Its not the worms.. chickens naturally carry coccidia and it's only a problem when too many have been ingested ie coccidioisis.. via fecal droppings.. they CAN (and too often do) get it in the brooder never having touched the outside ground.Never again will I feed worms to my chickens...I hope that I didn't just kill them.
Out of the 9-11 strains known to effect poultry only one or two will cause blood in droppings.
Medicated feed is a LOW dose of amprolium.. the active ingredient in Corid.. It may prevent coccidiosis.. but it also may NOT be enough to do so and treatment may still be required.