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The veterinary mystery of the California cluster
Equine researchers try to understand a spike in sudden deaths, possibly cardiac-related, among thoroughbreds.


Determining the cause of the California cluster

UC-Davis' Arthur says pathologists at California's Animal Health laboratory are conservative, so they won't categorize a sudden death as one diagnosis or another unless they have a definitive explanation.

"California has an extensive postmortem necropsy program," he says. "If you look at the nonmusculoskeletal sudden death incidents, there have been about 56 of them since July 1, 2010, but only 15 of those were from racing horses. As California has approximately 40,000 to 50,000 starts per year, that's about one out every 10,000 starts—less than 5 percent of all fatalities by starts. We also had during that same period about 40 horses die suddenly during or after training. Since a horse trains about 30 to 60 times per race start, having one of these untoward events when a horse is training is more likely."

A careful look at these cases indicates that a ventricular conduction anomaly occurring after training or racing (as per Physick-Sheard and McGurrin's work) may be at work, says Arthur. "We suspect that this represents a large portion of the sudden deaths where we do not have a definitive diagnosis, which is approximately one-third of the sudden deaths.

"One point to remember," he continues, "is that the pathologist frequently finds pulmonary hemorrhage in a lot of these horses, and myocarditis/myocardial degeneration as well, even if they were put down because of a fetlock fracture. So it is often difficult to determine the cause of death in every case due to one particular finding. A finding does not mean a cause of death."

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.


1. Lyle CH, Uzal FA, McGorum BC, et al. Sudden death in racing Thoroughbred horses: an international multicentre study of post mortem findings. Equine Vet J 2011;43(3):324-331.

2. Physick-Sheard PW, McGurrin MKJ. Ventricular arrhythmias during race recovery in Standardbred Racehorses and associations with autonomic activity. J Vet Intern Med 2010;24(5):1158-1166.

Suggested Reading

1. Gelberg, HB, Zachary JF, Everitt JI, et al. 1985. Sudden death in training and racing Thoroughbred horses. J Am Vet Med Assoc 1985;187(12):1354-1356.


Source: DVM360 MAGAZINE,
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