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Canine Demodicosis: Common But Treatable

by Karen B. Farver , DVM, ACVD (Dermatology)

Pinky presented to dermatology and Mainline Animal Rescue as an intact female of 4 years of age. Her history was unknown. She had severe dermatitis, pruritus, generalized lymphadenopathy, lethargy and was underweight.

Top differential diagnoses for her dermatitis included: ectoparasitism (demodex, sarcoptes with secondary bacterial infection, or dermatophytosis), severe bacterial folliculitis, hypersensitivity (atopy, flea allergic dermatitis, food allergy dermatitis), and autoimmune skin disorders.

Initial diagnostics and results included: a negative pinnal reflex, surface tape preparations which showed yeast, ear cytology which showed yeast otitis, pustule cytologic evaluation which showed excessive neutrophils, eosinophils and intracellular cocci. Her deep skin scraping was positive for all stages of the Demodex canis.

Diagnosis

Pinky had generalized adult onset demodicosis with secondary yeast, bacterial dermatitis and yeast otitis.

Demodicosis description

Canine demodicosis is a common skin disease caused by one of three types of mites: D. injai, D. canis, and an unnamed species. Demodex is a normal non-contagion commensal inhabitant of the hair follicle. Overgrowth and disease occur when the natural equilibrium is compromised. There are three classifications of canine demodicosis: localized, juvenile generalized, and adult onset generalized.

Juvenile onset demodicosis occurs in dogs under 18 months of age. Young dogs have inherently immature immune systems and are thus susceptible to the development of demodicosis without underlying disease. Typically, as their immune system matures, this mite infestation will clear. Juvenile demodicosis most commonly manifests as localized demodicosis. Localized demodicosis is defined as involving less than 4 areas of the body, and does not involve generalized pododermatitis. Approximately 90% of cases resolve spontaneously without treatment in 6-8 weeks. If a therapeutic intervention is desired, a benzoyl peroxide gel can be gently massaged into the alopecic area once daily. Generalized demodicosis is a more severe disease and has a hereditary component. It can be adult onset or juvenile onset. Treatment is recommended even in the juvenile form to facilitate recovery.

Clinical signs of generalized demodicosis include: alopecia, crusts and follicular papules/pustules which are initially on the face and feet, and then become generalized. Comedones, erythema, and seborrhea may also be present. Initially it is variably pruritic, but secondary infections will increase pruritus. Fever and lymphadenopathy are not uncommon. In Shar Peis excessive mucin may obscure the signs. Pododermatitis is always considered generalized demodicosis. CBC/blood chemistry abnormalities may include anemia, hyperglobulinemia, leukocytosis, and eosinophilia. Diagnosis is made with a deep skin scraping which allows for quantification of eggs, nymphs, larvae, and adults.

An older dog (adult onset) should not get demodicosis unless he or she has an underlying problem affecting the immune system. Investigation of the history and/or a more comprehensive medical work-up is recommended in these cases. Adult onset generalized demodicosis is the disease that Pinky had. The most common underlying causes are steroids (Cushings or exogenous), hypothyroidism, chronic infection or inflammation (e.g. heartworm, endoparasites, UTI, severe allergies, pyelonephritis), neoplasia, estrus, severe environmental stress or surgery. Pinky had multiple factors contributing to her development of the disease including coccidiosis, that she was an intact female, and that she came from a stressful environment.

Treatment for adult onset generalized demodicosis

Treatment for adult onset generalized demodicosis includes: Ivermectin 0.2-0.6 mg/kg (200-600ug/kg) orally once daily. Its use in treating demodicosis is not approved by the FDA. It should not be used in dogs with the MDR-1 mutant gene. There is a simple buccal swab test that may be used to determine if a dog carries this gene. Shelties, collies, Australian shepherds, collie type dogs, or Old English sheepdogs are the most common breeds with this mutant gene. In other breeds, adverse reactions from ivermectin can be seen occasionally; they include ataxia, bradycardia, mydriasis, respiratory arrest, salivation, stupor, and tremors. To avoid these side effects in a dog that has not previously received ivermectin; start on day one with 0.1 mg/kg and then increase the dose slowly by 50-100 mcg/kg/day up to the maintenance dose. If any side effects occur, the treatment should be decreased or stopped immediately. If there are no side effects, continue with maintenance therapy. Pinky was treated with .45mg/kg/day until resolution.

Milbemycin (Interceptor): This dose is 1-2 mg/kg PO daily.

Doramectin (Dectomax): is reported to be effective at 0.6mg/kg SQ once weekly. The cure rate is approximately 85%. Adverse effects are uncommon but include mydriasis, lethargy, blindness, and coma.

Amitraz (Mitaban) 0.03% to 0.05%: The hair coat should be clipped to obtain effective skin coverage. Sedation may be necessary. Eyes should be protected with ointment and the dog bathed thoroughly first to remove crusts. Then the Amitraz is applied without rinsing and should be applied every two weeks until resolved. The applicator should wear gloves and protective aprons and work in a well ventilated room. Toxic effects of Amitraz are sedation, edema, pain and pyrexia.

All treatment should be continued at maintenance dosing until the next scrape. Scrapes should be performed every 4-6 weeks with a count. If there has not been appropriate improvement then the treatment should be modified. After two consecutive scrapes are negative, treatment is discontinued.

Reducing physiological stress is an important factor in treatment. Ovariohysterectomy is recommended for intact females. The patient should be fed a quality dog food and be free of endoparasites and fleas. Skin infections are usually associated with these cases and antibiotics will likely be necessary. Antifungal therapy may also be needed. It is important that steroids and cyclosporine NOT be used in these cases as they will tip the immune balance in favor of the mite. Underlying systemic disorders should be treated appropriately. Antihistamines may help with some of the pruritus. Pinky was spayed, her secondary infections treated, and dewormed while on demodex therapy.

Prognosis

Pinky was cured of her demodicosis and adopted to a very loving family. To date, she has had no relapses.

The younger the dog, the better the chance of cure is. Most dogs less than 18 months of age recover completely. About 90% of patients can be cured. The average time to clinical and microscopic remission is approximately 4 months in reported studies, but it may take up to a year. The most common treatment problem is premature cessation of therapy. If there is not a relapse within the first 12 months the dog will usually not have a relapse. If there is consistent relapse, the dog may need to continue ivermectin (1-3x weekly) or be rescraped on a regular basis to prevent severe relapse.

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