Anticoagulant Rodenticide and Sudden Horse Deaths

From December 2012 through September 2014, six sudden deaths on four California racetracks were due to idiopathic hemorrhage. Necropsy results revealed traces of anticoagulant rodenticide in all six horses’ liver tissue.

Photo: Anne M. Eberhardt/The Horse

According to a 2011 study published in Equine Veterinary Journal, sudden death accounted for 3.5-19% of all flat horse racing fatalities. The main culprits might come as no surprise—cardiac failure, pulmonary (lung) failure, and pulmonary hemorrhage. But veterinarians with the California racing necropsy program have recently discovered another, less obvious cause: rat poison.

Rick Arthur, DVM, the equine medical director at the University of California, Davis, School of Veterinary Medicine, and colleagues took a closer look at sudden death incidents caused by rodenticide and racehorses’ risk of exposure to the poison. He presented his findings at the 2015 American Association of Equine Practitioners’ Convention, held Dec. 5-9 in Las Vegas.

Over 22 months of racing from December 2012 through September 2014, six out of 374 sudden deaths on four California racetracks were due to idiopathic (having no obvious cause) hemorrhage. Necropsy results revealed traces of anticoagulant rodenticide in all six horses’ liver tissue. In essence, these substances inhibit the horses’ blood from clotting, causing massive internal bleeding.

The levels of anticoagulant rodenticide found upon necropsy, however, were well below what’s considered toxic in horses, Arthur said. Thus, he hypothesized, “strenuous exercise might alter the toxic threshold for anticoagulant rodenticide in these horses.”

These post-mortem discoveries prompted the California Horse Racing Board (CHRB) to launch extensive investigations to understanding the poison’s source. They evaluated each track’s pest control program and found that three out of the four racetracks used commercial vendors to set bait out in sealed and secure stations, away from horse’s stalls. The fourth offered mechanical traps to trainers as needed.

“All commercial vendors reported that they had instances in which the bait stations had been broken open and the bait looted or otherwise removed,” said Arthur.

The CHRB also interviewed barn personnel, some of whom admitted to commonly distributing anticoagulant rodenticide around the barns. “From statements made to investigators, unauthorized anticoagulant rodenticide use occurs because barn personnel considered rodent control efforts inadequate and believed it was necessary to take matters into their own hands,” said Arthur. “Based on interviews, barn personnel were unaware that anticoagulant rodenticides presented a risk to the horses.”

For instance, one groom showed investigators where he had placed bait in the rat holes in his horses’ stalls. Another made mud poultices for use on the horses in a former rat poison bait bucket.

At the end of the investigation the CHRB established that rats are clearly a frustrating problem at the barns they investigated and that the most likely source of equine exposure is from barn personnel doing their own rat control.

Because toxicity thresholds are so low in horses, Arthur recommended strictly monitoring anticoagulant rodenticide use at racetracks and other locations stabling strenuously exercising horses. And “anticoagulant rodenticide toxicity should be considered in sudden death cases with idiopathic hemorrhage,” he said.

About the Author

Alexandra Beckstett, The Horse Managing Editor

Alexandra Beckstett, Managing Editor of The Horse and a native of Houston, Texas, is a lifelong horse owner who has shown successfully on the national hunter/jumper circuit and dabbled in hunter breeding. After graduating from Duke University, she joined Blood-Horse Publications as Assistant Editor of its book division, Eclipse Press, before joining The Horse.

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