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