Demodicosis is a common disease that affects dogs globally. Despite being a commonly encountered disease, treatment can still pose a challenge for a variety of reasons: 1) demodicosis is often complicated by concurrent secondary infections which can delay or impede resolution of clinical signs; 2) other dermatologic diseases often mimic demodicosis, for example dermatophytosis; 3) the number one cause of treatment failure seen in secondary referral practice is inadequate length of treatment; and 4) systemic diseases or other causes of immunosuppression can play a role in the development of adult-onset demodicosis which also need to be addressed in order to achieve cure.
Dexmodex canis mites are considered to be normal commensal organisms in the skin, dwelling in hair follicles and sebaceous glands. It is believed that aberrations of the immune system lead to their proliferation, thus resulting in the development of clinical signs. Two additional types of Demodex have been described in the literature. D. injai is a long-bodied mite often found in terrier breeds with greasy coats. The second is an un-named short-bodied mite; however, molecular testing has shown that this mite may simply be a morphological variant of D. canis.
Juvenile demodicosis develops secondary to an immunocompromised state resulting from endoparasiticism, malnutrition or debilitation. However, plenty of apparently healthy young dogs present with focal lesions without evidence of systemic illness. In older animals, endocrinopathies, neoplasia, or chemotherapy may trigger proliferation of Demodex mites.
Demodicosis is categorized as being localized or generalized. The localized form (figure 1a and b) carries a good prognosis and most cases spontaneously resolve without medical intervention. Generalized demodicosis (figure 2) is more severe and may be considered life-threatening in certain cases. The definition of generalized demodicosis, although a matter of some debate, is the presence of more than four lesions over 2.5cm in diameter.
Lesions often appear on the face and front legs, but progress to involve other sites. Clinical signs range from erythema, comedones, partial or patchy alopecia in mild cases to furunculosis, crusting, and generalized malaise, lymphadenopathy and fever in more severe cases. There is almost always a secondary bacterial component associated with demodicosis.
A deep skin scraping is the current diagnostic method of choice. A dulled scalpel blade, dental spatula, or curette can achieve acceptable results. In order to collect the best sample possible, mineral oil should be placed onto the sample site as well as on the sampling instrument to allow for better adhesion of cellular debris. The skin should be scraped in multiple sites until capillary oozing is noted. There is a better chance of finding mites in areas where primary lesions are present – such as alopecic areas, or areas where papules, pustules and comedones are found – than in secondary lesions such as ulcerations. Once the cellular material has been collected onto a glass slide, a cover slip should be placed and the debris examined under low power (4x or 10x magnification).
Other methods of diagnosis include trichograms (hair plucks), biopsy and histopathology, and impression smears of exudate. Trichograms are particularly useful for lesions in the periocular and interdigital regions.
Also it is critical to also identify whether a concurrent secondary infection is present. In severe cases involving furunculosis, bacterial septicemia is a risk if the infection is not addressed. Staphlyococcus pseudintermedius is most commonly encountered, although Pseudomonas aeruginosa or Escherichia coli may also act as secondary contaminants.
Resolution of clinical signs alone is not a reliable endpoint to therapy. It is important to aim for microscopic cure in addition to clinical cure to obtain long-term remission. Microscopic cure is defined as negative deep skin scrapings at least twice consecutively, usually one month apart. It is recommended that treatment continue for one more month beyond microscopic cure.
Amitraz is the only approved therapy labeled for use in dogs with demodicosis. Protocols vary from daily rinses to weekly rinses using concentrations that vary between 0.025% to 0.06%. Although amitraz is effective, the author has never used it due to potential side effects – which may affect the owner as well as the patient. In dogs, amitraz has been reported to cause depression, ataxia, polyphagia, polydipsia, vomiting and diarrhea. In humans, amitraz anecdotally causes headaches and induces asthma attacks. The spot-on preparation containing 15% amitraz and 15% metaflumizone is no longer recommended due to its potential to cause a pemphigus foliaceus-like drug reaction.
Ivermectin is not licensed for use in canine demodicosis. However, studies have shown it to be an effective therapy when used at 0.4-0.6mg/kg orally once daily. Ivermectin can cause a number of neurological side effects including lethargy, blindness, tremors, mydriasis and death in sensitive dogs. Collies and herding breeds are most susceptible to ivermectin toxicity, but other breeds can also be affected. The ABCB1-D1 (MDR-1) gene mutation is considered responsible for acute toxicity in Collies and Collie mixbreeds but is not the only mechanism that causes ivermectin sensitivity. A test for this gene mutation is available through Washington State University (http://www.vetmed.wsu.edu/depts-vcpl/).
Milbemycin oxime is recommended at a dosage of 1-2mg/kg orally once daily and has a higher safety margin in Collies. However, dogs with the ABCB1-D1 (MDR-1) gene mutation may still be susecptible to developing ataxia at 1.5mg/kg orally once daily. Milbemycin is currently unavailable.
Alternative therapies include doramectin administered subcutaneously or orally, or moxidectin in 2.5% spot-on formulation in combination with topical 10% imidacloprid given weekly to bimonthly. This treatment may be more effective in juveniles with milder forms of disease than in adults with late-onset generalized demodicosis. Additionally, adjunctive treatments such as shampoo therapy (2-3% benzoyl peroxide or 3-4% chlorhexidine) may provide relief. Finally, it is imperative that antibiotic therapy is administered concurrent with miticidal therapy.
Monitoring and prognosis
The long-term prognosis is good as long as the duration of treatment is adequate and underlying systemic disease is addressed. However, some dogs that have incurable underlying disease may require lifelong pulse therapy such as weekly ivermectin therapy or monthly amitraz dips. No resistance to treatment has yet been documented in Demodex mites.