I live in an area with a high red fox population. I see mange in about 10%-15% of the fox I catch. Mostly it is in the beginning stages affecting the tail, crotch and/or rear legs. I almost always put them up if the mange is not too advanced.

I get a kick out of the folks who recommend building a bonfire and incinerating them or burying them which is hard to do when winter has the ground froze as hard as concrete. I have put up many mangy fox with no issues at all. The relatively few I catch with advanced mange are discarded in an out-of-the-way place on the trap line. I don't think the mites survive very long in freezing conditions once the host is dead.

The percent of mangy fox I catch has been steady over the last 10-15 years so, while I believe it is almost always fatal, I don't think it spreads as easily as some think and it has certainly not caused any population crash in my area.