Ralf S. Mueller,     Munich, Germany

Clinical Signs and Diagnosis

Initially, the only clinical signs may be any combination of multifocal alopecia, comedones, mild erythema, and scaling. These are followed later by follicular papules and pustules. Furuncles, crusts, and draining tracts may develop if furunculosis results from the folliculitis. Lesions typically start on the face and feet and gradually generalize. Otitis externa can be caused by Demodex mites and infrequently is the only clinical sign in dogs. Secondary bacterial pyoderma is common; typically it is caused by Staphylococcus pseudintermedius, but Pseudomonas aeruginosa or other gram-negative rods also may be involved, particularly with deep furunculosis. Lymphadenopathy and fever may develop.

Demodicosis is diagnosed easily by deep skin scrapings. If three scrapings are negative for the mites, demodicosis typically is ruled out. False-negative results on scrapings have been reported in dogs with pododemodicosis and in shar-pei dogs. Because Demodex mites are part of the normal fauna, a single mite on a deep skin scraping may be an incidental finding. However, two or more mites on three scrapings usually are diagnostic of the disease. Trichography (in which hairs are plucked with a hemostat, placed in a drop of mineral or paraffin oil on a slide, and inspected under a coverslip) is an alternative diagnostic technique that is useful for lesions around the eyes or on the paws. In rare cases demodicosis may be diagnosed only on histopathologic examination of affected skin.

Secondary Infections in Dogs with Demodicosis

Cytologic evaluation to identify secondary bacterial infection usually is necessary. Systemic antibiotics are recommended for dogs with demodicosis and severe secondary bacterial pyoderma. Bacterial culture and susceptibility testing is recommended when rods are found on cytologic examination or in dogs with staphylococcal infections that have failed to respond to prior antibiotic treatment (see Chapters 101 through 103). However, recent evidence indicates that in many dogs with demodicosis and pyoderma antimicrobial shampoos are sufficient to treat the bacterial infection. Antimicrobial therapy should be continued until clinical and microscopic resolution of the pyoderma is achieved.

Miticidal Therapies

A number of successful miticidal therapies for canine demodicosis have been reported (Table 99-1). Amitraz and the macrocyclic lactones are the most commonly used, and their efficacy has been evaluated in numerous open-label studies.