Every scrap that isn't sold in the supermarket goes into hotdogs. We are talking about chicken butts and cow lips and everything in between. A long time ago the media did some reports of how nasty some of the factories were. I think the FDA or local health dept.s got involved. There were stories about workers throwing dead rats in the meat vats (probably internet rumor).
And fish commonly has worms and parasites in the meat. By the time the fish makes it to the market, they are usually dead. That is why you are suppose to cook things, especially pork.