True (primary) acanthosis nigricans is a genetic dermatosis found almost exclusively in Dachshunds. It is characterized by noninflammatory pigmentation of the axillary and inguinal areas. Postinflammatory hyperpigmentation can appear clinically similar; however, it is a reaction secondary to an underlying disorder. Dogs with primary acanthosis nigricans have no history of past or recent skin disease. Diagnosis is based on clinical signs. Treatment of postinflammatory hyperpigmentation includes topical anti-inflammatories, or antimicrobials if secondary bacterial or yeast infection is present, and resolution of the underlying inflammatory trigger.
True acanthosis nigricans (or primary acanthosis nigricans) is a genetic skin disorder, or genodermatosis, of dogs. It affects primarily Dachshunds, occurs early in life, and is noninflammatory.
"Secondary acanthosis nigricans" refers to a clinical reaction due to inflammation in dogs that is characterized by axillary and/or inguinal hyperpigmentation, lichenification, and alopecia. Although this postinflammatory hyperpigmentation can appear clinically similar to true acanthosis nigricans, dermatologists do not use the term "acanthosis nigricans" to describe the inflammatory reaction pattern.
Etiology of Primary Acanthosis Nigricans and Postinflammatory Hyperpigmentation in Dogs
"Acanthosis nigricans" is not a specific disease but a term used to describe a clinical sign (hyperpigmentation) of an underlying skin disease. "Primary acanthosis nigricans" is a rare heritable disorder of hyperpigmentation in dogs. The term "acanthosis" describes mild to moderate thickening of the stratum spinosum found on histological examination of skin biopsy specimens. The term "nigricans" is Latin for "becoming black" in color. Acanthosis and hyperpigmentation are nonspecific histological and clinical findings, respectively, found in many skin diseases.
Primary acanthosis nigricans occurs almost exclusively in Dachshunds and is believed to be heritable in a recessive pattern. The specific gene or genes involved remain unknown. It is a diagnosis of exclusion.
There is no sex predilection. Clinical signs usually develop by 1 year of age.
The term secondary acanthosis nigricans has been used to describe postinflammatory hyperpigmentation; however, because it is nonspecific and implies a diagnosis, the term "acanthosis nigricans" is not used in veterinary dermatology to describe this inflammatory reaction pattern. The proper term is "postinflammatory hyperpigmentation," which is a clinical sign of underlying disease.
Postinflammatory hyperpigmentation can occur in any dog breed and at any age; it is most common in breeds predisposed to conditions that result in inflammation of the axillary or inguinal regions because of the following:
conformational abnormalities
obesity
endocrinopathies (eg, hypothyroidism, hyperadrenocorticism, sex hormone abnormalities)
axillary and inguinal pruritus associated with atopic dermatitis
food allergies
contact dermatitis
primary keratinization disorders
skin infections (eg, staphylococcal pyoderma, Malassezia dermatitis)
Differentiating Between Primary Acanthosis Nigricans and Postinflammatory Hyperpigmentation
Primary Acanthosis Nigricans | Postinflammatory Hyperpigmentation | |
|---|---|---|
Breed | Dachshund | Any dog breed |
Age of onset | < 1 year | Any age, but usually > 1 year |
Pattern | Axillary and/or inguinal areas | Any site, any pattern; can be localized or diffuse |
Inflammatory lesions | Absent | Any combination of erythema, scaling, hair loss, lichenification |
Secondary infections | Absent | Common |
Pruritus | Absent | Common |
Skin surface texture | Smooth | Irregular, elevated, lichenified |
Concurrent skin disease | Absent | Common |
For comparison of the two types of the disorder, see the table Differentiating Between Primary Acanthosis Nigricans and Postinflammatory Hyperpigmentation.
Clinical Findings of Primary Acanthosis Nigricans and Postinflammatory Hyperpigmentation in Dogs
Clinical signs of primary acanthosis nigricans start with increased pigmentation in the axillary and/or inguinal regions. In primary acanthosis nigricans, hyperpigmentation is initially diffuse and noninflammatory; it tends to develop uniformly in affected areas. In addition, secondary inflammatory lesions (ie, lichenification) develop, usually as a result of skin-on-skin friction arising from a dog's conformation.
In postinflammatory hyperpigmentation, the distribution is patchy and often starts with a lacy appearance (see postinflammatory hyperpigmentation image). It might not develop in all areas at the same time. Inflammation is mild but becomes more severe with time. Lesions of postsecondary inflammation may not be present in both the axillary and inguinal regions, and they may might not be symmetrical.
Courtesy of Dr. Karen Moriello.
Postinflammatory hyperpigmentation is triggered by inflammation and/or friction. Lesions can develop into severe areas of hyperpigmentation, with marked lichenification, hair loss, and seborrhea. Often, these areas are odiferous and can be painful. The edges of these lesions are often erythematous, which is a sign of secondary bacterial and/or yeast pyoderma.
With time, lesions can spread to the ventral neck, groin, abdomen, perineum, hocks, periocular area, and pinnae. Pruritus varies and is usually due to the underlying disease or is the result of secondary microbial overgrowth (staphylococcal or Malassezia dermatitis).
Diagnosis of Primary Acanthosis Nigricans and Postinflammatory Hyperpigmentation in Dogs
For primary acanthosis nigricans: a diagnosis of exclusion after physical examination.
For postinflammatory hyperpigmentation: identification of the underlying disease
Suspicion of primary acanthosis nigricans is typically based on history, signalment, and physical examination, and diagnosis is largely one of exclusion. The physical examination findings compatible with a diagnosis of primary acanthosis nigricans include bilaterally symmetrical hyperpigmentation beginning in the axillary area and progressing to lichenification and alopecia.
Postinflammatory hyperpigmentation is a clinical sign of an underlying disease and should be evaluated through careful history and physical examination to identify the underlying trigger. Skin scrapings should be performed to exclude demodicosis, and impression smears should be performed to confirm suspected bacterial and/or Malassezia infections.
Pearls & Pitfalls
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Postinflammatory hyperpigmentation associated with endocrinopathies is not pruritic. Endocrine skin diseases are not pruritic unless accompanied by secondary skin infections. Older dogs with hyperpigmentation should be tested for thyroid and adrenal disease. Intradermal skin testing and/or a food trial also might be necessary, but they should be undertaken only after parasitic and other infectious causes have been ruled out.
Skin biopsies are usually not necessary to confirm primary acanthosis nigricans; histopathological changes can resemble those of chronic hyperplastic dermatitis due to various other causes. Skin biopsies are usually not helpful in identifying the underlying disease associated with secondary acanthosis nigricans (ie, postinflammatory hyperpigmentation).
Secondary bacterial and yeast infections are underdiagnosed with this condition, and bacterial culture and cytological examination of the skin are important initial tests.
In Dachshunds, many cases of cutaneous hyperpigmentation diagnosed as primary acanthosis nigricans are actually caused by underlying disease and not genodermatosis. Dogs that are pruritic and that do not have fleas or other signs of skin infection should be evaluated for allergic skin disease.
Treatment of Primary Acanthosis Nigricans and Postinflammatory Hyperpigmentation in Dogs
For primary acanthosis nigricans: supportive care
For postinflammatory hyperpigmentation: correction of the underlying cause
Primary acanthosis nigricans in Dachshunds is not curable. In some dogs, lesions do not progress beyond a cosmetic problem. Secondary inflammatory lesions may develop due to unrelated acquired inflammatory skin disease. In Dachshunds, the most common underlying problem is friction caused by conformational changes or obesity.
In addition to treatment of the concurrent trigger, treatment with antimicrobial shampoo (eg, chlorhexidine) is often helpful. Antiseborrheic shampoos (eg, coal tar, salicylic acid, and micronized sulfur) are often beneficial for removing excess oil and odor, but they must be used 2–3 times per week.
If lesions progress, aggressive systemic antimicrobial therapy might be needed. The choice of systemic antimicrobials should be based on culture and susceptibility testing.
In postinflammatory hyperpigmentation, most lesions resolve after the underlying cause is identified and corrected. Some residual lacy hyperpigmentation might remain. Treatment of secondary bacterial and yeast overgrowth is critical.
Patients not previously treated for staphylococcal bacterial skin infection should be treated with narrow-spectrum drugs selected on the basis of culture and susceptibility testing and/or with topical chlorhexidine/antifungal (miconazole or ketoconazole) shampoos 3 times a week, plus chlorhexidine or chlorhexidine/antifungal sprays applied on nonbath days. Culture and susceptibility testing is recommended to minimize development of methicillin-resistant staphylococci.
Yeast infections can be successfully treated with concurrent itraconazole (5 mg/kg, PO, every 24 hours for 10–30 days) or ketoconazole (5–10 mg/kg, PO, every 24 hours, for 10–30 days). Affected dogs benefit greatly from appropriate antimicrobial therapy and use of antiseborrheic shampoos (2–3 times/week).
If lesions are caused by friction, emollients can be beneficial.
Clinical signs resolve slowly, possibly over months.
Key Points
What is often diagnosed as primary acanthosis nigricans is usually postinflammatory hyperpigmentation, which is common in dogs with allergic skin disease and/or intertriginous friction (ie, skin-on-skin rubbing).
Topical therapy should be administered several times a week to relieve pruritus and discomfort and to control odor until the underlying skin disease can be identified and treated.
True (primary) acanthosis nigricans is a rare genodermatosis found in Dachshunds.
For More Information
Bajwa J. Cutaneous hyperpigmentation in dogs. Can Vet J. 2022;63:85-88.
Also see pet owner content regarding hyperpigmentation in dogs.
