Animals coming off of U.S. racetracks and out of pastures are injected with prohibited substances on a routine basis, and that alone makes this type of meat unsuitable for import. These animals were never bred or raised for the table, but for other purposes, and they should be disqualified from the meat trade.

http://www.nytimes.com/2013/03/12/opinion/global/questions-for-the-global-horse-meat-industry.html