I can't say that I have ever had any issues with parasites in blood worms. I used them regularly as a dietary staple for years and had no problems. Typically you run in to problems if the worms, or any food for that matter, is not kept at minimum temperatures over time. To kill parasites in foods a sub zero temperatur must be ahieved for 30 to 48 hours. Never feed a frozen food that has been thawed for any extended peiod of time. Keep stuff frozen!
Best wishes!