It can be quite worrying and sometimes disasterous if the water envirornment in which the fish live is rife with the parasite. Usually if the river section has a lot of water snail life, then a good number of fish, especially bottom feeders like barbel may suffer more so. However, barbel can live on totally blind using their barbels (barbules) and underslung mouths to find food and survive. The higher quality of river water, the less you will find them. Again most streams and rivers have sewage or chemical input so you can find these blind fish anywhere although not in great numbers, unless there is a water quality problem with the stream or river etc.
If the parasite attacks the eyes of mid-surface water, sight feeding fish, then they will find it difficult to find food and will die off much quicker due to starvation. In fact complete shoals of fish can be wiped out if the parasite takes hold especially in lakes and ponds.
I was studying this eye problem back in the 1980's/90s and have hundreds of pics of barbel eye problems of different fish from different rivers and have pieced these together like a jigsaw puzzle from the beginings of eye defects to the final blind outcome, if that makes sense.
Another earlier indicator is the cloudy/cataract pupil (black bit) which looks whitish. The whitish is the population of white worms inside the eye.
I am also sure that the parasite has evolved over time (from what is scientifically written) where the bird host has been eliminated from the cycle, hence the bursting of the eye in the water environment.
It is worth paying ongoing attention to the fish you are catching and the stretch you are fishing, as it might be indicating a problem that could get worse.
See below.