Keeping the house free from vermin is part of the implied warranty of habitability that's part of all residential leases. Essentially, the landlord must keep the rental fit for living.
If mice are getting in because of deficiencies in the home's structure, then it's the landlord's responsibility.
If a tenant is inviting mice by living in a dirty house, leaving doors open, not cutting the grass, etc., then it's the tenant's responsibility.
Good luck getting this sorted out!
Note: This is informational only and does not create an attorney-client relationship.
Leo M. Mulvihill, Jr.
Attorney and Counselor at Law
2424 E York Street, Suite 316
Philadelphia, PA 19125