Best guess is Cancer.
If it is Prions cleaning your gear can be really important. And almost impossible to do. Steaming and soaking in Chlorine might not get it done.
I've seen something similar at the base of the antlers, but not on the ribs. Usually associated with a castrated Buck, lost his sack on a barbed wire fence or some other accident.
Last edited by MudderChuck; 12-29-2016 at 06:39 AM.