Starting with speculation
Francisco Uzal, DVM, PhD, professor of clinical diagnostic pathology at UC-Davis School of Veterinary Medicine and the director
of the California Animal Health and Food Safety Laboratory System is trying to determine the cause of the racetrack deaths
in California. As a part of looking into the recent racetrack fatalities and those occurring over the past several years regarding
sudden cardiac death, he and his team have conducted pathologic and histologic examination of cardiac tissues and extensive
toxicologic studies have been conducted. "But we have not been able to find the cause of some of these recent cases of sudden
deaths," Uzal says.
According to the Daily Racing Form, traces of anticoagulant rat poison were found in two of the horses. Both horses had internal hemorrhage problems, though
whether this was related to the deaths has not been confirmed.
Despite this fact, Uzal does suspect that at least some of the sudden deaths seen in the general horse population may be produced
by arrhythmias. "But in many cases, we do not see morphological changes in the heart muscle or anywhere," he says. "Many times
we speculate that changes in the cardiac conduction system may be responsible, but it is pure speculation. What we frequently
see on postmortem is almost no cardiac pathology in sudden death in horses, making these cases somewhat of a mystery."
The cluster of sudden racetrack deaths in southern California is a different issue than atrial fibrillation (AF) ("How veterinarians
can address atrial fibrillation in horses," July 2013 dvm360). "With exercise, AF should not cause sudden cardiac death," says Virginia Reef, DVM, DACVIM, director of large animal cardiology
and diagnostic ultrasonography at the University of Pennsylvania's New Bolton Center. "You worry that if there is nothing
found at postmortem, or if they find changes in the myocardium, possibly there was a fatal ventricular arrhythmia that triggered
it," Reef explains. "Exercising horses have been shown to have ventricular arrhythmias that occur immediately after peak exercise,
just as they are slowing down.
"Why there was a cluster of apparent sudden cardiac death in southern California is unknown and probably very difficult to
determine," Reef continues. "It may be that a toxin, virus or drug interaction was involved, or it may be an adverse consequence
of the maximal effort performed. If any of these horses had ventricular arrhythmias, and if the arrhythmia was more malignant,
it could have triggered a cardiac death episode."
Just as with human athletes, the circumstances surrounding the episode and the postmortem findings may provide little information
on underlying cause, says Physick-Sheard. Reef agrees. "In these cases the term 'sudden cardiac death' is applied," she says.
"It is assumed the primary cause is a serious disturbance in heart rhythm, but evidence is lacking because affected animals'
ECGs are not being monitored at the time of death."