Clinostomum - Grugs in Fish

The grubs that sometimes infect bluegills or other panfish are larval stages of a parasite called clinostomum. The adult parasite lives in fish eating birds like the great blue heron. Parasite eggs pass out of the bird and the eggs hatch in the water. Larvae from the eggs swim in the water until they find a snail, and burrow into the snail, where they develop into second stage larvae called cercariae. When cercariae are mature, they leave the snails and swim in the water until they find a fish such as a bluegill or other panfish. The cercariae burrow into the muscle of the fish where they develop into third stage larvae called metacercaria. When a heron eats the fish, the metacercariae develop into adult parasites in the bird and the cycle begins again.

The metacercariae that you see in the fish are killed by cooking, and in any case, they cannot infect people, only birds. In recent years the number of fish eating birds has increased in the Midwest and snail populations have survived well over mild winters. These conditions favor the parasite's life cycle. Until there are changes in the bird, fish or snail populations, it is likely you will continue to see infected fish. In general, it is rare to see crappies infected with this parasite.