monensin poisoning in horses

It’s a terrible feeling to know something you’ve fed your horse has inadvertently made him sick. In some instances, however, it happens.

Monensin is an antibiotic feed additive mainly used to promote growth in cattle and poultry. While it can have positive effects on those and other livestock species, it’s highly toxic to horses. Although monensin can affect all cell types, the toxic effect is primarily observed in muscle cells. Clinical signs of consumption can include a loss of appetite, rapid or irregular heart rate, exercise intolerance, sweating, colic, and unexpected death.

Sporadic intoxications are usually caused by accidental feeding of commercial monensin-containing feed intended for cattle or poultry, or from contamination of horse feed with monensin during production.

Some horses that consume monensin eventually recover—a few even return to their previous performance level—but it hasn’t been clear how the toxicity impacted their hearts and subsequent athletic performance. So researchers recently gathered information on the cardiovascular and athletic outcomes of horses exposed to monensin. Clémentine Gy, DVM, IPSAV, an equine internal medicine resident at the University of Montreal Faculty of Veterinary Medicine, in St-Hyacinthe, Quebec, Canada, shared the results at the 2018 American College of Veterinary Internal Medicine Forum, held June 14-16 in Seattle, Washington.

Gy and colleagues conducted a prospective study on 76 horses exposed to monensin-contaminated feed. They evaluated four horses in the acute phase (less than two weeks after exposure), 29 horses in the subacute phase (15 to 45 days after exposure), and 70 in the chronic phase (after four to 10 months of rest following exposure). On each horse they performed a physical examination, exercise stress test, electrocardiography (ECG, recording of electrical activity as transmitted through electrodes placed on the skin) before and during exercise, and echocardiography (heart ultrasound) before and after exercise. They also followed up on 56 horses with telephone interviews about 16 months after exposure.

The team found:

  • Cardiac abnormalities (for example, increased or irregular heart rate, abnormal echocardiography) in 4/4 (100%), 19/29 (65.5%), and 31/70 (44%) horses during the acute, subacute, and chronic periods, respectively.
  • Sixteen months post-exposure, 34 of the 64 horses (53%) for which this information was available had return to their previous function, while 13 (20%) were exercise-intolerant, and three (5%) were retired.
  • Two horses died (one in the acute and one in the chronic period), and 12 horses were euthanized (two, one, and nine in the acute, subacute, and chronic periods, respectively) during this outbreak (22 %).

“When feed contamination occurs, it is generally recommended that exposed horses be quickly identified and rested in order to prevent further cardiac damage and protect potentially endangered riders,” Gy said. “An examination by the regular veterinarian is also recommended for cardiac evaluation.”