Right dorsal colitis
Right dorsal colitis (RDC) is a rare cause of colic in horses.
RDC is characterised by ulceration and inflammation confined to a specific region of the large colon. Although equine colitis may occur as a result of infectious disease, in response to antibiotic administration, and in the presence of certain nematodes, this manifestation of disease is directly related to toxicity associated with NSAID administration. The disease most commonly occurs after administration of high doses of NSAIDs, but some horses develop right dorsal colitis after administration of therapeutic doses. Toxicity from NSAID therapy is more likely to occur when horses are dehydrated and receive high doses of an NSAID. Because recent data have demonstrated that no greater analgesic effects are achieved even when doses as high as 8.8 mg/kg are used, there is no therapeutic advantage to using higher doses.
The mechanism of ulcer development results from inhibition of both cyclooxygenase (COX)-1 and -2. Under normal physiologic circumstances, the basal level of COX-1 is important in maintaining mucosal epithelial health through adequate blood flow, epithelial cell turnover, bicarbonate production, and mucous secretion. Because NSAIDs inhibit prostanoid (prostaglandin) synthesis, they can induce or potentiate inflammation in the gastrointestinal (GI) tract by causing mucosal injury. This injury results from epithelial surface damage that enhances mucosal permeability, leading to ulceration.
Acute right dorsal colitis occurs over hours to days and is typically represented by reduced appetite, colic, diarrhea, and dehydration.
Horses may develop endotoxemia manifested as the presence of dark mucous membrane color, tachycardia, tachypnea, and cool distal extremities. Clinical signs consistent with chronic right dorsal colitis include reduced feed intake, mild to moderate abdominal pain, soft to loose feces, weight loss, and ventral edema resulting from protein-losing enteropathy. Performance and miniature horses (or ponies) are most commonly affected with this condition because they are more likely to require NSAID administration and their body weights tend to be erroneously estimated. Although a history of NSAID administration is common in patients with right dorsal colitis, the condition does not usually occur with long-term administration of low doses of NSAIDs.
Evaluation of horses suspected of having right dorsal colitis should include a thorough physical examination, a complete blood count, a serum biochemistry profile, urinalysis, and abdominal ultrasonography. Hematologic abnormalities most commonly reveal anemia (associated with chronic disease), panhypoproteinemia, and hypocalcemia. Anemia of chronic disease is commonly moderate and rarely life threatening; however, chronicity of disease may not be the exclusive mechanism of blood loss because low-grade intraluminal blood loss may contribute to a reduced packed cell volume. Fecal occult blood is an unreliable diagnostic test because false-positive results may be obtained, particularly if the horse has been palpated per rectum within the previous 24 hours.
Protein-losing enteropathy may result in significant protein loss, potentially yielding a total plasma protein concentration of less than 3.5 g/dl and an albumin concentration of less than 1.5 g/dl. Determination of GI protein loss can be supported by ruling out protein loss externally (e.g., renal disease) or into a body cavity (e.g., peritonitis, pleuropneumonia). Hypocalcemia may result from reduced dietary intake as well as reduced protein-bound calcium associated with GI protein loss.
Because clinical signs and clinicopathologic abnormalities are not specific for right dorsal colitis, additional diagnostic testing may be indicated to confirm or support the diagnosis. Ultrasonographic examination of the right dorsal colon from the 11th to 15th intercostal spaces has been a useful technique for this purpose. From this perspective, the liver and duodenum appear in the upper (dorsal) portion of the image, whereas the right dorsal colon is positioned ventrally. Ultrasonographic analysis of the large colon has been successfully used to help detect increased colon wall thickness associated with large colon torsion10 as well as to indicate increased wall thickness involving the right dorsal colon. Reference intervals range from 0.27 to 0.55 cm from the 11th to 15th intercostal spaces, with clinically affected horses having values exceeding the upper limit of this range.
Other procedures that may be useful in the diagnosis of right dorsal colitis include nuclear scintigraphy, laparoscopy, and exploratory celiotomy. Although these techniques may be useful in selected cases, drawbacks exist for each of these diagnostic options. Radioactivity makes patient management a therapeutic challenge and is not a practical diagnostic method in all cases. Surgical exploration may be indicated if the source of the horse's abdominal pain cannot be reliably determined. It is important to note that horses with right dorsal colitis are not ideal surgical candidates because of their severely low plasma protein concentrations and the concurrent presence of endotoxemia.
Horses with right dorsal colitis may be treated with either surgery or appropriate medical management. Surgical treatment may involve bypass of the affected area or resection of stenotic lesions. Although horses with right dorsal colitis may recover from surgical intervention, those undergoing celiotomy in association with this condition have a more guarded prognosis. Surgical exploration provides the clinician with a definitive diagnosis, determines the severity of the lesion, and rules out other causes of abdominal pain.
Discontinuation of NSAIDs is paramount because the healing of ulcerated colonic epithelium may be impaired in the presence of continued prostanoid inhibition. When horses have concurrent inflammatory conditions (i.e., pulmonary abscess, peritonitis, osteomyelitis) that necessitate continued NSAID administration, ketoprofen may be the safest to judiciously administer. When analgesic properties are required for concurrent disease (e.g., laminitis, long-bone fracture), an agent of an alternative pharmacologic class should be selected. Butorphanol and fentanyl are opioid analgesics that effectively aid in managing severe pain and may be useful alternatives to NSAIDs. Short-term pain relief can be effectively provided by administration of a2-agonists (e.g., xylazine, detomidine).
Of primary importance in horses with right dorsal colitis is dietary modification to reduce the amount of colonic fill and thereby facilitate mucosal healing. Recommendations include elimination of long-stem roughage from the diet to reduce the physiologic load on the colon. Commercially available complete pelleted diets offered in several small meals throughout the day should provide the necessary calories and help avoid boredom. The exact amount to be fed depends on body condition and the specific product and should be initially based on the manufacturer's (label) recommendations and then modified based on response to therapy. An advantage of pelleted diets is that they are primarily digested in the small intestine and thus are low residue. However, these diets provide colonic enterocytes with constituents required for energy metabolism, cell turnover, and effective epithelial healing. Although the average duration of dietary modification is 3 to 6 months, serial monitoring of serum protein (total and albumin) concentrations and colonic wall thickness by repeated ultrasonographic examinations can help determine how long to maintain a horse on a pelleted diet. Some horses may eat other fiber materials (e.g., bedding) when long-stem roughage is restricted from their diet. In these cases, grass or chopped leafy alfalfa may be offered as a supplement to pelleted feed.
Psyllium mucilloid is an additive recommended for managing horses with right dorsal colitis. When psyllium mucilloid is provided over time, GI microbes metabolize psyllium to form short-chain fatty acids (i.e., acetate, propionate, butyrate) that subsequently aid in mucosal healing. Although the exact amount and efficacy of such supplementation have not been determined, the current recommendation is to feed approximately 2 oz daily divided between two of the horse's meals.9 Because of the reported benefits of dietary linoleic acid, a precursor of prostaglandin E2,14 horses with GI ulceration are supplemented with vegetable oil (e.g., corn oil, safflower oil) at a dose of approximately 200 ml/day/500 kg (1,100 lb). Prostaglandin E2 synthesis not only enhances mucosal healing but also may provide protection from gastric ulceration. Therefore, safflower oil has been recommended as a prophylactic supplement in horses receiving NSAIDs to protect against the development of gastric ulcers.
Hypovolemic, azotemic patients require intravenous fluid therapy to help restore adequate intravascular fluid volume and improve the glomerular filtration rate. Azotemia may result from prerenal causes, such as hypovolemia associated with diarrhea, or from direct toxic damage to renal tissue by NSAIDs. Monitoring of serum creatinine concentrations after initiation of fluid therapy can help distinguish between hypovolemia (prerenal) and primary renal disease. Hypoproteinemic patients may also require a plasma transfusion to restore adequate oncotic pressure and offset edema formation that would otherwise occur with concurrent fluid administration. Alternative colloidal agents (e.g., hydroxyethyl starch) may also be administered to maintain oncotic pressure in patients without evidence of altered clotting capabilities.
Therapy with synthetic prostaglandins, such as the prostaglandin E1 analogue misoprostol (2 to 3 µg/kg PO bid or tid), can help epithelial healing and protection. Because high doses of these synthetic prostaglandins may induce abdominal cramping, horses receiving these drugs should be monitored closely. Other complications associated with higher doses of misoprostol include straining during defecation or development of rectal prolapse. Furthermore, because misoprostol is an abortigenic, it is contraindicated for use in pregnant mares, and pregnant owners or handlers should use extreme caution when administering this drug to horses. Sucralfate (20 mg/kg PO qid) has been successfully used in humans with colitis associated with pelvic radiation therapy. Although the effectiveness of sucralfate has not been established in horses with right dorsal colitis, this therapy is effective not only for its potential colonic benefits but also in horses that may have concurrent gastric ulceration. If gastric ulcers have been diagnosed, specific therapy should include the proton pump inhibitor omeprazole (4 mg/kg/day PO) or a histamine type-2 receptor antagonist such as ranitidine (6.6 mg/kg PO tid). Therapeutic recommendations for gastric ulceration should be based on its severity as determined by gastroscopic examination.
The overall duration of medical therapy also depends on the severity of clinical disease. Monitoring should include periodic determination of serum total protein and albumin concentrations and ultrasonographic measurement of colon wall thickness; on average, 3 to 6 months of treatment and monitoring are required in horses with clinically significant disease. When the albumin concentration and colon wall thickness are within the normal range, standard feedstuff can be gradually reintroduced. Colonic stricture formation prevents some horses from tolerating long-stem hay without developing chronic colic; these horses must be fed a modified, low-residue diet for the rest of their lives.
Clinical outcome depends on the severity of ulceration and whether stenosis of the affected region of colon has occurred. Horses with ulcerative disease that are treated appropriately and promptly are generally given a favorable prognosis. In contrast, patients with continuing chronic protein loss, colic, and severe diarrhea have a more guarded prognosis. Horses that require surgery for resection or bypass procedures have a poorer prognosis than do horses with mild to moderate ulceration that have not developed a stricture. In recovered horses, future recommendations should include judicious NSAID administration because it is recognized that these patients have increased sensitivity to this class of agents.
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