Classification and causes
Canine demodicosis is separated into two categories according to the patient's age at the onset of disease. Juvenile-onset
demodicosis typically occurs in dogs less than 18 months of age. When a dog is considered an adult may vary based on its breed.
Animals that develop demodicosis after the completion of physical growth and development are considered to have adult-onset
demodicosis.
Juvenile-onset demodicosis is further categorized as localized or generalized according to the extent of the disease. A variety
of opinions exist on defining localized vs. generalized demodicosis. We consider a dog to have generalized demodicosis if
the condition involves the feet, an entire body region, or several sites; is spreading; or has persisted for at least six
months.
The exact pathogenesis of canine demodicosis is unknown but is suspected to be related to an aberration in the immune system.
Juvenile-onset demodicosis is associated with genetics, poor nutrition, stress, and various breeds. The breeds considered
to be predisposed to juvenile-onset demodicosis vary in different reports. Ultimately, any breed of dog can develop juvenile-onset
demodicosis.
Adult-onset demodicosis is associated with systemic diseases (e.g. hyperadrenocorticism, hypothyroidism, diabetes mellitus, neoplasia) or with immunosuppressive therapies. Thus, a full physical
examination, careful patient history taking, and laboratory testing (complete blood count, serum chemistry profile, urinalysis,
microscopic fecal examination, heartworm testing) should be performed in all dogs with adult-onset demodicosis. Both systemic
and topical immunosuppressive therapies can trigger demodicosis, so obtain a thorough drug history from the client. One of
us has witnessed a case of adult-onset demodicosis apparently triggered only by the application of an otic glucocorticoid-containing
product twice daily for one month.
Clinical signs
Figure 4. Nonpruritic spontaneous alopecia in a dog with adult-onset demodicosis.
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The primary lesions of demodicosis due to D. canis include spontaneous alopecia, scaling, follicular casts (keratosebaceous material adhered to the hair shaft), papules, and
comedones (Figures 4-6). Other lesions include crusts, erythema, hyperpigmentation, and lichenification. Demodicosis can occur with or without pruritus
but is usually more pruritic if concurrent secondary infections are present. Demodicosis due to the short-bodied form of the
mite (D. cornei) is often pruritic.
Figure 5. Diffuse comedones in a dog. (Photo courtesy of Lauren Pinchbeck, MS, DVM, DACVD.)
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Dogs with severe demodicosis involving concurrent deep pyoderma can present for evaluation of additional signs. If you note
swelling, pain, peripheral lymphadenopathy, lethargy, fever, and draining tracts, suspect a deep pyoderma. Likewise, evaluate
all dogs with deep pyoderma for demodicosis. A deep pyoderma is a potential dermatologic emergency because of the risk of
sepsis.
Figure 6. Follicular casting in hairs plucked from a dog with demodicosis.
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Demodex canis infection can also cause ceruminous otitis externa. Otitis can occur in conjunction with demodicosis lesions elsewhere on
the skin or be localized only in the ears.
Clinical signs of D. cornei infection are similar to those of D. canis infection, but D. injai infection can have a different presentation. Demodex injai infection typically does not cause alopecia but instead is most commonly associated with an oily coat on the dorsum of the
neck and trunk. This clinical sign is consistent with the observation of this mite histologically in the sebaceous glands
of the skin. Demodex injai infection has been observed more commonly in terrier breeds, such as West Highland white terriers, and only a low number
of mites may be found on skin scrapings.6
The clinical signs of canine demodicosis are from a folliculitis induced by the overgrowth of mites in the hair follicle.
Any other dermatologic condition that results in inflammation of the hair follicle region will cause similar clinical signs.
Differential diagnoses for folliculitis include demodicosis, bacterial skin infection (bacterial folliculitis or superficial
pyoderma), and dermatophytosis.
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