Moth balls are toxic. I read a story once where there was a mother, and her children. All were MR. Come to find out, the mother grew up in a home infested with vermin. Her mother combatted this infestation by scattering mothballs around the home liberally. (Instead of cleaning up the mess!). Well, the daughter grew up to do the same thing, so she and her children were nerve damaged from life long exposure to moth balls.
As for the mice or rats. If you see one, there are most likely 20 that you don't see. Set traps in such a way that the chickens or other animals can not get to them. A covered bucket trap might do well. Switch to fermented feed, and see to it that there is never any left over at night. Even if you remove your pellets or crumble every night, there will be plenty of feed that the sloppy chickens have billed out into their bedding. Be sure your feed supply is kept in a vermin proof container. Rats and mice will both chew through plastic. Rats will even chew through concrete.