Current Treatment Of Canine Demodicosis
Terese C. DeManuelle, BSc, DVM, Diplomate ACVD
Allergy & Dermatology Veterinary Referral Center, Milwaukie, OR
Adjunct Professor of Dermatology, Washington State
University College of Veterinary Medicine, Pullman, WA
Canine demodicosis is the sixth most common skin disease of dogs in North America. The generalized form of the disease can be frustrating to treat and requires dedication and commitment from the veterinarian and client. The prognosis for canine demodicosis is dependent on age, genetic factors, and underlying diseases.
BREED PREDISPOSITIONNumerous breeds have been reported to be at increased risk of demodicosis. Chinese Shar-peis, Afghan hounds, Boston terriers, Great Danes, weimaraners, English bulldogs, West Highland White terriers, Scottish terriers, Airdale terriers, Alaskan malamutes, rottweilers, Lhasa Apsos, Shih Tzus and miniature poodles are over-represented among the general population of dogs with generalized demodicosis. However, dogs of any breed can develop generalized demodicosis and dogs with generalized demodicosis should not be bred. If owners do not agree before initiation of therapy that a dog will be castrated or spayed, treatment of generalized demodicosis is not recommended.
PATHOGENESISThere is evidence of hereditary predisposition in dogs for juvenile onset (< 18 months of age) generalized demodicosis. Adult onset (>18 months of age) canine generalized demodicosis is most likely caused by suppression of the immune system. Multiple factors have been documented as initiating adult-onset canine generalized demodicosis. These factors include immunosuppressive drug administration, hypothyroidism, hyperadrenocorticism, diabetes mellitus, lymphosarcoma, mammary adenocarcinoma and hemangiosarcoma. Other conditions such as estrus, whelping, gastrointestinal parasite infestation, and heartworm infestation have also been associated with canine generalized demodicosis.
Dogs with generalized demodicosis exhibit a marked decrease in T-lymphocyte response as evaluated by in vitro lymphocyte blastogenesis. Normal T-lymphocyte response returns with resolution of the generalized demodicosis. Dogs with localized forms of demodicosis do not develop suppression of lymphocyte blastogenesis. Dogs with generalized demodicosis also have decreased interleukin-2 production. Interleukin-2 is important in leukocyte blastogenesis and activates and sustains T-lymphocyte function. It is hypothesized that demodex mite-induced cytokines may suppress T-lymphocytes.
DIAGNOSIS AND CLASSIFICATION
Because of the high incidence of demodicosis in the dog, all dogs with significant skin disease should have deep skin scrapings performed in affected areas. Skin scrapings are best performed with a dull no. 10 scalpel blade. The hair should be clipped from the affected areas and the skin squeezed to assist in extrusion of mites from the hair follicle. Skin scrapings should be performed in the direction of hair growth and the lesion should be scraped until capillary oozing is observed. Skin scrapings should be obtained from a minimum of three to five areas to determine the extent of the disease and to differentiate between localized and generalized forms of the disease. Areas that should be included in survey skin scrapings include lesions, interdigital spaces, and the lipfold areas. Dogs that have chronic fibrotic interdigital lesions and Chinese Shar-pei dogs may exhibit false negatives on skin scrapings and may require skin biopsy specimens for diagnosis of the disease. If skin scrapings are positive from two or more feet, from five or more individual lesions, or in one body region (i.e., trunk; head and neck) the disease is considered generalized. Dogs with generalized disease should have a complete medical evaluation performed for underlying disease appropriate for the age of the dog and in conjunction with other clinical signs. Tests to consider include complete blood count, serum chemistry profile, thyroid evaluation, adrenal evaluation and heartworm status. In juvenile-onset generalized demodicosis, fecal examination, heartworm test, and nutritional status should be evaluated.
Chronic generalized canine demodicosis is a frustrating disease to treat. General health and management of dogs with demodicosis should be addressed initially. Secondary pyoderma associated with localized or generalized disease should be treated with appropriate antibiotic therapy. Discontinuation of antibiotic therapy should be based on clinical re-evaluation. Corticosteroid therapy in any form is contraindicated in dogs with localized or generalized demodicosis.Localized demodicosis resolves in greater than 90% of dogs and usually does not require therapy. Amitraz (whole body application or spot treatment) is not recommended for localized demodicosis. Amitraz is very lipophilic and is well-distributed throughout the subcutaneous tissues. Treatment of localized disease with amitraz could lead to the development of amitraz-resistant mites and may mask the natural progression of localized demodicosis to generalized disease in some dogs.
Amitraz (Mitaban®) is the only drug approved by the Food and Drug Administration for treatment of canine demodicosis. It is approved for biweekly use at 0.025 percent (250 ppm; 10.6 ml in 7.6 L of water). It is imperative a total body clip is performed prior to treatment and the haircoat must be kept very short throughout the treatment period. The dip must be made fresh with each application as the solution quickly loses its efficacy and potentially degrades to more toxic compounds. Side effects of Amitraz therapy include sedation, ataxia, bradycardia, vomiting, diarrhea, hypothermia and transient hyperglycemia and may occur at an increased incidence in geriatric, debilitated, miniature and toy breed dogs. Amitraz appears to be seventy to eighty percent effective in curing generalized demodicosis in dogs with juvenile-onset disease. Adult-onset demodicosis may be more difficult to cure and may require a longer course of treatment. Amitraz therapy should be continued for one month after the second consecutive negative skin scraping is obtained.If mite counts are not improved after four to six treatments, alternative therapies are recommended. Amitraz dips may be increased to once weekly at 250 ppm concentration or avermectin therapy may be utilized. Dogs with generalized demodicosis treated with any therapy are not considered “cured” unless they are negative on skin scrapings one year after discontinuation of miticidal therapy. Ivermectin and milbemycin oxime are avermectins that are not approved for the treatment of canine demodicosis. The use of these drugs should be reserved for dogs that do not respond to Amitraz therapy. The end point for treatment with avermectin therapy is the same as for Amitraz; one month after the second consecutive negative skin scraping is obtained. Dogs must be heartworm negative prior to onset of therapy with avermectins. Side effects observed with avermectin therapy includes lethargy, ataxia, mydriasis, stupor, coma and death.
Milbemyin oxime appears to cause less serious side effects than ivermectin and side effects are usually reversible when the drug is discontinued. Milbemycin oxime may cause side effects at higher dosages in avermectin-sensitive collies and caution is recommended. The dosage range for milbemycin oxime is 0.5-2.0 mg/kg orally once daily; the dosage of 2.0 mg/kg daily appears to be most effective. The cure rate for dogs with generalized demodicosis is approximately 65-85% when results of studies utilizing different dosages are compared. As observed with Amitraz, a higher cure rate is seen in juvenile-onset generalized demodicosis.Ivermectin is used to treat canine generalized demodicosis at dosages of 400-600 micrograms/kg orally once daily. Ivermectin should not be used in breeds exhibiting sensitivity such as collies, Shetland sheepdogs, Old English sheepdogs, other herding dogs, or their crosses. Some dermatologists recommend a test dosage of ivermectin which begins therapy at 100 micrograms/kg orally daily. The dosage increases by 100 micrograms/kg on each subsequent day until the target dose is achieved. If signs of toxicity are observed, treatment is discontinued.
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