I have arrived in Australia to see my aunt who has not been well recently. It was her and my uncle’s 50th wedding anniversary at the weekend and I was delighted to be able to share this event with them and all their family and friends. My own parents are 50 years married in February so I will have to put some effort into our event too.

I arrived in Adelaide on Saturday morning and am getting over my jetlag. It was raining heavily when I arrived. Is this definitely Australia? Weather has been quite cool but warmer than Liverpool.

On Tuesday night 8.30pm I will be doing the second of my derm case studies for Royal Canin. I hope you can attend.  The first one on canine dermatology seemed to go well and on Tuesday I will be doing feline dermatology.

We got lots of questions from the canine derm webinar last week so I will try to answer them in this blog.

Some thoughts on demodicosis

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.

Treatment

As previously mentioned, dogs with localised demodicosis do not usually need treatment. I recommend waiting to see if they progress before starting treatment.

In generalised treatment, it is very important to get the client’s co-operation from the start. This is a frustrating and expensive disease to treat but success levels have risen dramatically since I qualified. Most dogs will have a secondary infection and will need at least one month’s antibiotics at the appropriate dose. Good nutrition and treatment of fleas and worms is also important.

Amitraz has been the only licensed drug for many years. I am not a huge fan of this drug because of its potential to contaminate the environment and also because it can leave the patient quite unwell too. Amitraz is a formamidine and acts by inhibition of monoamine oxidase. It is also a prostaglandin synthesis inhibitor and alpha adrenergic agonist. If a dog has a deep pyoderma, I recommend 7-10 days of antibiotics to begin healing the skin before applying the amitraz usually at 500ppm, although 250ppm may be used in small dogs. If the dog has long hair it is sensible to clip the dog before instigating therapy. A benzoyl peroxide shampoo( Paxcutol, Virbac) should be used first to get rid of all the crusts and flush the follicles where the mites reside.

Commonly treatment will need to be performed weekly for at least 8-12 weeks to effect a cure. Monthly skin scrapes should be performed to count mite numbers as well as egg and larvae numbers. If no eggs are seen it suggests that the mites have stopped breeding. After I have seen no mites on scrape I continue for 4 weeks and then stop. I tell the owners to watch the dog very closely for the next year. If they do not relapse in the first 12 months they will probably have no further problems unless treated with immunosuppressive drugs such as steroids.

Promeris is a product made by Fort Dodge which contains amitraz in a spot on form to counteract ticks. They also have a recommendation for use in demodex. This product seems to reduce numbers rather than eradicate them. There has been mention of reactions to this product. It is not one I would routinely use.

Macrocyclic lactones

These drugs are produced as a by fermentation of various actinomycetes. This class of drugs includes ivermectin and selamectin. They work by potentiating the release and effects of GABA. GABA is a peripheral neurotransmitter in susceptible nematodes, arachnids and insects. They are also agonists of glutamate gated chloride channels. In mammals, GABA is limited to the CNS. These drugs do not cross the blood/brain barrier and are, therefore, safe at usual doseage levels. However, high doses of ivermectin can be toxic in dogs under 3 months or certain breeds particularly collies and collie crosses.

PLEASE DO NOT USE THESE DRUGS IN THESE BREEDS FATALITIES HAVE OCCURRED

However, in most dogs it is wonderfully tolerated and I have found it to be a wonderful drug in the treatment of juvenile onset demodicosis. I usually see referral cases were other treatments have failed and I feel ok to reach for the ivermectin. You need to be careful of the cascade before doing this and obviously request informed consent. Iddex can perform a test called the Ivermectin hypersensitivity test which looks for the MDR1 mutation. This is a useful test to do before starting the dog on ivermectin. The usual dose is 600 micrograms/kg or 1ml per 17 kg. This is given orally each day until no mites are seen and one further month has passed.

Milbemycin can be used at a dose rate of 1-2 mg/kg sid. It is better tolerated by ivermectin sensitive breeds. However, it is very expensive when used in this way. We do not have milbemycin on its own in the UK so it would need to be imported under licence from the VMD. Some dermatologists use cat milbemax tablets at 1-2mg/kg of the milbemycin and ignore the praziquantal dose. This is given orally and daily

Bayer’s product Advocate contains the milbemycin moxidectin. It has a claim for demodicosis and was presented in Hong Kong by Ralph Mueller from Germany.
In the study, the advocate was applied every 2 weeks. 72 dogs were included in the study: 52 with juvenile onset, 20 with adult onset disease. 23 juvenile onset dogs went into remission 3 with adult onset. Mean time until remission was 12.5 weeks. He also showed that dogs with mild signs showed a better success rate than those with severe disease.

This products seems to be better at controlling rather than eradicating the mites. Dogs will often look better but are not microscopically cured.

In cases that do not cure, then maintenance treatment will probably be required.

Hope this is a helpful review on demodicosis and is my understanding of the condition. Please check dosage and always get informed consent for un-licensed drugs. Please feel free to contact me at Anthony@thewebinarvet.com

I think it is important to make a diagnosis. Using advocate if you are not sure of your diagnosis is a mistake. In my opinion advocate reduces rather than eradicates demodex mites. Selamectin does not work against demodex as far as I am concerned.


One Comment

Jenny Darmstadter

Great update; very helpful. Still find demodex very frustrating to treat. Have found Amitraz works well for the juvenile form but not adult onset. Reaching for the ivermectin more and more ……..

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