Some Thoughts on Demodicosis: Part 1

Demodicosis… where are we now?

I was fortunate to attend the World Dermatology Congress in November 2008 in Hong Kong. These gatherings are always fantastic opportunities to meet with colleagues and learn new ideas in the field of dermatology. I want to report on some of the lectures as well as give my own thoughts on the subject. I am a private practitioner and hopefully my way of doing things is helpful for you in general practice.

Newer treatments have come onto the market for demodicosis which are licensed. These usually promise to lower but not eradicate the mite. As part of the cascade we probably need to try these first for efficacy before moving onto off –licence drugs. I will go over some of these new products later on in the week

Demodicosis is a reasonably common inflammatory, parasitic disease of dogs characterised by the presence of larger than usual numbers of demodectic mites. The initial proliferation of the mites may be due to a genetic or immunological disorder. Mites can be found in young and older dogs.

Most dogs have some demodex mites but in some individuals these mites multiply in their thousands to cause erythema and alopecia. The mites most commonly live in the hair follicles( Demodex canis) although, a short stubby mite recognised as a separate species lives in the stratum corneum.

The demodex mite spends all of its life in the skin and is very rarely believed to be contagious. Demodex is spread from mum to pup during lactation over the muzzle and face. Mites have been found puppy hair follicles within 16 hours. In the majority of cases these mites do not cause a problem but in some cases may develop into disease.

The pathogenesis is not clearly understood. In the early part of the last century, it was believed that the demodex mite acted as a transmitter of staphylococcal infection in the dog. Another researcher reported the development of generalised demodicosis in pups receiving anti-lymphocyte serum. Danny Scott hypothesized that dogs with generalised demodicosis have a specific T-cell deficit. This allows the multiplication of the mites which then induce a serum immunosuppressive factor which causes generalised T-cell suppression. As the mites are eradicated, measurable T-cell suppression disappears.

There are 2 forms of demodectic mange:

  1. localised
  2. generalised

Localised is the most common form and is seen in young dogs under a year old. It is often seen on the face or around the eyes and manifests itself as small patches of alopecia. It is usually not pruritic. If there are more than 12 areas of alopecia then it is almost certainly generalised, less than 6 and it is localised. Dogs in the middle have to evaluated on an individual basis.

If a dog has localised demodex I usually do not feel that it is necessary to treat them. A small percentage of these will go on to develop generalised disease.

In generalised disease, large areas of the body can be affected. There is usually a secondary pyoderma and the dog may be itchy. This pyoderma may develop into a deep infection requiring long term antibiosis. Staph pseudintermedius and the demodex mite share a symbiotic relationship and treating just the bacteria may lead to a big improvement but once stopped the condition will often flare quickly. Scale and comedones can occur. The dog is often erythematous and alopecic.

Some dogs may only have mites in their feet. This is still classed as generalised disease. Skin scraping may miss the mites and the dog is misdiagnosed as having only a bacterial infection. Biopsies are sometimes necessary in these cases to rule demodex in or out.

Juvenile v Adult-onset demodicosis

Demodex canis is most commonly seen in young dogs. It is believed to have an hereditary basis and sire, dam and puppies should not be used for breeding after diagnosis is made. In my hands, treatment, although, expensive is usually successful and very few dogs are euthanased compared with 30 years ago. Young dogs can relapse later on in life especially if immunosuppressive drugs are used.

In older dogs, treatment is usually less successful and the dog may need to stay on maintenance therapy. Finding the underlying cause is always helpful as this makes the prognosis much more hopeful if treatment can be given. Underlying causes include: neoplasia, hypothyroidism, hyperadrenocorticism, serious systemic disease and allergy

Diagnosis

Made by clinical impression and the presence of large numbers of mites on skin scraping or hair plucks. The mites are usually found in the hair follicles so skin scraping should be deep and capillary ooze should be noted and some of this sample placed on your slide. Sometimes scrapings are negative but you are very suspicious of the mite. This can especially happen in feet and in certain thick skinned dogs such as mastiffs and Shar peis. (I usually biopsy most of my shar peis with skin disease as it helps me very quickly differentiate the different types of skin disease this dog commonly gets). Don’t feel bad if you miss it on a skin scraping.