Generalized demodicosis

Generalized disease in dogs shows widespread patchy hair loss with redness, papules and pustules, comedones, scaling or oily seborrhoea, swelling, darkened thickened skin and crusts that may ooze blood or pus. Itch ranges from mild to intense, and inflamed feet (pododemodicosis) are frequent. Severe cases develop deep infection and cellulitis and can become systemically ill. In cats, generalized D. cati disease causes hair loss, crusting and pyoderma over the whole body, sometimes only ceruminous ear inflammation, while D. gatoi is usually itchy.

Demodicosis is disease caused by excessive multiplication of Demodex mites, which live in small numbers as normal commensals in the hair follicles and sebaceous glands of most dogs. In dogs the main species is Demodex canis, with D. injai and the short, surface-dwelling D. cornei also recognised; cats carry follicular D. cati and the superficial D. gatoi. Puppies acquire mites from the dam while nursing in the first days of life, and the whole life cycle is spent on the host. Disease follows when the immune system fails to hold the population in check: juvenile-onset generalized disease is linked to an inherited, mite-specific defect in cell-mediated immunity, while adult-onset disease points to acquired immune compromise from illness or medication. Follicles packed with mites become inflamed, hair falls out, and secondary bacterial infection is common.

In a large UK study of dogs under veterinary care, juvenile-onset demodicosis (under 2 years) had a one-year prevalence of 0.48%, about ten times that of adult-onset disease (over 4 years, 0.05%); median age at diagnosis was 7 months, with an interquartile range of 4 to 13 months. British Bulldog, Staffordshire Bull Terrier, Chinese Shar-Pei, Dogue de Bordeaux, Pug and French Bulldog had the highest odds of juvenile disease, and Shar-Pei, Shih-Tzu and West Highland White Terrier of adult disease. Hereditary predisposition to generalized disease is strong. Adult-onset cases in dogs are associated with hyperadrenocorticism, hypothyroidism, diabetes and cancers such as lymphoma; in cats, with feline leukaemia, feline immunodeficiency, diabetes or neoplasia. Canine demodicosis is not contagious; feline D. gatoi is.

The mite species themselves must be separated: follicular D. canis is found by deep skin scraping or hair plucking, whereas the short-bodied D. cornei lives in the stratum corneum and is found by tape impression or superficial scraping; in cats, superficial and contagious D. gatoi is distinguished from follicular D. cati the same way. In cats dermatophytosis can resemble and coexist with demodicosis, so fungal culture is essential. Secondary bacterial pyoderma, confirmed by cytology, often accompanies generalized disease and should not stop the search for mites.

Skin, hair follicles, sebaceous glands, stratum corneum, feet (pododermatitis), ear canals (cats), cell-mediated immune system

Do not breed dogs that had juvenile-onset generalized demodicosis. Avoid corticosteroids in affected animals. Look for and manage underlying immunosuppressive disease in adult-onset and feline cases.

Generalized disease can be life-threatening untreated. Once guarded, outlook in dogs is now good with isoxazolines or macrocyclic lactones given until repeated monthly scrapings are negative. Adult-onset and feline cases hinge on the underlying disease.