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Demodicosis, demodetic mange, and red mange, are all terms that apply to a skin disease caused by a proliferation of the mite Demodex canis. At least once a year I receive questions from a breeder or an owner concerning a Border Terrier that has been diagnosed with demodex.
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The Demodex mite is a normal resident of the skin of all dogs. Puppies become infested within 16 hours of birth during normal nursing. Mites can first be found on the muzzle and head of neonates. Puppies born by caesarian section and hand reared away from other dogs do not become infested, and mites are not present on stillborn pups. Demodex canis is an obligate parasite of the hair follicles and skin and spends it entire life cycle on the skin. Mites feed on cells, sebum, and epidermal debris. They do not survive for long periods off the skin of the dog. They are not contagious from one dog to another.
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In some individual dogs the environment of the skin becomes ecologically favorable and mites begin to proliferate to great numbers, they damage the hair follicles, and lesions appear. Demodicosis is defined as either localized or generalized.
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Diagnosis: Demodicosis is diagnosed by finding multiple mites in hair plucks and skin scrapings of the lesions of affected dogs. The skin scrapings need to be done properly, the skin should be squeezed to express the mites from the hair follicles, and the scrape needs to be deep enough. Demodicosis is diagnosed if there are many adult mites present, or adults and a high number of immature forms dead or alive. Skin biopsies may be needed in some circumstances. The skin should be re-scraped at monthly intervals until no live mites or immature forms are present. The Dermatology service at Cornell is continually surprised by referral cases that missed the diagnosis because either scrapings were not done, or not done correctly.
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Localized Demodicosis is characterized by mild erythemia (redness), with partial alopecia (hair loss) in young dogs. The lesions are most common around the eyes, on the face, and on the forelegs. The lesions are normally small, distinct, reddened, or hyper-pigmented (slate blue) scaly areas of hair loss. Demodicosis is most common in puppies from 3 to 8 months of age, but may occur up to 12 months of age, and usually resolves without treatment. Hair begins to grow back usually within one or two months, although larger lesions may take longer to resolve.
The lesions can occur in the ear canal as well, and in this case should receive appropriate treatment since secondary infections can readily occur in the moist, humid environment of the ear canal.
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Generalized Demodicosis: Lesions that grow larger and eventually coalesce, or are scattered all over the body, characterize generalized demodicosis. Secondary bacterial infections are common. There is deep follicular involvement, and plaques and pustules may occur. The dog may be extremely itchy and then painful. In some breeds, and some individuals, the mites are found on the foot and between the toes, this is termed pododermatitis and it may be extremely painful. Dogs with demodectic pododermatitis are often presented to the veterinarian because they are limping or reluctant to walk. Biopsies may be necessary to diagnosis demodectic pododermatitis. Amazingly enough, 30 to 50% of cases of generalized demodicosis in dogs under a year old, can resolve spontaneously, especially if secondary infections are controlled. However, spontaneous resolution is uncommon where there is a known familial history of demodicosis, and where there is adult onset demodicosis.
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Inheritance: There is a hereditary predisposition for the development of demodicosis. Dermatology services tend to see more cases in certain breeds; some breeders have told dermatologists that they can predict which litters will have affected puppies. Generalized demodicosis probably occurs because of specific defects in the immune system. Some experts theorize that there is a Demodex canis specific T-cell defect. In a predisposed dog, there may be a reoccurrence of demodicosis in bitches in estrus, and in dogs undergoing chemotherapy or other immunosuppressive stress. Adult onset demodicosis is the result of severe immuno-suppression caused by systemic disease (such as Cushing's disease, corticosteriod overdose, hypothyroidism, diabetes, or cancer.) The underlying cause must be identified and treated before there can be successful resolution of the demodicosis. Juvenile localized demodicosis may occur because of an immature immune system - especially where there are stresses of poor nutrition or concurrent disease. However, spontaneous occurrence and resolution of localized demodicosis has been reported where there is no obvious stress other than being a young dog. Where a breeder has either localized or generalized demodicosis occur in more than one litter out of one bitch, or from one sire, or in many puppies in a litter, hereditary demodicosis should be considered.
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The inheritance is not clear, in two kennels data were consistent with a simple recessive mode of inheritance. Removal of carrier dogs (sires, dams, and sibs of affected dogs) from the breeding population solved the problem for these kennels. However, other dermatologists feel that the disease is polygenic, or that if there is one major gene, there is variable expression because of modifiers (pers. communication, Dr. Vet. Med. Genevieve Marignac.) Generalized demodicosis is not an outcome of untreated localized demodicosis. The inheritance of localized demodicosis is unclear, it may be the result of the immature immune system but if there is a familial pattern it should be considered hereditary. The generalized form indicates a specific immune system impairment. Dermatologists will not treat the generalized form unless the owner agrees to neutering the affected animal.
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Recommendations: Puppies that develop demodicosis should be given a chance to self-cure. This allows you as a breeder to determine if you are dealing with the localized form or the more serious generalized form. Waiting out the course of localized demodicosis is frustrating - you want to treat. The pet owner and their vet may want to treat immediately. Asking them to wait is important for your breeding program. The pup should have monthly skin scrapings to monitor the progress. If the population of mites decreases, it indicates self-cure might be happening, and in most cases treatment is not necessary.
If the population of mites increases, and the lesions become larger, you and your veterinarian may consider treatment. Scott, Miller and Griffin in Small Animal Dermatology, 6th Ed., recommend different treatments. These dogs can be put in remission, should be neutered, and can live normal lives without recrudescence of the disease.
Dogs that develop the generalized form should not be used in a breeding program, their parents should be considered carriers of the defective genes, and their sibs have a 50% chance of being a carrier. It is important that the owners of both the sire and the dam be informed of dogs that develop both localized and generalized demodicosis. Breeding decisions concerning dogs with localized demodicosis, and their sire and dam, should consider the number of affected littermates, and the family history of other cases of demodicosis. Where a family history is evident, such dogs should not be used in breeding programs.
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References:
Muller's & Kirk's Small Animal Dermatology, 6th Edition. Danny W Scott, DVM, William J Miller, DVM and Gary E Griffin DVM. W. B. Saunders, 2001. Pp 457-474.
Personal Communication: Dr vet Geneviève Marignac , Consultante en Dermatologie Vétérinaire Chargée de consultation à l'ENV, Alfort, France
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