From: Rabbi Moshe Sofer
To: Unknown recipient
A rabbi identified as P.R.M. (son of a distinguished Gaon) ruled that a chicken bitten by a rat on the head could be retained for twelve months, claiming the Rema's prohibition in Siman 57 was merely a Polish custom. The Chasam Sofer firmly rejects this, ruling the bird must be slaughtered immediately and its skull examined, and insists the Rema's rulings bind all Ashkenazi communities universally, warning that geographic limitation of the Rema's authority would undermine traditional practice.
Source: Shu"t Chasam Sofer YD §59