Posted By:
Dan Bretta"A study in the Equine Veterinary Journal found hemorrhaging in the lungs in 95% of horses checked during two post-race examinations. An article in the Veterinary Clinics of North America: Equine Practice Journal states that hemorrhaging in the lungs is "a condition affecting virtually all horses during intense exercise worldwide….there is no treatment that is considered a panacea, and the currently allowed treatments have not proven to be effective." Another study in the Equine Veterinary Journal noted that as long as a horse continues to undergo training and racing, the lungs cannot heal."
http://www.chai-online.org/racing.htm
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So even if they're bred for racing it's apparent their bodies weren't made for racing.