Just a quick comment or 2. I’ve seen the “yellow grub” in just about every fish at Pelican and have known that they were harmless to humans. The grub is less during Winter and spring. I’ve also seen the red worm in Mantua just like shown - in the body cavity,- I think. I’ve seen those “red worms” at Blue Lake ever since going there for many years and have asked the DWR about them but never got a good answer. My question is - at Blue Lake they are in the flesh like the “yellow grub” and appear as black/dark spots that when cut out and examined are curled up “red worms”- not in the body cavity, at least that I’ve noticed??? They maay have been in the body cavity and I just didn’t notice since when I clean em I’m filleting and they are much more apparent then. Are they the same? I agree, I’ve not seen them in the Tilapia, wonder why? I always cut em out of the flesh with the point of my fillet knife just to be sure but aam sure I’ve eaten a few that I’ve missed.
Leaky
I know this will be hard to beleive but some fish especially ocean fish carry certain worms. You might not be able to see them but they are there trust me i know . They are harmless to humans i know this because there no cases of people getting sick from eating a Mcdonalds fish sandwhich.The larger the fish the more likely they carry worms of some type. Certain fish like amberjacks and almacos carry worms once they past a certain size. I love to eat amberjacks and almacos but i only would eat the smaller ones its just a preference. I think if there was a problem with the worms we would have known by now.To eat or not to eat is the question!! 
Re question 1; "at Blue Lake they are in the flesh like the “yellow grub” and appear as black/dark spots that when cut out and examined are curled up “red worms”
The number of species in the genus Philometra is fairly large. Each species has a slightly different pattern of infestation. Some preferentially encyst in the gonads, some in the eyes, the flesh, others live free in the peritoneal cavity or in multiple sites at the same time. The link I posted before did list that red worms do migrate into flesh. Here are two additional links describing Philometra species living in the flesh of fishes.
http://www.int-res.com/…/dao/15/d015p081.pdf
http://fwie.fw.vt.edu/…is/lists/M010093.htm
The below quote from this link is informative. The flesh from a species of sole is the subject. “A “wormy” appearance presents a similar marketing problem when adults are infected with the nematode Philometra americana. Although no mention was made of their effect on the health of the fish, neither of these conditions is harmful to human consumers.*”
Re question 2; “I’ve not seen them in the Tilapia, wonder why?” I have no idea on that one.
As I did with the yellow grubs, I would be happy to submit affected tissue to a veterinary fish pathologist. I would need to have a fresh frozen sample and about $15 to defray the cost of the pathology reading. I still am confident that these parasites are not a threat to human health.