would likely result from iatrogenic administration in the horse, it is imperative that the clinician is cognizant of clinical signs associated with CNS excitation then depression, GI stasis causing episodes of colic, and decreased pulmonary/cardiovascular activity.
Dermorphin administration.
Electrolyte imbalance.
CNS Stimulants (amphetamine, methamphetamine).
CNS depressants (benzodiazepine).
Colic of other etiology.
Ethanol.
Ethylene glycol.
Ivermectin.
Marijuana toxicosis.
CBC/Serum Chemistry
Hypoglycemia.
Elevated BUN.
Other Diagnostic Tests
Arterial blood gas.
LC‐MS.
Detoxification
Remove the source of the toxicosis.
Provide supportive care as needed.
If suspected, oral overdoses could be evacuated via nasogastric intubation. Once evacuated, administer activated charcoal at 1 g/kg with mineral oil and water to help decrease absorption of the medication, and assist with the potential GI stasis/impaction.
Appropriate Health Care
Monitor cardiovascular and respiratory activity. Assisted ventilation may be essential if severely overdosed.
Gastrointestinal activity may be greatly compromised and monitoring for possible GI stasis should be considered.
Antidotes
Naloxone 0.01–0.02 mg/kg up to 0.05 mg/kg IV bolus. Note: the half life is 1–1.5 hours and may have to be repeated because its half‐life is shorter than morphine.
Butorphanol (for pure mu agonist) – 0.01–0.1 mg/kg IV, IM.
Drugs of Choice
IV fluids as needed for volume expansion and dehydration.
Cardiovascular:Atropine (anticholinergic, but will decrease gut motility) – 0.02 mg/kg IV.Glycopyrrolate (anticholinergic, but will decrease gut motility) – 0.005 mg/kg IV.
GI Protectants:Omeprazole 2–4 mg/kg PO q24h.N‐methylnaltrexone (research only) – does not cross blood–brain barrier and may reduce negative GI effects when a mu agonist such as morphine has been given.
CNS signs:Detomidine 0.02‐0.04 mg/kg IV or IM.Xylazine 1.1 mg/kg IV; 2.2 mg/kg IM.
Precautions/Interactions
Pethidine (Meperidine) is short‐acting. Can cause seizures if administered IV. Diazepam or pentobarbitone would help control these clinical signs.
Avoid use in horses with renal insufficiency.
Fentanyl has been noted to be a heavy respiratory depressant, and mechanical ventilation should be used with gas anesthesia.
Caution should be used if administering other depressants with opioids as the negative side effects could increase. Benzodiazepines should be avoided.
Ethanol is contraindicated.
Respiratory depression in neonates has been described when morphine has been administered to mares prior to birth.
Prevention/Avoidance
Limit opioid use and monitor closely when used.
Educate clients regarding adverse effects in horses.
Possible Complications
Seizures.
Hyperthermia.
Expected Course and Prognosis
Prognosis for recovery is good if respiratory and cardiovascular functions are maintained.
Abbreviations
See Appendix 1 for a complete list.
Suggested Reading
1 Boscan P, Van Hoogmoed LM, Farver TB, et al. Evaluation of the effects of the opioid agonist morphine on gastrointestinal tract function in horses. Am J Vet Res 2006; 67:992–997
2 Boscan P, Van Hoogmoed LM, Pypendop BH, et al. Pharmacokinetics of the opioid antagonist N‐methylnaltrexone and its effects on gastrointestinal tract function in horses treated or not treated with morphine. Am J Vet Res. 2006; 67:998–1004
3 Combie JD, Nugent TE, Tobin T, Pharmacokinetics and protein binding of morphine in horses. Am J Vet Res 1983; 44(5): 870–874.
4 Gupta GC. Veterinary Toxicology, 2nd edn. San Diego: Elsevier, 2012.
5 Matthews NS, Carroll GL, Review of equine analgesics and pain management. AAEP Proc 2007; 53:240–244.
6 Reed SM, Bayly WM, Sellon DC, ed. Equine Internal Medicine, 4th edn. St Louis: Elsevier., 2018.
7 Roger T, Bardon T, Ruckebusch Y. Colonic motor responses in the pony: relevance of colonic stimulation by opiate antagonists. Am J Vet Res 1985; 46:31–35.
8 Van
