Demodicosis

What it is

Demodex canis is a normal inhabitant of the canine hair follicle in small numbers. Puppies acquire it from their dam by direct contact during the first days of life; it is not contagious between adult dogs. Demodicosis develops when the mites multiply uncontrollably because the host's immune control of them fails. Two other species are recognised: D. injai, a long mite that lives in the sebaceous glands and causes greasy dermatitis on the dorsal trunk, and a short form found in the stratum corneum.

The disease is classified by age at onset and extent:

• Juvenile-onset: in young dogs, with a hereditary predisposition. Localised forms often resolve without treatment.
• Adult-onset: in dogs with no previous history; it usually reflects an underlying condition such as hyperadrenocorticism, hypothyroidism, neoplasia or immunosuppressive treatment.
• Localised (a few patches) or generalised (many lesions, large areas or involvement of the paws).

Who it affects

Juvenile-onset disease occurs in puppies and young dogs, often in breeds such as the shar-pei, bulldogs, pug, Staffordshire bull terrier and Dobermann. Adult-onset disease affects older dogs with predisposing conditions.

Anatomy involved

The mites live in the hair follicles of the skin (cutis), head down in the follicular lumen. Their overgrowth distends the follicle and causes follicular plugging, hair loss and folliculitis. If the follicle ruptures, keratin and mites in the dermis cause furunculosis and deep pyoderma. Early lesions are typical around the eyes, on the lips and on the forelimbs; pododemodicosis affects the paws.

Clinical signs

Patchy alopecia, erythema, scaling, comedones, follicular casts on the hair shafts and hyperpigmentation. Pruritus is absent or mild unless there is secondary infection. Generalised disease may progress to deep pyoderma with swelling, draining tracts, lymphadenopathy, fever and malaise.

Diagnosis

• Deep skin scrapings after squeezing the skin, until capillary bleeding, to bring mites out of the follicles.
• Trichograms (plucked hairs) and tape impressions after squeezing; useful in sensitive areas such as around the eyes and the paws.
• Biopsy for chronic pododemodicosis or for breeds with very thick skin, such as the shar-pei.
• Investigation of underlying disease in adult-onset cases.

Treatment

Generalised demodicosis is now usually treated with isoxazolines (afoxolaner, fluralaner, sarolaner, lotilaner), which are effective and safe. Older options include amitraz rinses and macrocyclic lactones; ivermectin and similar drugs are dangerous at high doses in dogs with the ABCB1 (MDR1) mutation, common in collies and related herding breeds. Treatment continues until repeated skin scrapings are negative. Secondary pyoderma is treated at the same time, glucocorticoids are avoided, and the underlying disease is managed in adults. Dogs with juvenile-onset generalised demodicosis should not be used for breeding.