Can you please provide information about the parasite only being in the heads of these fish. We lived on the NW coast of BC of 10 yr and were always under the impression that it was the whole fish.
What I would like to suggest is that you contact Fish and Game in the Seattle, WA area and ask that. Several years ago this was the issued warning on steel heads and I doubt much have changed. At that time the warning specifically mentioned that the parasite was located in the brain of the fish.
Please check with them