Acanthosis Nigricans

Introduction

Acanthosis Nigricans is a dermatological pigment condition. It is a cutaneous reaction pattern characterised by axillary and inguinal hyperpigmentation, lichenification and alopecia. Acanthosis nigricans should be thought of as a clinical sign and not the diagnosis in some cases (secondary) as signs are usually due to inflammation caused by friction of the skin. Primary (idiopathic) disease is very rare and is thought to be a genodermatosis.

Primary disease occurs in Dachshunds, of either sex, but it has become increasingly rare as the ability to diagnose more common causes of axillary hyperpigmentation has increased (allergies and microbial infections). Clinical signs are usually manifested by around 1 year of age.

Secondary acanthosis nigricans is relatively common and can occur in any breed of dog, especially those with a predisposition to the underlying causes. Such underlying causes include obesity, conformational abnormalities, endocrinopathies, axillary and inguinal pruritus associated with atopy, food allergy and some skin infections.

Clinical Signs

Clinical signs include bilaterally symmetric axillary or inguinal hyperpigmentation and lichenification. The edges of these lesions are often erythematous and this is often due to secondary pyoderma. Seborrhea may also occur as a secondary condition. With time, lesions may spread to the ventral neck, groin, abdomen, perineum, hocks, periocular area, and pinnae. Pruritus is variable and is caused by either the underlying disease or a secondary infection. As the disease progresses, alopecia may then become evident. If the condition is presented in its chronic form, then lichenification may be present in the areas mentioned above.

Diagnosis

Clinical signs are indicative of the disease.

Full physical examination should be performed and all underlying causes should be investigated, by skin scrapings, smears, endocrine tests, intradermal skin tests etc. The diagnosis of primary acanthosis nigricans can only be made by exclusion, after all causes have been eliminated from the differential list.

Upon microscopic examination of the skin, one will see a thickened epidermis by mainly acanthosis, hyperkeratosis and increased melanin in the epidermis.

Treatment and Control

Primary acanthosis nigricans in Dachshunds is not curable. Some cases, if caught early, may respond to shampoo therapy and local topical glucocorticoids, eg, betamethasone valerate ointment. As the condition progresses, more aggressive systemic therapy may be useful. Vitamin E, systemic glucocorticoids, then on alternate days; melatonin, then weekly or monthly as needed, has been used as a treatment regime in more severe cases and has proven effective. Antibiotics or treatment of Malassezia is required prior to commencement of treatment in most cases.

In secondary acanthosis nigricans, the lesions will spontaneously resolve after identification and correction of the underlying cause. However, this will not occur if secondary bacterial and yeast pyodermas are not treated appropriately. Cephalexin and concurrent itraconazole or ketoconazole is an effective treatment regime. Affected dogs benefit greatly from appropriate antimicrobial therapy and antiseborrheic shampoos (2-3 times/wk).

Prognosis

Will be a lifelong condition in primary disease. In secondary disease, clinical signs should resolve in a few months depending on underlying cause.

References

Foster, A, and Foll, C. (2003) BSAVA small animal dermatology (second edition) British Small Animal Veterinary Association

Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial

Tilley, L.P. and Smith, F.W.K.(2004)The 5-minute Veterinary Consult (Third edition) Lippincott, Williams & Wilkins.