I too would like to know what it is. I was at Red Feathers Dowdy 08/04- 08/05 and every rainbow trout I caught had red lesions on the body. It looked like there was a clear worm in the center of the red spot. I fished Dowdy three weeks back and all the fish were fine. I released all the fish.
Lots of stress on front range waters due to high water temps, this stress will cause some fish to be more vulnerable to disease and parasites , but the wounds very well could be from predators trying to make them a meal, it is a shame but the strongest will survive .
like FOM said, you will rarely see this in winter or even fall or spring, but dog days of summer breed bacteria, weather it's a wound from a preadator that gets infected or caused by an actual parasite I couldn' say but you see it MUCH more in summer. Back home we used to call them warm water marks.