I can tell you about two different types of US Slaughterhouses: The typical beef slaughter that Only slaughters cattle who have reached market weight, and the opposite extreme: Cull dairy plants. There are of course, slaughter plants that do a little of everything, or whatever comes in, or mostly beef and some odd balls, or what is called custom slaughter, which is what the owners bring in and take home for their own use.
Okay, so the beef animals typically come from a feedlot. Almost all of them have had some degree of respiratory illness (ex- 'Shipping Fever') and were treated with antibiotics (most or all, depending on the feedlot). You generally have great carcasses, with little pathology. But you can still have different diseases in otherwise healthy animals (say an abscess that was not noticed or is newly developing). Trim and eat, and no problem. You might get a cow or steer that was failing to thrive in the feedlot, or didn't recover from that shipping fever, or whatever. Sometimes these animals don't look that bad UNTIL YOU CUT THEM OPEN. They hide illness if they can, either because they are stoic or they don't want predators seeing them sick, domestic or not. It has nothing to do with ethics, necessarily (there are ethical and non-ethical producers and slaughterhouses, but that's got nothing to do with this discussion per se.) Now, can you have a baldie steer with cancer eye? Heck yes. 'Cancer eye' is technically a cancer of the tissues around the eye. Trim and eat, assuming no spread of the cancer. Got a cow with lymphoma? Usually condemned because lymphoma spreads fast. Got a healthy animal with a single lymphoma like lesion but is otherwise healthy? They might do some testing, but otherwise it is trim and pass. I was not exaggerating when I said if you've eaten meat in the US, you've had an animal with cancer at some point. Some of these cut offs for cancer in the US is more a technicality of when, visually, do we want to throw something away vs save it. It's not like a steer with one lump makes good/okay meat and a steer with two lumps has suddenly turned into a poisonous thing.
Now, a cull dairy plant? You name it, they've got it. It's really hideous. First of all, some cows come in and their feet have not been done in months (and it's VERY obvious). They have slipper feet, are in pain, and can barely walk from the pain and the mechanics of having horrible feet. Any idiot farmer can learn to do his cow's feet very nicely, but apparently many don't want to bother or their cows are 'too wild' or whatever. Anyways, lactation is very draining on the metabolism, especially for cows breed to produce, produce, produce. So the cull cows are usually skin and bones with some meat holding them together. Then there is so much pathology, the USDA inspectors are kept very busy. So focusing on the cancers, it's the same thing. Anything they can have they will. Now, any animal that is wasting away actively and rapidly such that the fat is breaking down systemically and pathologically (you can see this difference between the fat) means the carcass is getting tossed out.
I've heard that with systemically ill animals you have to cook them well to get the bacteria/virus/ AND TOXINS out of them, but I've not researched that to see if it's true. For a simple/basic/early infection, I'm sure it is, because there's no visual way to identify them in the millions of animals inspected and they go out in the food chain. I'm guessing anyone trying to survive off wild animals is just going to cook well and go for it.
