If the earthworms came from water that was infected with ich, then yes, they could. A single drop of water infected with the parasite, could cause an outbreak.
Interested to know the answer to this also...imo I've never had an infected fish get reinfected, but I think that's due to learning my lesson with that particular group and quarantining etc
I would say yes they can get "infected" again. Ich is not a virus or bacteria that an immune systems can build up against. it is a parasite . healthy fish with strong immune systems may not become infected with ich but they can definitely be a carrier