Moved from Cuterebra.

In WI you have two subspecies of mouse bots. You would have needed to rear this in soil for the winter to get an adult fly- in order to seperate out the two species. Generally you see Cuterebra fontinella fontinella in white footed mice and Cuterebra fontinella grisea in deer mice. But it is for sure one of these. Fairly common in mice in Aug-Sept in WI. See other posts on Bugguide to see what the adults look like. The maggot will emerge quickly after the death of the host and try to pupate in the soil. Yours look like they might have been big enough to overwinter. So readers will be happy that the fly will likely survive...can't say as much for the mouse.

We remove these larvae from rabbits, cats, and dogs. Not common but always a crowd pleaser. We have a sick staff of people in our veterinary office who are fascinted by this kind of stuff!

Nice find, but seriously, thats revolting

I have seen these before in mice and squirrels. Maybe someone else here can help you out more on an exact name.