The old timers in my area always had shellcrackers in their ponds to decrease/stop this. Apparently they eat the snails that are required for the life cycle of the parasite. It does seem to help in my ponds. We hadn't had this problem (had shellcrackers) and then the otters killed out one of my ponds. When the fish recovered, there were no shellcrackers and we had these parasites for the first time.