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Adult onset canine generalised demodicosis

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01 April 2017, at 1:00am

David Grant continues his series looking at dermatological conditions.

CANINE DEMODICOSIS IS A COMMON SKIN DISEASE in which there is an increased number of Demodex mites, normal inhabitants of canine skin. Three mites have been identi ed, with their usual site in brackets: Demodex canis (hair follicle and sebaceous gland), Demodex injai (hair follicle and sebaceous gland) and Demodex cornei (stratum corneum). 

Development of the disease is associated with immune deficiency (Paterson, 2008). Demodex canis is the most commonly seen mite and is described in this article. Demodex cornei is less common although clinically indistinguishable (Miller, Grif n and Campbell, 2013).

Demodex injai has a different clinical presentation, mainly a greasy seborrhoea that tends to be dorsal, with a breed predisposition for terriers, especially the West Highland white and Shih-Tzu. This form is not described further in this article and interested readers are referred to other texts (Miller, Grif n and Campbell, 2013; Paterson, 2008).

Clinical presentations

Demodicosis occurs in various clinical manifestations. These are localised, generalised (further divided into juvenile onset and adult onset) and pododemodicosis, which may be seen in both localised and generalised forms.

Authors vary in their definitions of what constitutes localised and generalised, and also concerning the age of onset. Paterson (2008) defines localised demodicosis as having fewer than five patches on the skin, and one foot only involved in localised pododemodicosis. 

Generalised demodicosis is defined as more than five patches of affected skin and two or more feet in the pododemodicosis form. Juvenile onset disease is de ned as occurring between three and 18 months of age and adult onset after three years. There is no uniformly accepted standard for the above definitions but this author prefers those described by Paterson. 

Adult onset generalised canine demodicosis 

Signs 

  • Age of onset is after three years and frequently older than this 
  • Care needs to be taken with dogs presenting at around the three-year age group, as it is not unusual for the disease to remain undetected for several years if careful sampling has not been done. A detailed history will be essential in these cases. 
  • Regardless of the age of onset the generalised forms are clinically indistinguishable. Early signs are erythema, alopecia, scaling, comedones, crusts and haemorrhagic lesions. Generalised erythema accounts for the old description of the disease “red mange”. This colour is highly suggestive of demodicosis but may be misdiagnosed as atopy.
  • With chronicity, secondary infection with Staphylococcus pseudintermedius is common. Occasionally Proteus and Pseudomonas can be involved. Bacterial infection in untreated cases is frequently severe with cellulitis, furunculosis and generalised lymphadenopathy. In chronic skin cases the skin is often a blue-grey colour and should suggest the disease and prompt skin scrapings.
  • Lesions tend to be most severe in the predilections sites – the facial area, neck and feet, but ultimately may involve the entire body.
  • Depression, lethargy and anorexia develop in advanced neglected cases and fatality is possible. 

Underlying causes

  • In all cases of generalised demodicosis the immune system is depressed.
  • In adult onset cases, an immune suppressing disease occurring later in life is responsible for the sudden appearance of demodicosis.
  • These cases are much rarer than the juvenile onset cases.
  • Possible underlying causes include hyperadrenocorticism (both naturally occurring and iatrogenic), hypothyroidism, diabetes mellitus, neoplasia, leishmaniasis, and immunosuppressive treatments for neoplastic or immune-mediated diseases.
  • Underlying trigger diseases may become apparent some time after cutaneous lesions are apparent, thus these dogs should never be considered cured and frequent monitoring is advisable. 

Diagnosis

  • History and physical examination. 
  • Identification of mites from hair plucks, acetate tape impression smears, deep skin scrapings, and in chronic cases, where the skin is lichenified particularly in interdigital areas, and biopsy.
  • Complete blood count, biochemistry, endocrine function tests, radiography and ultrasonography depending on clues from the history and physical examination.

Treatment

  • Successful treatment in adult onset cases is more difficult than in the juvenile onset cases.
  • Treatment of underlying causative diseases is essential and if successful will result in success rates comparable with juvenile onset cases.
  • In those cases where the underlying disease has not been diagnosed or has not yet manifested itself, some improvement and control may be possible but relapse will quickly occur on cessation of treatment. As many as 50% of cases may fall into the category.

Licensed treatments

  • Amitraz (Aludex, MSD). This is applied as a 0.05% solution (50ml in five litres) of water weekly. The dip is left on the coat and not rinsed off. Clipping long-haired dogs will aid penetration. Treatment is continued until two weeks after clinical cure and negative skin scrapings. Sedation is common particularly after the first application and tends to diminish as a problem subsequently. When secondary infection is confirmed cytologically, anti-bacterial treatment is essential. The product should not be used on Chihuahuas. 
  • Moxidectin/Imidacloprid (Advocate, Bayer). This treatment is in the form of a spot-on applied weekly, and is continued for two weeks beyond clinical cure and negative skin scrapings.

Success rates with both these treatments vary according to the literature. In the author’s experience and with good compliance the success rate in first opinion cases is in excess of 90% in juvenile onset cases and a similar percentage in adult onset cases where the underlying cause is determined and successfully treated. If this is not possible such cases need maintenance therapy and frequent monitoring. 

Unlicensed treatments

Most of the unlicensed treatments that have been used are avermectins. Two are mentioned here: 

  • Ivermectin. This drug is the most widely used at a dose of 0.2-0.6mg/ kg by mouth daily. It is often started at 0.1mg/kg and gradually built up over a few weeks to monitor for possible neurological side-effects. These are particularly common in Collie dogs and their crosses but can occur in other breeds.
  • Milbemycin. This is given at a dose of 0.5-2mg/kg by mouth daily. Neurological side-effects may occur but are less common compared to ivermectin. With both these drugs the success rate is equivalent to the licensed treatments.
  • Recent attention has focused on a product licensed for fleas and ticks in the UK. 
  • Fluralaner (Bravecto, MSD). This product is a chewable tablet given once every three months. Fourie and others (2015) reported on a study involving 16 dogs that were treated for generalised demodicosis either with uralaner or imidacloprid/ moxidectin spot on. Both treatments resulted in clinical cure but uralaner eliminated mites at 56 and 84 days, whereas mites were still present at these times with imidacloprid/ moxidectin. However, the product was administered monthly in this study. Weekly administration according to the more recent license has been shown to give better results. 
  • A larger open study in Poland (Karas-Teczay and Dawidowicz, 2015) treated 163 dogs with uralaner for generalised demodicosis. The overall response to therapy was 100% with 87.1% skin scrape negative after one month and the remainder after two months; 37.4% of these dogs were over two years of age when the disease was diagnosed, which suggests that it may be effective in dogs with true adult onset disease. This definition of adult onset disease differs from that of Paterson described in this article. Further studies are under way to see if these promising results can be replicated.

References and further reading 

  • Fourie, J. and others. (2015) Ef cacy of oral administered uralaner (Bravecto) or topically applied imidacloprid/moxidectin (Advocate) against generalised demodicosis in dogs. Parasites and Vectors 8: 187. 
  • Karas-Teczay, J. and Dawidowicz, J. (2015) Ef cacy of uralaner for the treatment of demodicosis. In: Proceedings of the Annual Congress of the European Society and College of Veterinary Dermatology, pp124-125. Miller, Grif n and Campbell (2013) In: Muller and Kirk’s Small Animal Dermatology, 7th edition, p305. Elsevier.
  • Paterson, S. (2008) In: Manual of Skin Diseases of the Dog and Cat, 2nd edition, pp104-109. Blackwell.