The condition is hereditary. Other tests will be done, especially in older animals, because it is important to screen them for underlying disease. Your veterinarian may also culture the skin for a secondary bacterial infection. Unfortunately, a generalized infection is difficult to treat and the prognosis is guarded. Be aware that the treatment course is long and the disease may never resolve completely.
Younger animals do have a better prognosis than older ones.
The dogs older than 2 years may have concurrent systemic diseases and some require long-term maintenance therapy. Therapy will entail treatment of the underlying disease, treatment of the mite infestation, as well as treatment of the secondary bacterial infection. The latter two are best controlled with veterinarian-prescribed antibiotics and miticidal dip therapy. Your pet will not be a beauty queen for a while after treatment. If your pets do not have signs of hair loss and inflammation, they don't even know they have Demodex mites, just as you would never know that you have Demodex mites on your own body.