There aren't very many parasites that are carried by earthworms. Its chemical that you need to worry about. Like Maharg said dyes, artificial scents, etc. Get q small bowl and place some de-chlorinated water in it and then grab a few paper towels. Dip the selected worm in Ty. water to rinse the soil off and then dry it a little with a paper towel. This helps remove some of the slime from the worm as well. Then offer appropriately sized pieces of the worm to your frog.