Canine Demodicosis
Canine demodicosis is a disease encountered commonly in small animal practice. Although in the past severe generalized demodicosis may have prompted euthanasia, current miticidal therapy allows the disease to be cured, or at least managed, in almost every patient.
Demodex mites are part of the normal cutaneous fauna in dogs. They live in the hair follicles and cause a problem only when they proliferate abnormally. The predisposition to develop canine demodicosis is an inherited trait (presumably a defect in cell-mediated immunity); thus dogs with generalized demodicosis must not be bred, and the author strongly advocates neutering of all affected dogs. Other predisposing factors in young dogs include nutritional deficiencies, endoparasites, and stress. Adult-onset demodicosis is associated with neoplastic disease, chemotherapy, hyperadrenocorticism, and hypothyroidism. Diagnosis and treatment of these predisposing factors results in improved treatment outcome for demodicosis.
Demodicosis may occur as localized or generalized disease. Up to four small lesions in any body location are considered to be localized disease by the author. Involvement of a large area on the trunk or any affected paw always constitutes generalized disease. Spontaneous remission is seen in the vast majority of dogs that develop localized demodicosis; thus specific miticidal therapy is not indicated. Use of antiseptic shampoos or gels minimizes the chance of a secondary bacterial infection. Dogs with localized demodicosis that do not achieve spontaneous remission without miticidal therapy also should not be bred.
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.
TABLE 99-1
Recommended Treatment Options for Canine Demodicosis
| Drug | Trade Name | Dosage |
| Amitraz | Mitaban | 0.025% rinse q14d |
| Ivermectin* | Ivomec | 0.3 (up to 0.6) mg/kg q24h PO |
| Milbemycin oxime* | Interceptor | 1-2 mg/kg q24h PO |
| Moxidectin* | Cydectin | 0.2-0.4 mg/kg q24h PO |
*Treatment of demodicosis is an extralabel use of these drugs; appropriate education should be provided and consent should be obtained. Extralabel use of drugs should take into account local and regional restrictions on their use.
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