Demodicosis is a very common skin disease of dogs caused by an abnormal proliferation of demodex mites. Each animal (including humans) harbors its own specific mite species. Demodex mites rarely cause disease in cats, horses, cattle or man and it is not contagious.
The mite causing disease in the dog is called Demodex canis
. These mites live in the hair follicles and sebaceous oil glands, feeding on cellular debris. The mites, once off the dog, do not live for very long – only about an hour. The entire life cycle of the demodex mite consists of 25 to 30 days and is completed on the host. Four stages of the life cycle may be evident in skin scrapings. They include spindle-shaped eggs, six-legged larvae, eight-legged nymph, and eight-legged adult.
The pathogenesis of the disease states associated with the proliferation of demodex mites is not completely understood. Most animals harbor very low numbers. In affected animals, the mites proliferate in very large numbers within hair follicles.Symptoms Skin lesions suggestive of demodicosis include redness, papules, pustules, patches of hair loss (alopecia), blackheads, scaling and areas of excessive skin pigmentation. Pruritus (itchiness) may or may not be present.
Some dogs with demodicosis do not develop balding areas but develop pruritus instead. These dogs can be easily misdiagnosed as allergic dogs, especially due to the distribution of the lesions on the face and feet.
There are two vastly different major clinical forms: the juvenile onset (less than two years of age) and the adult onset demodicosis (age of onset more than two years of age).
Juvenile Onset Demodicosis
The juvenile onset version is further classified into localized and generalized. Prognosis and therapy varies for each condition. Several dogs breeds have a higher incidence of demodicosis including Afghan hounds, Boston terrier, boxer, Dalmatian, Chihuahua, English bulldog, Doberman pinscher, American pit bull terrier, German shepherd dog, Old English sheep dog, pug, Shar-Pei, American Staffordshire terrier, collie, and the Great Dane.
Localized form. In this form only one body area is affected, based on the results of skin scrapings. The disease is usually benign and self-limiting and may consist of one to several patches of circumscribed, erythematous (reddened), scaly hair loss. Commonly affected areas include the face and feet. Bacterial infection may be present and is responsible for the development of pustular eruption and/or draining tracts depending on the depth.
Prognosis is good because over 90 percent undergo spontaneous remission in three to eight weeks in spite of therapy. Only 10 percent become generalized.
Generalized form. Lesions are present in more than one region of the body. Dogs with pododemodicosis (affected feet) are classified as generalized. Breed predilection exists; commonly affected breeds include pit bull, bulldog, Boston terriers, beagle, collie, Dobermans, boxers, dachshunds, pointer, Dalmatian, shar-pei, shih tzu and Lhasa apso.
This form is a much more severe form than the localized form and affected dogs have a genetically inherited predisposition for the disease.
Relapses are common and prognosis is guarded. However, up to 50 percent of these cases may still recover spontaneously without antiparasitic therapy provided that the secondary bacterial infection is addressed, as bacteria may be immunosuppressive.
Both sexes should be neutered, even if the disease cleared spontaneously without treatment.
Adult Onset Demodicosis
Demodicosis is secondary to another disease or immunosuppressive therapy. Conditions commonly associated with adult onset demodicosis include endocrine disease like hypothyroidism and Cushing's syndrome, metabolic disease and cancer such as lymphoma.
Demodicosis can also be triggered by prolonged steroid therapy. This is the most common cause of adult onset demodicosis.
Prognosis of adult onset demodicosis depends on the nature of the underlying diseases. In some cases (up to 50 percent) no underlying disease is detected at the time of diagnosis, as demodicosis may be the first sign of immunosuppression.