vaccination and supplementation is not within USDA standards for human consumption.
There is no way to tell how or what has been fed to or injected into a horse or other domestic equine species. Many race horses and some show horses are (illegally) pumped full of steroids and all sort of performance enhancing, anti-inflammatory and pain relieving drugs. My own horse has been given Banamine on occasion for visceral gut pain due to colic-like symptoms as have most horses. Many horses are given herbal and joint, hoof and coat supplements. Many of the horses which are slaughtered are very old, very thin and weak or very sick (bacterial infections, fungal infections, cancer, viruses, etc.)
I got into a HUGE flame war about this issue the other day in another DU forum.
We don't eat our pets here in the USA. We don't raise horses, dogs and cats for human consumption.
The horse slaughter houses are owned by foreign companies and the meat is sold in Asia and Europe. Why should we allow OUR horses to be sold for meat in other countries.
BTW, my cats do not eat canned food horse meat.
edit: Add link to previous and very long thread on the topic:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=102x2091857