Affects: Dogs
Canine demodicosis is a common parasitic skin disease in dogs caused by the excessive proliferation of the hair follicle mite Demodex canis. It can occur locally or all over the body. Demodicosis in older animals only occurs in connection with disorders of the immune system, in young animals the development of the disease is not fully understood. Demodicosis usually begins with hair loss and without itching. As the disease progresses, a secondary bacterial infection can lead to severe skin changes up to purulent skin inflammation (pyoderma). The disease is detected by microscopic detection of mites. Treatment is carried out with mite-effective medication.
Cause Of Illness: Demodicosis is mainly triggered by Demodex canis. Demodex canis is a slender mite, about 250 to 300 micrometers long and 40 micrometers thick, which parasitizes the hair follicles and sebaceous glands. There it feeds on sebum, tissue fluid and the naturally rejected cells. In small numbers, these mites also occur as commensals in many clinically healthy animals (the human-specific sister species D. brevis and D. folliculorum are very common and almost always commensals). The female mites lay eggs, which develop into adult mites via a larval and nymph stage. The entire development cycle takes place in the hair follicles and lasts 20 to 35 days. Outside the host, hair follicle mites are not able to survive and die quickly as a result of dehydration. Hair follicle mites do not produce feces but store metabolic degradation products in cells of the intestinal tract, so that they hardly provoke an immune response.
More recently, other Demodex canis-like mites have been described, which are larger and smaller. The shorter mite was called Demodex cornei, the longer one Demodex injai. Demodex cornei lives mainly on the surface of the skin and can occur in combination with Demodex canis. Demodex injai seems to be mainly found in the sebaceous glands. However, D. injai and D. cornei may only be morphological variants of Demodex canis and not independent species.
Pathogenesis And Spread Of The Disease: The transmission of Demodex mites usually takes place at the age of a few days from the mother to the puppies during suckling. However, this infection usually remains asymptomatic. Transmission from dog to dog after the third day of life is considered unlikely. The outbreak of demodicosis only occurs much later, when these mites multiply strongly. Affected young animals do not seem to have any disorders of the immune system, as they are not more sensitive to other diseases, so it is assumed that their immune competence is reduced mite-specific. Only a temporary reduction in T cell immunity is observed, but this may only be a consequence of the disease. In older animals, demodicosis usually occurs due to disorders of the immune system such as tumors, hyperactive adrenal glands, hypothyroidism, leishmaniasis, malnutrition, and treatment with glucocorticoids, other immunosuppressants, progesterone or chemotherapeutic agents.
Demodicosis occurs worldwide. An increased tendency to the disease in certain dog breeds (breed predisposition) is not observed in Europe, in contrast to the United States. In the United States, English Bulldogs, French Bulldogs, Pugs, Dobermans, German Shepherds, Miniature Schnauzers, and some terriers (Pit Bull, West Highland White, Jack Russel, Staffordshire Bull Terrier) are particularly affected. This is explained by the fact that immunocompromised animals ("caretakers") are not used for breeding in Europe. In addition, there seems to be an individual genetic predisposition.
Treatment: Local demodicosis in young animals regresses spontaneously in 90% of cases. Whether a treatment is useful or not is controversial in the literature. On the one hand, it is recommended to avoid generalization, and on the other hand, it is recommended to wait for the possible generalization in order to be able to use it as a breeding exclusion criterion. Local external (topical) treatment, for example by applying a gel with benzoyl peroxide, chlorhexidine or rotenone, is usually sufficient. Benzoyl peroxide penetrates well into the hair follicles, but has a strong drying effect and sometimes irritating to the skin.
In the past, regular washing treatment with amitraz had proven effective in both local and systemic demodicosis. However, some dwarf dog breeds (Chihuahua, Maltese) react very sensitively to this active ingredient, so that its use in these is not recommended. In the case of severe infestation, full shearing is recommended for long-haired dogs, as the active ingredient must wet the skin well in order to penetrate deep enough into the hair follicles. In the case of severe secondary bacterial infection, it should be treated first, e.g. by shearing the affected areas, cleaning with disinfecting washing solutions and systemic administration of antibiotics, as amitraz should not be applied to larger wounds. Since June 2009, a spot-on preparation with amitraz was also approved for the treatment of demodicosis, which only needs to be applied 14 times a day. Especially in the case of local demodicosis, a 14-day therapy control is useful in order to prevent premature discontinuation of treatment and thus the risk of generalized demodicosis. An emerging treatment success is visible in the decrease in the number of live mites, the increase in crippled mites and the decrease in larvae. Complete healing is indicated by regrown hairs and lack of evidence of live mites and succeeds with amitraz in about 80% of cases. In about 40% of cases, side effects such as fatigue and itching occur, which can be reduced by increasing the dilution or reducing the frequency of treatment. Occasionally, more severe side effects such as reluctance to eat, ataxia as well as increased thirst and urination can occur. Since amitraz also causes an increase in blood sugar levels, its use in diabetic dogs is contraindicated. However, as of August 2020, preparations containing amitraz are no longer approved for use in dogs.
Systemic treatment with ivermectin, moxidectin, or milbemycin oxime is also highly effective. These active ingredients are administered daily orally until successful therapy control. Some dog breeds and puppies under 12 weeks of age react very sensitively to some avermectins due to the insufficient blood-brain barrier (→ MDR1 defect) and that there is only one avermectin preparation approved for dogs in Germany, moxidectin. Treatment with milbemycin oxime is also possible for dogs that are sensitive to avermectin. However, it usually has to be carried out over about 70 days and is therefore very cost-intensive. Current studies show good efficacy of isoxazolines such as Fluralaner, Sarolaner, or Afoxolan in generalized demodicosis. Since 2018 and 2019, preparations of these three isoxazolines have been approved for the treatment of demodicosis.
In the case of secondary bacterial infections, local treatment with disinfectant solutions (benzoyl peroxide, chlorhexidine, povidone iodine) or antibiotics is indicated in addition to mite control, and in severe pyoderma, the systemic administration of antibiotics before the actual mite control is also indicated.