Earthworms can carry Hepatic worms (Capillaria sp.) that can affect liver function in snakes. As far as avoiding them, the best way is to raise your own worms or make sure that you get them from a dealer that who has good healthy stock. As far as sweezing the "muck" out of them, I've heard different things on this. It won't help prevent any parasite problems, but some say to let the worms "empty out" before feeding them. I honestly disagree with this. As long as the worms were raised in a good healthy media (another thing for raising your own) this is probably beneficial to the snakes, by providing them with all kinds of minerals and such in the digesting matter in the worm's gut. I try to even make sure that my media is very rich in minerals and such so that I'm actually "gut loading" my worms to be fed to the snakes. Hope this helps.
Roy