Common Causes of Colic in Horses

Any condition causing abdominal pain in the horse is referred to as colic, which remains among the most common medical conditions affecting equids. Because of the prevalence of colic and its complexity and potential for severe complications or even death, prompt recognition, appropriate treatment, and early referral when necessary (TABLE 1) are key to successful outcomes.

Causes of colic can broadly be classified as gastrointestinal or extragastrointestinal. The equine digestive tract is extremely complex and can be affected by a wide range of issues.

Common gastrointestinal causes can be separated by location (e.g., gastric, small intestine, large intestine) and include impactions, gas or tympany (e.g., obstruction), displacements or torsions, inflammation (e.g., diarrhea, colitis), and parasitism.

Extragastrointestinal causes of colic may involve reproductive tract disorders (e.g., uterine or testicular torsion, ovarian disorders such as large granulosa theca cell tumors, ovarian cysts, estrus during routine heat cycles), urinary tract conditions (e.g., stones, infection, bladder rupture, urethral obstruction), liver or splenic conditions (e.g., organ failure, enlargement, rupture, abscessation, neoplasia, infarction), and peritoneal or abdominal wall issues (e.g., neoplasia, peritonitis, adhesions, hernia, trauma).

Signs of musculoskeletal disorders such as laminitis or rhabdomyolysis (tying-up) may resemble signs of colic and are commonly misclassified due to overlapping clinical signs such as restlessness, sweating, and reluctance to move.

Colic Workup

History

In working up a routine colic case, obtaining a thorough history from the owner or caretaker is critical and should be completed first. A detailed history can help distinguish the severity of the pain as well as the duration and progression of the colic episode. Information that can provide context for the physical examination and diagnostic findings as well as identifying the specific cause of colic includes any predisposing factors (e.g., diet changes, recent deworming, administration of medications such as NSAIDs), previous or current health issues, recurrent colic episodes, activity level or routine exercise practices and discipline, recent transport or stressful event, and changes in manure output or consistency.

Physical Examination

After a detailed history is obtained, a physical examination should then be performed efficiently and systematically. All physical examinations should begin with simply observing the horse and its surroundings. Note the horse’s attitude, behavior, and posture to identify any signs of discomfort or agitation (e.g., up-and-down behavior, inability to stand quietly [depending on the horse’s natural behavior], pawing, flank watching, flehmen response, sweating, muscle fasciculations or tremors, abrasions on distal extremities or bony prominences along the face or body, abnormal abdominal contour, fecal staining along the rear or tail). In addition, the surrounding environment and housing area should be examined for things such as disruption of bedding secondary to rolling or thrashing, manure output and consistency, uneaten feed, and quality of feed or hay.

For the hands-on portion of the physical examination, clinicians should develop their own systematic process and ensure that all components are addressed during each examination. Interpretation of vital parameters can indicate severity of the colic episode as well as potential for systemic compromise.

Auscultation of the heart and lungs, aside from just obtaining heart and respiratory rates, is a crucial part of the colic workup and is often overlooked when the focus is on the abdomen. Auscultation of these areas can provide vital information about both extragastrointestinal causes of colic and overall systemic status. Abdominal auscultation should include assessment of borborygmi as absent, hypomotile, normal, or hypermotile, as well as any pinging sounds.

Every physical examination of a horse should include obtaining rectal temperature, regardless of the nature of the visit (i.e., healthy/preventive or sick). In addition, asking the client whether temperatures are routinely taken on the farm can be useful for identifying any trends or differentiating between possible infectious or systemic disease.

Bounding digital pulses, weight shifting, or reluctance to lift feet or walk when asked may suggest laminitis, and these signs in addition to stiff or sore muscles may indicate rhabdomyolysis, both of which are frequently confused for episodes of colic.

Differentiating Between Gastrointestinal and Extragastrointestinal Cause

Differentiating between gastrointestinal and extragastrointestinal causes of colic may require further diagnostics. Although each case is unique and may necessitate alternative diagnostics, a routine colic diagnostic workup may include transrectal palpation, nasogastric intubation, ultrasonography, blood work, and abdominocentesis. Although all of these diagnostics together will provide a comprehensive evaluation of the horse, the ability to perform them depends on patient compliance, case presentation, availability of supplies, financial considerations, and clinician preference.

Transrectal Palpation

Transrectal palpation can often yield a diagnosis and is also useful for determining the degree of bowel distention, although inadequate restraint or lack of amenability of the horse can preclude the ability to perform.¹ In addition, the diagnostic usefulness of transrectal palpation is limited to evaluation of the caudal one-third of the abdomen; therefore, more cranial abnormalities may be missed.

Nasogastric Intubation

Nasogastric intubation can be both diagnostic and therapeutic, depending on the underlying cause. Assessment of the amount, consistency, contents, and odor of the reflux may be useful. Abnormal findings include > 2 L of reflux for an average-sized adult horse, foul- or fetid-smelling reflux, excessive feed quantity or volume of gas, hemorrhagic reflux, or overall difficulty with advancing the nasogastric tube. Portable meters for measuring L-lactate and serum amyloid A have also become more accessible and can provide beneficial information during a colic examination and be used to help determine if referral is needed.

Ultrasonography

Abdominal ultrasonography is less invasive and can yield a plethora of information. Ultrasonography equipment is becoming more accessible in ambulatory settings, although inadequate user comfort and experience may deter clinician use. During ultrasonography, examining the abdomen as well as the caudal thoracic area may aid in identifying conditions such as pleuropneumonia, diaphragmatic hernias (FIGURE 1), or other thoracic pathologies that may mimic or complicate abdominal disorders.

Figure 1. Diaphragmatic hernia.

Abdominocentesis

Abdominocentesis is a sensitive way to evaluate pathology in the abdomen because the composition of peritoneal fluid changes rapidly in response to pathophysiologic changes.¹ Fluid samples can be collected with a needle, teat cannula, or bitch catheter, and collection is performed in a standard manner with few associated complications. Evaluation of peritoneal fluid (FIGURE 2) can also be useful for deciding whether to refer the case.

Figure 2A. Abdominocentesis.

Figure 2B. Normal abdominocentesis fluid.

Figure 2C. Abnormal abdominocentesis fluid (serosanguinous).

Causes of Colic

Given the large size of horses as well as the limitations of diagnostic modalities available, a definitive diagnosis may or may not be reached. However, understanding the common causes of colic and their pathophysiology are essential for guiding a focused and efficient workup and implementing appropriate treatment plans.

Recognizing causes that are most likely given the horse’s scenario will help tailor clinical evaluation, improve efficiency during evaluation, improve communication with clients, and guide therapies or timely referral. The most frequently encountered causes of colic are impaction colic, gas colic, and displacements.

Impaction Colic

Risk for development of an impaction is increased for horses that consume coarse hay or low-quality forage or have poor dentition, reduced exercise or activity levels, and/or abrupt changes in water availability or intake.

Impactions can occur in all parts of the gastrointestinal tract. Gastric impactions are rare and account for < 5% of causes of colic in horses.² Small intestine impactions are more commonly associated with ingestion of coastal Bermuda grass hay, accumulation of dead ascarid worms after deworming therapy, or inflammatory bowel disease. The most frequently encountered location for impaction is the large colon, which accounts for the main cause of colic in horses.² The most common location for large colon impactions is the pelvic flexure due to its anatomic design, which includes a hairpin turn as well as decreased lumen size. Impactions can also occur in the small colon, although similar to gastric impactions, they are less likely. Small intestine impactions may be appreciated during abdominal ultrasonography, and a history of diet or recent deworming may be useful to support the diagnosis.

Diagnosis of gastric impactions requires endoscopy, although suspicion can follow nasogastric intubation, depending on the quantity and content of the reflux obtained. Transrectal palpations are often useful for diagnosing large and small colon impactions, although depending on the location, the impaction may not be within physical reach of the clinician. A history of reduced water intake, reduced manure output, or manure consistency that is dry or mucus-covered can be supportive.

Medical Management

Medical management of gastrointestinal impaction varies based on the site of the obstruction but focuses on relieving the impaction, controlling pain, and maintaining hydration. Gastric impactions are managed with repeated gastric decompression and lavage via nasogastric intubation, along with careful monitoring for rupture. Small intestinal impactions often require fluid therapy; anti-inflammatories; and, in some cases, gastric decompression, though surgical intervention is more common if the obstruction does not resolve. Large colon impactions are typically treated with enteral and intravenous fluids as well as laxatives. Small colon impactions may require enteral and intravenous fluids, laxatives, enemas, and careful monitoring for endotoxemia or salmonellosis. Across all sites, supportive care, analgesics, and close monitoring are essential. Referral for surgical management may be necessary.

Gas Colic

Gas colic, or tympany, is excessive gas accumulation and often results from dietary changes or grain overload. The excessive gas accumulation distends the gastrointestinal tract (FIGURE 3), leading to secondary pain. Fermentation in horses takes place in the hindgut, where an abundant microbial population breaks down carbohydrates and other components for energy. During the fermentation process, gas is produced and can accumulate if production is excessive or expelling is impeded. Common causes of increased production include rapid transition in feeding regimens or diets that are high in nonstructural carbohydrate content. Causes for inability to expel gas can include a physical obstruction that blocks the exit of gas or decreased motility from stress, illness, reduced exercise or physical movement, and/or pain.

Figure 3A. Distended small intestine in ventral abdomen.

Figure 3B. Distended small intestine.

Medical Management

Sufficient medical management may include antispasmodic therapies to relax the smooth muscles of the gastrointestinal tract, thereby reducing spasms and pain; exercise to improve gastrointestinal motility; analgesics; and feed restriction. However, additional therapies such as enteral fluids, intravenous fluids, and enteral laxatives may also be useful. Although gas colic is usually mild and responds well to medical management, if left unmanaged it can lead to more serious conditions.

Displacement

If severe enough, excessive gas buildup can lead to displacement or twisting of the gastrointestinal tract. Because some segments of the gastrointestinal tract are not attached to the body wall, a segment of the gastrointestinal tract may be physically moved from its natural location. Displacement occurs when a segment moves out of its normal anatomic position but does not twist, which in the horse typically involves the large colon and can result in left dorsal displacement of the large colon (also known as nephrosplenic entrapment) (FIGURE 4) or right dorsal displacement of the large colon (FIGURE 5). With left dorsal displacement, the left dorsal and ventral colon migrate dorsally and become trapped between the spleen and the left kidney within the nephrosplenic space. In right dorsal displacement, the left dorsal and ventral colon migrate across the abdomen toward the right side over or around the cecum.³ Diagnosis of large colon displacements are commonly appreciated during transrectal palpation, although abdominal ultrasonography can be useful as well.

Figure 4. Nephrosplenic entrapment or left dorsal displacement of the large colon.

Figure 5. Right dorsal displacement of the large colon.

Medical Management

For large colon displacements, medical management is often successful, although surgical correction and decompression may be necessary.

Phenylephrine, an α₁ adrenergic receptor agonist, can be used to facilitate splenic contraction and correction of left dorsal displacement of the large colon.³ However, several adverse effects of this medication have been reported (e.g., hemorrhage, bradycardia, hypertension, arrhythmias) and may deter its use.^3,4

Another mode of medical management that can cause splenic contraction and is a safer alternative is exercise. Rolling techniques while the patient is under general anesthesia have also been described. If desired, exercise and rolling techniques can be performed concurrently with phenylephrine administration.³

If surgical intervention is necessary, the approaches are laparoscopy and ventral midline celiotomy. Additional supportive therapies (e.g., enteral and/or intravenous fluids, enteral laxatives, analgesics, antispasmodic agents, feed restriction) are often beneficial.²

Treatment for right dorsal displacements is similar, although administration of phenylephrine would not be indicated. Transabdominal or transrectal trocharization of the cecum or large colon have also been performed as adjunctive therapies for a low-cost method when surgical intervention is not feasible.² If large colon displacement progresses, volvulus or twisting of the large colon can result in the need for immediate surgical intervention.

Inflammation

Inflammation or infection of the small intestine is known as enteritis; of the large intestine, colitis; and of both, enterocolitis.⁵ Most cases of enterocolitis are infectious, although some may be inflammatory, and either can contribute to signs of colic.

For infectious etiology, bacterial, viral, and parasitic causes can all be implicated. For infectious cases, many of the pathogens involved pose zoonotic risk and warrant appropriate biosecurity measures. Relying on a thorough history from the owner or caretaker in addition to watching for any signs of loose manure can be useful. In such cases, abdominal ultrasonography can help identify thickening of the small and/or large intestinal wall. In addition, fluid content within the lumen may also be noted. If appreciated, fecal diagnostic testing is suggested to appropriately implement therapy as well as guide herd-health recommendations and biosecurity protocols.

Medical Management

Medical management may include antimicrobial therapies, although therapy should be based on clinical appearance and fecal pathogen testing as use of antimicrobials is not indicated for every patient. Blood work including a CBC and chemistry profile can be useful for systemically evaluating the horse and guiding therapies. In addition, depending on the degree of fluid loss, fluids may be given enterally to account for such loss, although intravenous fluid administration is often needed depending on the circumstance or patient response. Enterocolitis cases may need to be referred due to the intensity of medical management required and the need for appropriate isolation facilities.

Parasitism

Control of gastrointestinal parasitism has become increasingly more difficult over time due to the development of anthelmintic resistance. Parasite control strategies have been poorly understood and inappropriately implemented, resulting in overtreatment and emergence of resistant parasites. See TABLE 2 for common equine gastrointestinal parasites.⁶

The most prevalent group of gastrointestinal parasites affecting horses are cyathostomins,⁷ nematodes that can infect all horses of all ages and do not elicit development of immunity in response to infection. Clinical signs can be absent or can result in colic signs, diarrhea, weight loss, or death secondary to acute larval cyathostominosis (mass synchronous excystment of larvae, resulting in generalized typhlocolitis).^6,7

Large strongyles, most commonly Strongylus vulgaris, have largely been eliminated from clinical detection secondary to anthelmintic treatment, although cases have been intermittently reported and can lead to thromboembolism and peritonitis.⁶

Anoplocephala perfoliata tapeworms can also lead to colic signs due to their predilection for the ileocecal region, resulting in mucosal erosions at the site of attachment, intussusceptions, or impactions (FIGURE 6).⁶

Figure 6. Tapeworms noted within the cecum during a typhlotomy.

Parascaris equorum roundworms more commonly affect younger horses (typically less than 6 to 8 months of age unless immunocompromised); in contrast to cyathostomins, they do elicit an immune response to mitigate future infections and can also lead to more severe clinical signs when worm burden is high.⁷ Colic episodes associated with roundworm infections most commonly occur in heavily parasitized foals that have been given a dewormer and resulted in a large worm die-off with secondary accumulation and impaction of dead worms within the intestinal lumen.⁸

Medical Management

Anthelmintic strategies should be tailored specifically to each farm based on population, pathogen burden, and fecal diagnostics. According to numerous studies, fecal egg counts for identifying parasite burden can be misleading. However, to avoid contribution to resistance and to estimate resistance within a particular population, a fecal egg count should be performed at least 1 to 2 times per year, before anthelmintic treatment, to stratify horses into low-, medium-, or high-shedder categories to reduce pasture contamination. To assess therapy effectiveness, a fecal egg count reduction test should be performed 10 to 14 days after anthelmintic treatment.

Principal strategies for management should focus on eliminating pasture contamination to prevent the parasite from completing its life cycle within the host. The daily infective dose for grazing horses is increased by high stocking density, overgrazed pastures, presence of horses with high fecal egg counts, presence of younger horses, spreading manure on pastures, and warm damp weather.⁹

Surgical Management of Colic

Although most clients and clinicians prefer to medically manage colic episodes, surgical intervention may be warranted for some cases. Early referral for surgical intervention decreases postoperative morbidity and mortality as well as reduces cumulative cost for hospitalization.¹ One of the biggest hurdles encountered when deciding whether to pursue colic surgery is the financial cost for the surgical procedure and in-hospital postoperative management as well as the costs associated with loss of use, stall rest, and at-home management.¹⁰ In a recent retrospective study evaluating long-term outcome after colic surgery, it was noted that 72.1% of horses that were athletically active before surgery continued their athletic activity after surgery. In addition, the decision to proceed for colic surgery was rated at 4/5 for overall satisfaction by the owners polled in the study.¹⁰ Although prognosis and survival vary according to the severity and underlying cause of the colic episode, many horses recover fully and return to their previous level of activity and lifestyle. Prompt diagnosis and appropriate referral are critical for improving outcome for patients with lesions requiring surgical correction.

Colic Detection Technology

To improve prompt recognition of colic, technology progression with the use of live-stream cameras and wearable devices that monitor vital parameters and activity have been useful. Such devices are especially valuable for monitoring overnight or during times without direct supervision when early signs of colic might otherwise go unnoticed for a prolonged time. As technology and our understanding of equine veterinary medicine advance, so too will the ability to improve therapies and outcomes for horses affected by colic.

Summary

Colic is associated with multiple causes, which are either gastrointestinal or extragastrointestinal. Diagnosis involves history-taking; physical examination; and any combination of transrectal palpation, nasogastric intubation, abdominal ultrasonography, blood work, and abdominocentesis. Successful outcomes depend on timely recognition, proper medical management, and early referral when indicated.