and i guess nobody picks up on the deafness ??
Hard to tell with ferrets for the most part mate, especially if you don't have many, or you havn't had many, you probably assume it's normal. They are very good with their sense of smell etc. I have a blind old girl here, works fine, runs about the court fine, does everything the other ferrets do. But you need to speak to her when you approach her. And she knows the difference between me and my friends, my girlfriend or my old man, by voice and smell.
I'm also myself not 100% sure it always causes total deafness, but it definatly wea