The demodex mange mite is a normal parasite to dogs. When the immune system is suppressed due to immaturity or illness the mites can become overpopulated causing hair loss. The hair loss can be localized resulting in patchy bald areas on the face and the legs or generalized all over the body. They do not usually cause itchiness. Demodectic mange is diagnosed by scraping the skin and looking for the mites under a microscope. This is what they look like.
The form we most often see is the localized which is more common in juvenile dogs. It usually develops in puppies less than six months of age. This disease can often resolve spontaneously without treatment as soon as the immune system matures. Here are some photos of local patches of hair loss.
The generalized form is usually due to an illness which is suppressing the immune system or genetics. This form of the disease can require aggressive and extended treatment. If you notice any fur loss on your pet make sure you get a diagnosis from your veterinarian.