It is known by a variety of names such as red mange, and is caused by a mite known as Demodex canis. The mite is cigar shaped when viewed under a microscope, but is not visible to the naked eye. It is generally believed that all dogs have a small number of these mites living in their hair follicles at all times. The mite is not contagious from dog to dog in the usual sense, nor is it transmissible to humans.
Dogs acquire the mites during the first two to three days of life while nursing their dams.
In a large number of dogs, the mites cause no problem, but in some individuals, skin disease does result. Two theories 1 exist about why certain dogs develop demodectic mange, both of which deal with suppression of the immune system. While not proven to be inherited, most researchers believe the tendency to develop demodectic mange is hereditary, and most breeders seem to find evidence of certain families that are more prone to the problem. The second theory holds that it is not the mites, but a tendency to develop a bacterial infection along with the mange, which results in the bacteria suppressing the immune system. Either way, the result is the same, the mites and bacteria proliferate on the skin, causing hair loss, redness, etc.