Objective - To evaluate the effects of morphine administration for 6 days on gastrointestinal tract function in healthy adult horses. Animals - 5 horses. Procedures - Horses were randomly allocated into 2 groups in a crossover study. Horses in the treatment group received morphine sulfate at a dosage of 0.5 mg/kg, IV, every 12 hours for 6 days. Horses in the control group received saline (0.9% NaCl) solution at a dosage of 10 mL, IV, every 12 hours for 6 days. Variables assessed included defecation frequency, weight of feces produced, intestinal transit time (evaluated by use of barium-filled spheres and radiographic detection in feces), fecal moisture content, borborygmus score, and signs of CNS excitement and colic. Results - Administration of morphine resulted in gastrointes tinal tract dysfunction for 6 hours after each injection. During those 6 hours, mean ± SID defecation frequency decreased from 3.1 ± 1 bowel movements in control horses to 0.9 ± 0.5 bowel movements in treated horses, weight of feces decreased from 4.1 ± 0.7 kg to 1.1 ± 0.7 kg, fecal moisture content decreased from 76 ± 2.7% to 73.5 2.9%, and borborygmus score decreased from 13.2 ± 2.9 to 6.3 ± 3.9. Mean gastrointestinal transit time was also increased, compared with transit times in control horses. Conclusions and clinical relevance - Morphine administered at 0.5 mg/kg twice daily decreased propulsive motility and moisture content in the gastrointestinal tract lumen. These effects may predispose treated horses to development of ileus and constipation.
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