Demodex is a mite that is a normal inhabitant of the skin of dogs. It is NOT contagious to other dogs, that's why your other 2 have not been affected. Most dogs can keep the mite populations in check and never have a problem. Other dogs seem to have more trouble. They may have a localized infection, which often resolves on its own without problems, or they can have a generalized infection which involves multiple areas of the body. It sounds like your dog has the latter. Sometimes the condition will resolve once they reach adulthood, in other cases, however, it's a lifelong problem.
Unfortunately, Dogue's are predisposed to generalized Demodex infections. You need to have a talk with the breeder, as this is something in the line. Dogs with generalized Demodex should NOT be used for breeding purposes, as there is a hereditary component to the disease.
I've included a web site link that should be helpful to you. At the bottom of this link is another for Mar Vista Animal Medical Center- this is full of great information for you.
http://www.canadasguidetodogs.com/mastiff/doguearticle2.htm
I hope this was helpful.
