While using canned tuna to make salad, I remembered a Youtube video about sushi-grade tuna. If I recall correctly, the main criterion was if it contains parasites and that it's rather hard to find a clean fish, which makes them so highly priced. Especially the tail was heavily infested when they sliced up an example.  Now, since I have nothing better to do while eating, I thought I could ask how likely it is, that I'm currently eating some cooked tuna parasites? 
TL;DR - Yes, the "monster soup" is pretty much a real thing. To elaborate, I assume that we are talking about canned food in general, as there's nothing specific about canned tuna except that it's meat, which means it'll definitely go through Pressure Canning  The industrial canning process is all about Preservation by Sterilization i.e. putting the food in the can, sealing it, ensuring that it's leak proof (air tight) and then heating the under high pressure, high temperature (more than ~ 120°C / 248°F) so that all pathogens and bacteria in it die Retort packaging e.g. Capri Sun also undergoes a similar process Tetrapac is very similar to retort packaging except it relies on air tight / leak proof packaging and the food is only heat sterilized before packaging (part of the reason why orange juice in such packaging is always devoid of vitamin C) As long as the container isn't damaged and within it's expiry date, it should be ideally safe for consumption  If you're concerned about The Dead Parasites  Yes, some parasites die & remain in the fluid as dead matter and keep floating in Brownian motion (click the link to watch the video) Bacteria spores survive canning which remain dormant until the conditions become favorable again i.e. until you open the can (that's why they advise you to consume as soon as possible or refrigerate immediately)  PS:  About the criterion you mentioned about quality of tuna / infested with parasites, I guess, that must be regarding the concern for transporting. Tuna fish is too big, too hard to catch & contain, and it is is expensive too. Tuna is usually captured & transported from Japan and if it's already infested with parasites, then it may not survive the journey and the dry ice doesn't help much as the fish is exposed. For the same reasons, some top chefs prefer (small) fishes shipped alive, Tuna fish is too big for that.  I became familiar with the Victorian age fantasy word The monster Soup while watching Manu Prakash's Foldscope. The word is a reference to the microbes present in water observed under a microscope.  Boiled Dirty Water under the Microscope - This video is cool. I'd definitely recommend watching.  In a comment, user: @roetnig has said it correctly - Parasites can be found in any living being (including humans). On the exterior e.g. skin, hair, orifices and even inside the digestive tract. But they are never (or rarely) found in the flesh or blood because the body's defense mechanism gets rid of them. If an animal's flesh or blood contains parasites, then it can mean 2 things: either that animal is suffering from some serious infection or it has been cut open. Most frequently, the latter is the case. Consider it similar to how people get Salmonela, they usually ingest it by consuming contaminated food or water. The doctors surgeons steralize their equipment before the surgery just to prevent the patient dying from bacterial or other parasitic infections.   