Well that sounds like an odd thing, I have no Idea the only thing thst crosses my mind is some kinda worms snakes can have worms, you could take a sample of his potty and have it lookef at by a vet. On those live Rodents to many times the snake gets bit he might just be afraid of his food at some point on my Balls I got them all switched over to F/T its alot more safe, Ive seen some pretty bad damage that live rats and mice have done to Snakes it scared me after seeing the pictures.

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