Demodicosis

Description

An inflammatory parasitic disease of dogs and cats caused by Demodex. The mites are host-specific and can occur in dogs, cats, cattle, sheep, goats, pigs and horses. There has to be an increased number of immature mites to classify as an infestation. The mite is transmitted from dam to offspring during nursing. The mites live in hair follicles and infestation may lead to secondary bacterial infection and furunculosis.

Signalment

Commonly affects the dog and more rarely the cat.

Breed predilections: Demodicosis is usually a disease of short haired dogs and is more common in pedigree breeds. Within dog breeds, the Staffordshire bull terrier, German Shepherd dog, Old English sheepdog, Collies and Dalmations are at risk. Additionally within cat breeds it has been reported that there is an increased incidence in Siamese and Burmese cats.

Age predilections: Often animals less than 12 months of age are affected although adult onset is recognised.

Diagnosis

History and clinical signs will often indicate demodicosis. Deep skin scraping reveals Demodex mites, eggs and larval forms in high numbers. Hair plucks can also demonstrate mites microscopically. In all cases of adult onset demodicosis a detailed history and medical investigation must be undertaken to identify an underlying systemic disease.

Clinical Signs

Crusting, hair loss, grey hyperpigmentation and plugged follicles are typical findings in animals suffering from demodicosis.

Dogs: May be localised or generalised.

Localised demodicosis: Typically seen in dogs 3-6 months of age and 90% of cases will heal spontaneously. Signs are of a mild non- pruritic alopecia in some cases erythema and a fine scale. Lesions are most commonly found on the head particularly around the periocular region and the forelimbs.

Generalised demodicosis: Most commonly occurs in dogs between 3-12 months of age. Can be widespread from the beginning and lesions can range from multiple poorly circumscribed areas of erythema, alopecia and scale to generalised pustules often resulting in rupture of the hair follicle (furunculosis). A lymphadenopathy and pyrexia are also common features. Pododemodicosis can occur with the generalised form and can persist after other lesions have resolved or appear in isolation. This manifests as erythema and secondary infection of the feet.

Adult Onset Demodicosis: Can present as generalised disease or focal alopecia . Onset in adulthood is often associated with internal disease such as (hyperadrenocorticism, immunosuppression or neoplasia. Long term use of glucocorticoids in the West Highland white terrier has been associated with adult onset demodicosis.

Cats: Often multifocal alopecia particularly of the eyelids, periocular region, head and neck. Erythema scale and crust may be present with varying degrees of pruritis. Otodemodicosis which manifests as an erythematous ceruminous otitis has also been reported in the cat.

Cattle: Pea-sized nodules are present over the body. Each nodule contains several thousand mites. Affects hide quality and is economically important in Australia.

Laboratory Tests

Haematology and Biochemistry may be useful in identifying an underlying cause in adult onset demodicosis.

Biopsy

Cutaneous biopsy may be necessary if lesions are chronic, granulomatous or fibrotic.

Treatment

Localised demodicosis in young dogs will often resolve spontaneously. In cases of generalised demodicosis: Amitraz (Aludex) should be used as a dip weekly until there is a resolution of clinical signs and skin scrapes are negative for the mite. The mites are not easily accessible to acaricides due to their deep location in the skin and to aid amitraz penetration, clipping a dog's coat and washing with benzoyl peroxide shampoo prior to application is recommended. Side effects of this treatment include lethargy, anorexia, depression, vomiting and diarrhoea. A spot on treatment containing imidacloprid and moxidectin (advocate) is also licenced to treat demodicosis. Any secondary pyoderma present on the animal must also be treated with appropriate antibiotics. If the amitraz and imidacloprid-moxidecin combinations are ineffective or not tolerated ivermectin or milbemycin can be given daily. This treatment is off-licence and requires informed owner consent as well as monitoring of the animal for signs of neurological side effects.

Prognosis

Prognosis is good for localised cases. Generalised demodicosis is often managed medically however many cases are controlled rather than cured. Owners must be educated about the disease and be advised not to breed from animals with the generalised form. Adult-onset dogs are often refractory to treatment.

References

Tilley, L.P, Smith, F.W.K. (2004) The 5-minute veterinary consult. canine and feline. (3rd edition). Lippincott Williams and Wilkins.

4th year BvetMed Dermatology Notes. RVC 2008