Recent study results showed donkeys only had mild and brief pain relief when given a low dose of morphine designed for horses. Higher doses of morphine, though, led to better and longer-lasting analgesia—without the higher heart rates, increased body temperature, excitability, and other side effects that occur in horses on high morphine doses, said Jill Maney, VMD, of the Ross University School of Veterinary Medicine’s Clinical Sciences Department, in Basseterre, St. Kitts, the West Indies.

“I would encourage using higher doses of morphine in donkeys than horses in order to achieve a clinically useful analgesic effect and duration of action,” she said.

Researchers have previously shown donkeys respond differently than horses to many drugs, Maney added. “When we are treating a painful animal, it is extremely important to choose drugs and doses effective for the species,” she said. “Morphine is a common opioid analgesic, and this information will improve veterinarians’ ability to alleviate pain in donkeys.”

To investigate this concern Maney and her fellow researchers gave eight healthy, donkey geldings ranging from 3 to 9 years-old—all belonging to the Ross University teaching herd—two different doses of morphine: The typical horse “low dose” of 0.1 mg/kg and a high dose of 0.5 mg/kg. The donkeys were randomly assigned to an intravenous saline group or an intravenous morphine group and switched to the opposite group after one week.

After each morphine administration, a researcher unaware of the donkeys’ treatment status assessed the animals’ pain thresholds four times during the first hour after administration, then every 30 minutes for the next three hours and once an hour for two more hours. To do this, the veterinarian placed a pressure boot on each donkey’s forelimb that provided gradually increasing pressure and, presumably, pain when the donkey lifted his leg.

Meanwhile, the researchers took blood samples from the donkeys at 2, 5, 10, 15, 30, 45, 60, 90, and 120 minutes after morphine administration to check for concentrations of morphine and morphine metabolites, Maney said.

They found no significant increases in heart rate, respiratory rate, or temperature in these donkeys following morphine administration, she said. The animals also showed no signs of colic—abdominal pain—or lack of appetite. In addition, neither treatment caused significant abnormal reactions.

The donkeys showed only mild pain-relief effects with the low dose of morphine, Maney said. At the higher dose the donkeys’ pain threshold increased—without negative consequences—for about five hours. “The high dose provided a long-lasting change in pain perception without the more significant side effects seen in horses,” she said.