I got some salmon that also had worms.  I cut it open and saw them squirming around.  I saw some tuna in a store that had a sign that said shushi grade.   I asked them what that meant, and the man told me that shushi grade is fish that had been frozen.  I also checked on the Internet and found this    http://www.sushifaq.com/sushi-sashimi-info/sushi-grade-fish/   

I think everyone already has parasites, especially if you're going to eat raw meat, so I take food grade diatomaceous earth to keep them in check.  If you decide to take diatomaceous earth, make sure it's food grade.