Generalized demodicosis is a non-contagious disease of dogs and a potentially contagious disease in cats that is associated with an uncontrolled increase in the number of Demodex mites within the hair follicles. The mite itself is a normal inhabitant of all animals. However, some cats and dogs cannot control the numbers of mites and the mite population grows out of control. This inability to control mite population probably relates to an abnormal immune system.
Abnormalities of the immune system have many causes. Some examples are an inherited weakness, hypothyroidism (low thyroid hormone), excessive cortisone production or use, cancer, or unknown causes.
When generalized demodicosis occurs in young dogs (<6 months) it is probably an inherited condition (even though the parents may not have exhibited the disease). In cats and older dogs, it may result from internal diseases affecting the immune system. Testing to help find the cause is very important.
Localized demodicosis occurs as one or more small, round, red, scaly, occasionally itchy areas of hair loss. This form of the disease usually resolves with just topical treatment, but in dogs it may progress due to an inherited weakness. Any area of the body may be affected, however the face, forelegs and feet are the most commonly affected areas in dogs and cats (cats also tend to have the ventral abdomen affected).
In dogs, a chronic demodectic pododermatitis affects the paws and may occur with or without the rest of the body being affected. This is an extremely difficult form to treat or cure. Many times demodicosis is complicated by a secondary skin infection. In these cases, cultures and long-term antibiotics are needed.