Wow, Beth,... I am so sorry to hear about what happened to Festivus!
If the mouse was the wire carrier, it would seem (to me) that the wire must have been on the outside of the mouse ... especially to affect Festivus' mouth/throat.
I don't know but ... it is, certainly, not something that someone would expect to be there &/or would be looking for.
This is such a freak accident, and is a bit of a shock, so that I just keep thinking ... Wow.
I know how difficult this must be, for you, and my heart goes out to you.
I really pray that Festivus remains on the mend and gets well soon!