Earthworms do carry amphibian parasites. They can sometimes be found in the soil where the worm lives. Of course once the worm eats the soil it also ingests the parasite's eggs which they become larvae within the worm and once eaten by the true host they become adults and breed. Even store bought worms can contain them depending on the source of the worms. Parasites are literlly everywhere and are encountered everyday, but most of the time the body destroys them before they can become a threat.