California racing saw a sharp increase last year in the number of fatalities due to cardiac failures. Eleven horses were reported as having died in this manner during fiscal year 2012 (July 1 through June 30) compared with six for the same period in 2011 and four in 2009, according to a California Horse Racing Board (CHRB) annual report.

Equally concerning is that a reported seven out of the 11—or 64%—of the horses that died from cardiac failure statewide had been stabled with leading Southern California trainer and Hall of Famer Bob Baffert, according to several trainers and exercise riders who watched the horses drop during training hours.

Baffert did not respond to questions by email or return phone calls. The identifications of the horses that died have not been released by the CHRB.

Cardiac failure in Thoroughbred racehorses is a relatively rare occurrence, according to trainers and racetrack veterinarians. A study published in 2010 in the Equine Veterinary Journal on sudden death in racing Thoroughbreds found it was responsible for 9% of fatalities in California. This same study showed 96 reported sudden deaths between Feb. 1990 and Aug. 2008 in California among Thoroughbreds while they were exercising, or an average of five per year. During the 18-year period, a total of five were reported in Pennsylvania; 23 in Victoria, Australia; 16 in Sydney; four in Hong Kong; and none in Japan.

"It’s disturbing and unnatural," said one trainer based at Betfair Hollywood Park, in Inglewood, Calif., who spoke on the condition of anonymity about the number of sudden deaths he’s seen. One of his own horses