What Causes Demodectic Mange?
Demodectic mange is caused by the demodectic mite. Demodectic mite is part of the normal fauna of skin and presents in small numbers in most dogs. It lives in the hair follicles and feeds on the skin cells, oils and skin debris. The mite appears like an alligator with eight legs under the microscope. The mites cause no harm as long as the dog's body immune system is functioning.
Demodectic mange most often occurs when a dog has an immature immune system allowing the mites to grow rapidly. Hence, this disease occurs mainly in dogs less than 12 to 18 months of age. In most cases, as a dog matures, the immune system also matures and mange becomes less common in the more mature dogs. Adult dogs that have the disease usually have a defective immune system (which is common in dogs on long term cortisone treatment).
Why Doesn't the Immune System Mature Correctly in Some Dogs?
Development of the immune system is under genetic control. An affected dog usually comes from a litter containing other affected puppies. It is, therefore, advisable not to breed the puppies further or their parents.
What are the Signs Shown in a Dog with Demodectic Mange?
There are two categories of demodectic mange, the localised and the generalised forms.
Localised demodectic mange usually presents as mild red bald patches where the hair has fallen off. The patches may have fine silver scales covering the area and they are normally not itchy. The most common site is the face and most cases occur at 3 to 6 months of age and heal on their own. Up to 10% of localised demodectic mange will progress to the generalised form.
Generalised demodectic mange is more common in pure breeds rather than cross bred dogs, with hereditary breeding predisposition being regularly observed in breeding kennels. Affected dogs show patches of hair loss over the entire body with fine scales but not itchy. It is not uncommon for little follicles to be present over the bald areas which are indicative of a secondary skin infection.
How is Demodectic Mange Diagnosed?
Diagnosis is made via deep skin scrapings and examination of the scrapings under microscope to reveal large numbers of demodex mites.
What is the Treatment for Demodectic Mange?
For localised demodectic mange, 90% of cases will heal spontaneously. Localised demodectic mange is usually benign while generalised demodectic mange is a severe illness often due to an inherited defect in the immune system. It is often advisable not to treat the localised forms. This allows the differentiation of self-healing ones from the generalised ones.
Where veterinary advice is given to treat the localised forms often a benzyl peroxide gel or shampoo is used for four weeks with repeat skin scraping to be done at the end of the four weeks to ensure the mites are gone.
In the case of generalised demodectic mange, approximately 30 - 50% of all dogs under 1 year of age recover spontaneously and early treatment does not seem to influence the recovery. Nevertheless, there are several treatment options used with the most common being the following:
(i) Washing weekly with Amitraz.
(ii) Use of oral Ivermectin daily.
(iii) Use of Dectomax injections weekly.
For all these treatments, the procedures must be continued until two successive biweekly skin scrapings fail to reveal live demodectic mites.
Will Demodectic Mange be Likely to Recur After Successful Treatment?
10% of dogs with generalised demodectic mange that respond to treatment may eventually relapse, and periods of oestrous and pregnancy as well as stress or cortisone treatment can trigger a relapse. Dogs with generalised demodectic mange should be desexed as oestrus may trigger new outbreaks. Puppies of affected dogs have a higher incidence of generalised demodectic mange.