palpable mass characteristic of impaction. Colonic displacement produces different rectal findings and may require surgical correction. Large colon volvulus causes more severe signs with cardiovascular compromise. Cecal impaction creates a mass palpable in a different location. Sand colic may complicate or mimic simple impaction. Small intestinal conditions typically produce more rapid deterioration with nasogastric reflux. Enterolithiasis, intestinal stones, can obstruct the colon and mimic impaction. The veterinarian integrates all findings to determine the specific diagnosis and appropriate treatment approach.

Emergency and immediate treatment for colonic impaction focuses on pain relief, rehydration, and initiating therapy to soften and mobilize the impacted material. Analgesics including flunixin meglumine, xylazine, or detomidine provide pain relief and allow thorough examination. Nasogastric intubation confirms no reflux is present and provides a route for enteral administration of fluids and laxatives. Intravenous fluid therapy begins to restore systemic hydration and increase water available for intestinal secretions. Initial assessment determines impaction severity and establishes a treatment plan. Feed is withheld to prevent additional material from entering an already obstructed system.

Medical management successfully resolves the majority of colonic impactions without surgical intervention. The cornerstone of treatment involves aggressive intravenous fluid therapy using balanced electrolyte solutions, typically at rates two to four times maintenance or higher during active treatment. This systemic rehydration increases water secretion into the intestinal lumen, helping to soften impacted material. Enteral administration of water via nasogastric tube delivers additional fluid directly to the gastrointestinal tract. Mineral oil, administered via nasogastric tube, lubricates intestinal contents and may help passage of softened material. Magnesium sulfate or sodium sulfate solutions act as osmotic laxatives, drawing water into the intestinal lumen. Dioctyl sodium sulfosuccinate, a surfactant, helps break up impacted material when used judiciously.

Surgical intervention becomes necessary when medical management fails to resolve the impaction within an appropriate timeframe, typically 48 to 72 hours, or when clinical deterioration suggests complications. Surgery is performed through a ventral midline celiotomy under general anesthesia. The surgeon locates the impaction and performs an enterotomy, creating an incision in the colon to manually evacuate impacted material and lavage the intestinal lumen. Colonic viability is assessed throughout, and any compromised sections require resection and anastomosis. The abdomen is thoroughly lavaged before closure. Surgical correction of impaction carries good success rates when performed before significant intestinal compromise develops.

Supportive care during impaction treatment addresses patient comfort and systemic needs while working toward resolution. Continued pain management maintains comfort and allows assessment of treatment progress through pain resolution. Electrolyte supplementation corrects imbalances that may develop. The horse is typically maintained without feed during active treatment, with grazing or small amounts of grass offered once progress becomes evident. Hand walking may encourage gut motility in stable patients. Monitoring for complications, particularly laminitis, involves regular hoof palpation and observation for shifting leg lameness. Serial rectal examinations track impaction size and consistency to gauge treatment effectiveness.

Rehabilitation and return to work following impaction resolution requires careful dietary reintroduction and gradual activity resumption. Once the impaction has resolved, as confirmed by return of gut sounds, appetite, and fecal production, feeding begins with small frequent meals of easily digestible forage. Hay is often soaked or replaced with hay cubes or complete feeds during the initial refeeding period. Grain is reintroduced gradually after forage tolerance is established. Water intake is encouraged and monitored closely. Activity increases progressively from stall rest to hand walking to turnout and eventually normal work, with the timeline depending on case severity and whether surgical intervention was required.

Treatment decision factors guiding the choice between medical and surgical management include impaction characteristics, duration of clinical signs, response to initial therapy, and cardiovascular status. Small, soft impactions detected early typically respond well to medical management. Large, firm impactions present for multiple days or those failing to soften despite aggressive fluid therapy may require surgery. Worsening pain, deteriorating cardiovascular parameters, or progressive abdominal distension during medical management indicate the need for surgical evaluation. Financial considerations, the horse's age and intended use, and owner preferences factor into decisions, with the veterinarian providing realistic assessments of prognosis under different treatment scenarios.

Recovery timeline for colonic impaction depends on severity and treatment approach. Mild impactions resolving with outpatient medical management often show improvement within 24 hours, with complete resolution over two to three days. More severe impactions requiring hospitalization and intensive fluid therapy typically resolve over 48 to 72 hours, with several additional days of careful management before return to normal feeding. Surgical cases require longer recovery, typically 8 to 12 weeks of controlled activity with gradual increases before return to full work. Most horses recover completely without lasting effects, though some require ongoing management modifications to prevent recurrence.

Post-treatment care and monitoring following impaction resolution focuses on ensuring complete recovery and preventing recurrence. Feeding reintroduction proceeds gradually over several days, beginning with small amounts of easily digestible forage and progressing to normal diet. Water intake is monitored to ensure adequate consumption, with flavoring or warming of water used if needed to encourage drinking. Observation for return of normal gut sounds and fecal production confirms functional recovery. Any recurrence of colic signs warrants prompt veterinary reassessment. Surgical patients require incision monitoring with suture or staple removal typically occurring at 14 to 21 days postoperatively.

Prognosis factors for colonic impaction include the impaction's size and location, duration before treatment, response to therapy, and presence of complications. Impactions detected early and responding well to medical management carry excellent prognosis, with survival rates exceeding 95 percent. Cases requiring surgery have good prognosis when intestinal viability is maintained, with surgical survival rates of 70 to 90 percent. Cases complicated by colonic rupture are typically fatal. Underlying factors predisposing to impaction, such as dental disease or management issues, influence recurrence risk. Compliance with post-recovery recommendations and preventive measures affects long-term outcomes.

Long-term soundness outlook for horses recovered from colonic impaction is generally excellent. Most horses return to their previous level of performance without permanent effects. Recurrence rates vary depending on underlying predisposing factors and management modifications implemented after recovery. Horses with correctable issues such as dental disease often have reduced recurrence once these problems are addressed. Those with uncorrectable factors such as chronic reduced water intake may experience periodic repeat episodes despite optimal management. Overall, colonic impaction represents one of the more favorable surgical colic diagnoses when surgery becomes necessary, with most affected horses enjoying continued comfortable, productive lives.

Management practices for preventing colonic impaction focus on maintaining adequate hydration and supporting normal intestinal function. Ensuring constant access to fresh, clean, palatable water represents the single most important preventive measure. Water should be checked multiple times daily for cleanliness and appropriate temperature. During cold weather, heated water buckets or frequent water changes help maintain drinkable temperatures. Consistent feeding schedules with gradual changes when diet modifications are necessary allow the gut to adapt appropriately. Regular exercise and turnout promote normal intestinal motility. Avoiding sudden stall confinement when possible, and providing maximum movement during necessary confinement periods, helps maintain gut function.

Nutritional prevention strategies specifically address factors contributing to impaction development. Providing high-quality, appropriately processed forage reduces the risk of impaction from coarse, stemmy material. Soaking hay before feeding adds moisture directly to ingesta and may be particularly beneficial for senior horses or those with history of impaction. Salt supplementation encourages water intake and should be provided either as a salt block or added to feed. Some practitioners recommend periodic psyllium supplementation to support intestinal content movement, particularly in areas with sandy soil. Avoiding excessive grain relative to forage prevents fermentation abnormalities. Ensuring adequate daily forage intake maintains normal gut motility patterns.

Exercise and conditioning practices support the intestinal function necessary to prevent impaction. Regular physical activity promotes normal gut motility and helps maintain appropriate body condition. Horses in consistent work programs have lower overall colic rates than sedentary animals. Turnout allowing free movement complements structured exercise. When stall rest becomes necessary for medical reasons, hand walking should be implemented when possible to maintain some gut stimulation. Avoiding prolonged immobility, such as during long trailer rides without breaks, helps prevent transit-related impaction. Gradual changes to exercise programs prevent the digestive disruption that can accompany sudden alterations.

Environmental factors influencing impaction prevention include housing, water source management, and forage quality control. Water sources should be clean, easily accessible, and maintained at drinkable temperatures year-round. Multiple water access points in large pastures ensure horses do not become dehydrated due to distance from water. Hay storage should prevent excessive drying that reduces moisture content. Bedding choices should minimize ingestion risk, with some horses requiring alternatives to straw if they eat bedding material. Sand accumulation in paddocks and feeding areas should be minimized through appropriate ground management and use of feeders that prevent ground feeding.

Dental care and health monitoring play essential roles in impaction prevention. Regular dental examinations, typically annually for adult horses and more frequently for seniors, identify and correct abnormalities that prevent proper feed processing. Sharp enamel points, wave mouth, and missing teeth all compromise chewing efficiency. Senior horses with significant dental disease may require diet modifications including soaked feeds, chopped hay, or complete pelleted feeds that require less chewing. Monitoring body condition helps identify horses not processing feed efficiently. Early attention to any signs of colic allows treatment before impaction becomes severe. Collaboration with a veterinarian to develop individualized preventive strategies addresses each horse's specific risk factors.

Daily management adjustments for horses with history of colonic impaction emphasize hydration support and digestive care. Water availability and intake should be monitored closely, with daily consumption tracked when possible. Fresh water should be available at all times, with buckets or troughs cleaned regularly to maintain palatability. Adding water to feeds increases moisture content of ingesta directly. Feeding practices should emphasize forage-based nutrition divided into multiple meals throughout the day rather than two large feedings. Hay may be soaked or replaced with higher-moisture alternatives for horses prone to impaction. Monitoring daily fecal output provides an easily observable indicator of gut function, with changes in quantity, consistency, or moisture content warranting attention.

Housing and turnout considerations for impaction-prone horses prioritize movement and hydration access. Maximum turnout time consistent with safety supports gut motility through natural movement. Multiple water sources in turnout areas ensure adequate access. Social housing with compatible companions reduces stress-related digestive disturbances. Horses requiring stall time should have water available continuously, with buckets positioned to encourage drinking. Bedding choices should prevent ingestion of inappropriate material. When weather conditions discourage water intake, proactive measures such as warming water or adding flavoring encourage consumption. Gradual environmental transitions help horses adapt without digestive disruption.

Exercise modifications for horses recovered from colonic impaction depend on the severity of the episode and treatment required. Medically managed cases typically return to normal activity within days of resolution, with gradual work resumption over several sessions. Surgical patients require longer recovery periods with progressive exercise increases over weeks to months as determined by the veterinarian. Throughout return to work, monitoring for any signs of recurring discomfort guides advancement pace. Some horses benefit from permanent incorporation of regular exercise into their management, as consistent activity supports gut function more effectively than sporadic work.

Monitoring and ongoing care for horses with impaction history includes owner vigilance and regular veterinary input. Owners should know normal patterns for appetite, water consumption, and fecal production, enabling rapid recognition of deviations. Any sign of colic warrants prompt attention given the known predisposition. Regular dental examinations ensure ongoing ability to properly process feed. Periodic veterinary examinations allow assessment of overall condition and opportunity to discuss any concerns. Maintaining detailed records of colic episodes, dietary changes, and management alterations helps identify patterns or triggers for individual horses.

Quality of life and use considerations for horses with impaction history recognize that most affected animals live normal, comfortable, productive lives with appropriate management. Colonic impaction rarely represents a career-ending diagnosis, with most horses returning to their previous level of performance. Horses requiring surgical correction typically resume normal function after adequate recovery. Those with recurring impactions may need permanent management modifications but often remain functional for their intended purpose. Quality of life remains excellent for most impaction survivors, with the condition representing a manageable concern requiring ongoing attention rather than a limiting factor in the horse's usefulness or comfort.

High-risk breeds for colonic impaction relate primarily to management circumstances and individual factors rather than breed-specific genetic susceptibility. No particular breed demonstrates inherent predisposition to colonic impaction. However, certain breed-related factors may influence risk indirectly. Miniature horses and ponies, particularly those prone to obesity and metabolic issues, may face elevated risk associated with altered gut motility. Draft breeds processing large amounts of feed might have increased exposure to impaction-promoting factors. Senior horses of any breed face age-related risk factors including dental deterioration and reduced activity. Breed-related management practices, such as intensive feeding regimens for performance horses, may create periods of increased susceptibility.

Use and discipline considerations affect impaction risk through associated management factors. Performance horses experiencing travel stress, varying feed sources, and competition pressure may face elevated risk during intensive campaign periods. Horses undergoing stall rest for medical conditions face inherently increased impaction risk from reduced movement and altered routine. Retired horses transitioning from active to minimal exercise require careful management during the adjustment period. Breeding stock, particularly horses with seasonal breeding schedules, experience management changes that may affect gut function. No specific discipline appears uniquely associated with impaction risk when management factors are appropriately controlled.

Genetic testing and breeding recommendations are not applicable to colonic impaction as this condition lacks hereditary basis. However, certain heritable factors may indirectly influence susceptibility. Horses inheriting poor dental conformation may be predisposed to impaction through inadequate feed processing. Breeding for overall robust constitution supports gastrointestinal health. Selection against extreme body types that might have altered gastrointestinal anatomy may theoretically reduce risk. Practically, management factors remain far more important than breeding considerations in preventing colonic impaction. Individual horses with repeated impaction episodes warrant careful management regardless of breed, with attention to identifying and addressing specific predisposing factors.

Commonly co-occurring conditions with colonic impaction reflect both predisposing factors and potential complications. Dental disease frequently underlies impaction development, with inadequate chewing allowing coarse material to reach the colon. Dehydration from any cause, including concurrent illness or inadequate water availability, contributes to impaction formation. Gastric ulceration commonly occurs in horses under stress and may accompany or predispose to impaction through reduced appetite and altered gut motility. Sand accumulation complicates impaction in horses grazing sandy pastures or fed on sandy ground. Electrolyte imbalances may develop during impaction from reduced intake and altered intestinal function. Laminitis represents a potential complication of severe impaction with endotoxemia.

Conditions with similar symptoms requiring differentiation from colonic impaction include various causes of mild to moderate colic. Gas colic produces comparable intermittent discomfort but lacks the palpable mass of impaction and typically resolves quickly with treatment. Colonic displacement causes positional changes detectable on rectal examination and may require different management. Cecal impaction presents as a mass in a different location on rectal palpation. Enterolithiasis, intestinal stones, may produce similar intermittent obstruction but with a hard, distinct mass rather than doughy material. Small intestinal conditions typically produce more rapid deterioration with reflux. Ascending colon volvulus causes more severe pain with cardiovascular compromise.

Potential complications of colonic impaction extend from mild treatment-related issues to life-threatening events. Prolonged impaction can cause colonic wall damage with compromise of blood supply and tissue devitalization. Severe distension may lead to rupture with fatal peritonitis. Endotoxemia developing from compromised intestinal barrier function triggers systemic inflammatory response. Laminitis secondary to endotoxemia may cause permanent lameness. Treatment complications can include gastric distension from excessive enteral fluid administration. Post-surgical complications in cases requiring surgery include incisional infections, adhesion formation, and prolonged ileus. Recurrence of impaction represents a common concern, particularly in horses with uncorrectable predisposing factors.