Canine Demodicosis – Clinical Signs and Options for Therapy

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CANINE DEMODICOSIS – CLINICAL SIGNS AND OPTIONS FOR THERAPY

Author : Sue Paterson

Categories : Vets

Date : August 22, 2011

Sue Paterson discusses this skin condition in dogs, highlighting the symptoms and diagnostic methods as well as management choices

DEMODEX species are commensals of the canine skin. The mites are thought to be passed from mother to pups within the first few days of birth and three forms of the mite are recognised in dogs.

The most common form is Demodex canis – a long thin mite that is a resident of the hair follicle and sebaceous and apocrine glands (Table 1). Demodex corniei is a much shorter, truncated mite that lives in the superficial layers of the stratum corneum, and it is most commonly found in the interdigital spaces. The most recently discovered form of Demodex is the very long, thin, almost “snake-like”, Demodex injai.

Clinical signs

Demodicosis can be localised or generalised. The localised form is seen most commonly in young dogs. Localised disease is defined as involvement of five or fewer areas of skin. Lesions appear most often as well-circumscribed scaling areas with associated alopecia. The skin is usually a blue/grey colour due to the presence of confluent comedones. (Figure 1).

Areas commonly affected include the face, neck, forelimbs and the trunk. Of the dogs that develop localised disease, 90 per cent will go on to self-cure, while only 10 per cent of cases progress to

1 / 5 generalised disease. Dogs with generalised demodicosis have widespread disease involving multiple areas, a whole body region or all four feet (Figures 2 and 3).

Clinical signs overlap with those of localised disease, however secondary infection is more common. Dogs can have superficial pyoderma with pustules and papules or deeper infection leading to areas of furunculosis and cellulitis. When secondary infection occurs, dogs usually manifest signs of systemic ill health. Demodicosis can be further divided into juvenile and adult onset disease. Juvenile onset disease, as its name suggests, occurs in young dogs usually less than three years of age. In immature dogs, it is thought to be brought on by the stresses of weaning, vaccination and sexual development. With appropriate treatment as affected individuals mature and grow, the disease will usually go into remission.

In adult onset disease, there is usually an underlying trigger. The adult onset disease can be iatrogenically induced by over-zealous prescription of glucocorticoids or other immunosuppressive therapy, or may be associated with systemic problems such as endocrinopathies, systemic neoplasia or other debilitating diseases. Where the immunosuppressive trigger can be removed or successfully treated, the prognosis for dogs with adult onset demodicosis is good. In cases where such therapy is impossible, the prognosis is much more guarded.

Diagnostic methods

A diagnosis of demodicosis can often be made on the basis of a clinical history and the presenting skin lesions, especially where comedones are present. However, a variety of different techniques have been described to identify Demodex mites. For the precise methodology of each technique, the reader is referred to Table 2.

Management of demodicosis

– Licensed drugs

Amitraz has been the mainstay of most therapy for many years and is the oldest of the drugs licensed to treat demodicosis. Although it is a highly effective form of therapy, it is time-consuming and unpleasant to apply. Dogs need to be clipped (which usually requires sedation) and diluted solution is applied as a leave-on dip (ensure administrators wear protective apron and gloves). Applications need to be repeated weekly until resolution, which can be up to six months. Its success rate is said to range from zero to 90 per cent, but it can produce side effects in some dogs of gastrointestinal disease, erythroderma, bradycardia and hypotension. It should not be used in chihuahuas.

Amitraz has been produced in a spot-on formulation combined with metaflumizone. There is no doubt topical application works well, although long-term studies are needed to assess the continued progress of animals.

2 / 5 A study has suggested that this topical spot-on formulation of amitraz with metaflumizone can cause cutaneous drug eruptions. Work by Oberkirchner et al in 2010 suggested this combination produces a contact induced pemphigus foliaceus (PF)-like disease. This can occur both at the site application of the spot-on and also at distant sites. Despite the fact the lesions of the PF-like disease was associated with drug therapy, some of the dogs in the report needed immunosuppressive therapy despite withdrawal of the drug to bring their disease under control.

A spot-on formulation of 10 per cent imidacloprid with 2.5 per cent moxidectin is licensed for treatment of demodicosis – initially for monthly use. Changes to the label for this product (weekly for severe cases) has improved its effectiveness for topical therapy. Studies have shown that adverse effects were very uncommon when given at this frequency.

– Unlicensed drugs (Table 3)

Although there are numerous reports in the literature of the use of both milbemycins and avermectins to treat demodicosis, use of most of these drugs currently constitutes an offlicence use of therapy and their use should be justified under the veterinary cascade. Certain breeds have been identified with an MDR1 gene defect that leads to an inability to pump these drugs from the brain when they penetrate the blood brain barrier. When this occurs, profound, and often life- threatening, side effects have been recorded, which include inappetence, vomiting, lethargy and ataxia.

A test has been developed and is available in the UK to check for the presence of this genetic defect.

Where a dog is not screened for the MDR1 gene, then drugs within this group should only safely be used under their licence application, for example, selamectin, milbemycin and moxidectin.

Susceptible breeds include the Australian shepherd, border collie, German shepherd dog, long- haired whippet, Old English sheepdog, rough collie, sheltie and smooth collie.

Groups of endectocides

– Avermectins

Ivermectin can be given orally at a dose of 300ug/kg to 600ug/ kg daily or every other day to produce good clinical cure rates of up to 85 per cent. However, it commonly causes adverse effects, especially in collies and related breeds, even at low-dose rates. There is a single report in the literature of doramectin being used to treat demodicosis. Although no side effects were noted in this case, it may be this was a non-susceptible animal and it should be anticipated that side effects to this drug will probably be the same as ivermectin.

3 / 5 – Milbemycins

Milbemycin has also been given orally at a dose rate of 0.5mg/ kg to 2mg/kg daily with good success rates of 60 to 90 per cent. Recommendations from clinical work suggests the lower end of the dose range of 0.5mg/g to 0.75mg/g should be given initially and increased if a clinical response is not seen. Treatment should be continued until two consecutive negative skin scrapes, taken one month apart, are obtained.

Adverse effects of this drug are rare and even collie-type breeds seem to tolerate it well. When side effects occur, they are reported to be low grade and consist of mild ataxia tremors and gastrointestinal disturbances. The monovalent form of milbemycin is now available through veterinary wholesalers in the UK, but a special import certificate is required to justify its use outside cascade. Vets can contact Novartis Animal Health Technical Services for further advice on 01276 694402.

A useful link for veterinary surgeons wishing to obtain milbemycin in this way is www.vmd.defra.gov.uk The reader should go to the Veterinary Medicines Guidance Note No 7 – Import Certificate Scheme.