I have been researching why my chickens have been doing the head shaking and that open mouth thing and I have concluded that after a summer for indulging in earthworms a few of my birds have gapeworm. I don't see any signs of respitory infections. No eye or nasal discharge. I haven't lost a bird. My birds are mostly hatchery stock just used for eggs so I haven't brought in any started birds. What is the best treatment or wormer to use for this?