Arms Runner injured his right front leg and fell in the Grade 3 race on turf Sunday, two days after Santa Anita reopened to racing after being closed for nearly a month.
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That caused a trailing horse, La Sardane, to fall. The 5-year-old mare got back on her feet and didn't appear to sustain any injuries. Jockey Ruben Fuentes didn't ride the rest of the day.
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Arms Runner was tended to by track veterinarians and vanned off. Track stewards said the horse was euthanized.
The accident occurred as the horses crossed the dirt surface during the transition from the hillside turf course to the main turf course.
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