Look up black spot disease. It’s a parasite. Infected catfish look like they have 1/8” ink spots on them.
A bird eats an infected fish, it grows in them and is passed to the water through their droppings. From there it goes to, typically, a snail and then to the fish when it eats the snail. Infected fish are supposed to be ok to eat if cooked enough to kill the parasite. This is just based on what I’ve read on it yesterday, so I could be misinformed. We caught 5 or 6 catfish yesterday and all had it.