Cutaneous hyperpigmentation in dogs

Hyperpigmentation is a common skin condition that can be concerning to pet owners. Hyperpigmentation occurs when skin produces increased melanin (melanoderma; Table 1), and may be localized, multifocal, or generalized. Hyperpigmentation of the hair may also be noted (melanotrichia). Hyperpigmentation can affect dogs of all breeds, hair coat types, and ages. The term hyperpigmentation is commonly used based on grossly increased pigmentation, beyond the animal’s normal skin color. Histologically, hyperpigmentation is associated with increased evidence of melanin in the epidermis and corneocytes. Dermal melanin pigment deposition may also be noted histologically (1). Cutaneous hyperpigmentation has various causes, including genetic and/or acquired.

Table 1.

Definitions of cutaneous hyperpigmentation in dogs.

Term	Description
Hyperpigmentation	Increased pigmentation of cutaneous structures, including skin and hair
Melanoderma	Increased pigmentation of skin
Melanotrichia	Increased pigmentation of hair

The most common cause of hyperpigmentation in canine clinical practice is post-inflammatory cutaneous change (1). Post-inflammatory hyperpigmentation may be focal and circumscribed, patchy, or diffuse (2). Increased melanin pigment is produced in or around areas of inflammation. Chronic erythema and/or papular lesions from any primary cause may undergo hyperpigmentation as active inflammation is cleared by the body’s immune system, anti-inflammatory or antimicrobial treatment, or resolution of the inciting cause. This change often occurs concurrent with active inflammatory changes within epidermal collarettes, as central melanoderma accompanying alopecia with peripheral crust and erythema (Figure 1). Similarly, the center of healing annular dermatophytosis lesions frequently has hyperpigmented skin. When generalized crusts or epidermal collarettes are present, concomitant patchy multifocal to generalized hyperpigmentation may also be noted. More diffuse hyperpigmentation may result from chronic diffuse inflammation, e.g., ventral trunk of an atopic dog, or with hormonal disorders such as hyperadrenocorticism (Figure 2) or hypothyroidism. When generalized erythema is present, controlling the inflammatory change will often lead to post-inflammatory hyperpigmentation with a lattice-like appearance. Such a change may also occur with resolving generalized superficial bacterial pyoderma (Figure 3) and/or malassezia dermatitis. Chronic or healing lesions of parasitic diseases such as demodicosis and scabies also lead to hyperpigmentation. Almost any acute or chronic cutaneous insult can lead to the development of post-inflammatory hyperpigmentation (1).

Figure 1.

Epidermal collarette exhibiting central melanoderma with peripheral crust and erythema.

Figure 2.

Diffuse hyperpigmentation on ventral trunk of a dog affected by hyperadrenocorticism.

Figure 3.

Periocular hyperpigmentation with resolving pyoderma.

Darkening of the skin is often distressing to pet owners and they will report it. Post-inflammatory hyperpigmentation may occur at the same time as active skin disease, or it may occur in association with improvements in cutaneous health. Generally, post-inflammatory cutaneous hyperpigmentation is a positive change when pruritus and secondary infection are either well-managed or resolved. In the author’s dermatology practice, such hyperpigmentation is regarded as a routine and expected change in an improving patient. Primary disease and other secondary factors, including pruritus and infection, are treated while monitoring hyperpigmentation and post-inflammatory skin changes. Upon adequate control of primary and secondary factors affecting cutaneous health and inflammation (primary, secondary due to self-trauma, chronic changes, infection, etc.), hyperpigmentation slowly resolves, with the patient’s normal skin pigmentation becoming more apparent over time. It may take weeks to months to return to a normal appearance and is usually only cosmetic if patient comfort and other secondary factors remain well-managed or have resolved. However, for incurable skin conditions such as canine atopic dermatitis or canine cyclical flank alopecia, hyperpigmentation can be expected to return with recurrence of cutaneous abnormalities such as pruritus, inflammatory changes, hair loss, etc. Although post-inflammatory hyperpigmentation is most likely in dogs exhibiting hyperpigmentation during ongoing therapy for a responsive skin condition, thorough review of patient history, and clinical assessment, including appropriate diagnostic testing, should always be considered to assess the potential cause(s) of hyperpigmentation, as it may suggest development of novel dermatological disease and/or secondary infection.

The exact mechanism of hyperpigmentation in inflammatory, metabolic, or hormonal diseases is unknown. Stimulated or stressed keratinocytes may be able to locally stimulate melanogenesis by releasing melanocyte stimulating factors; they may be present in low concentrations in normal epidermis, but their concentrations and activity are increased in response to stimulation or keratinocyte stress (3). Melanocytes may also be affected by direct effects of hormonal changes. Corticotropin and other pituitary lipotrophic hormones stimulate melanogenesis, accounting for some cases of hyperpigmentation in hypercorticism and adrenal sex hormone patients. However, direct actions of other hormones on melanocytes have not been well-defined.

Ultraviolet (UV) light exposure also likely has a role. Seasonal hyperpigmentation in warmer months may be noted in dogs that spend considerable time outdoors and is likely related to UV light exposure of sparsely haired skin such as ventral caudal trunk and axilla. Alopecia of any cause, including hormonal and non-hormonal conditions, is often accompanied by hyperpigmentation, indicating that skin exposure to increased UV light may contribute to pigment changes. Such conditions include hypothyroidism, hyperadrenocorticism, pattern alopecia, follicular dysplasia, traction alopecia, as well as alopecia associated with inflammatory and infectious causes. Canine flank alopecia is another alopecic condition that is visually distinctive, with hyperpigmentation affecting hairless skin. In addition, Alopecia X in plush-coated breeds is another visually striking skin condition that includes bilaterally symmetric hair loss with usually significant generalized hyperpigmentation of the skin. Protection of alopecic skin from light exposure decreases hyperpigmentation in some animals.

Focal or multifocal post-inflammatory melanotrichia may occur with healing of deep cutaneous inflammation from conditions such as sebaceous adenitis, vaccine reaction, panniculitis, wounds, or after punch biopsy sampling of affected skin (1). Hair regrowth in alopecic conditions may also occur with accompanying melanotrichia. Melanotrichia usually resolves at the next shedding cycle (2).

Like post-inflammatory hyperpigmentation, post-inflammatory hypopigmentation can also occur in dogs, but is less common as a sequel to cutaneous inflammation. Cutaneous pigmentation usually normalizes after control and/or resolution of underlying cause(s) of post-inflammatory pigmentary changes.

Other acquired causes of hyperpigmentation in dogs include:

• Papilloma virus associated hyperpigmentation is present within various clinical forms in association with the virus. Hyperpigmentation can be noted within canine oral papillomatosis lesions, cutaneous (exophytic) papillomas, and multiple pigmented plaques (4,5) in pugs and miniature schnauzers as non-regressing lesions on the ventral trunk and medial thighs. Lesions begin as pigmented macules and plaques that progress to scaly and hyperkeratotic flat masses. If undiagnosed or left untreated, some lesions may undergo malignant transformation into squamous cell carcinomas.

• Comedone associated hyperpigmentation may be noted in association with conditions that lead to comedone formation such as pyoderma, chronic pressure points, topical corticosteroid use, endocrine disease, demodicosis, actinic keratosis, etc.

• Cutaneous tumor-related hyperpigmentation is noted with melanocytoma and melanoma (6). These tumors occur in dogs at an average age of 9 y; breeds at risk include miniature and standard schnauzers, Scottish terrier, and Irish setter dogs. When suspected, therapy of choice is surgical excision and histopathology. Various other tumors may also exhibit hyperpigmentation including basal cell tumors, trichoblastomas, fibromas, epidermal nevi, and epithelial nevi. Apocrine gland cysts and tumors appear bluish. Vascular tumors may appear red, port wine, red-blue, or dark blue to black. Histiocytic, lymphocytic, and plasmacytic tumors often appear pink, red, or purple.

• Drug-induced hyperpigmentation is considered rare, as most hyperpigmentation is believed to be post-inflammatory, as a result of effective drug therapy. Drug-induced hyperpigmentation has been documented in association with mitotane, minocycline (7), cabergoline (8), and ketoconazole (1).

Some genetic causes of hyperpigmentation may be present at a young age or may develop as the patient ages. Genetic factors may also cause increased or more dramatic development of hyperpigmentation in the event of patient being affected by cutaneous disease. Some genetic causes of hyperpigmentation are:

1. Lentigo is an asymptomatic condition characterized by 1 (lentigo) or more (lentigines) flat macules or patches of sharply circumscribed hyperpigmented skin. These are benign skin changes and a cosmetic problem only, with highest incidence in middle-aged to older dogs. Lesions are most common on the ventral abdomen and chest (2). When noted, the lesions should be differentiated from pigmented tumors such as melanoma, papilloma virus-induced lesions and pigmented nevi. Differentiation can be achieved by careful clinical assessment (including palpation and measurement of lesion size), monitoring for progression, or histopathological analysis (1).

2. Canine acanthosis nigricans is an uncommon cutaneous reaction pattern characterized by axillary hyperpigmentation, lichenification, and alopecia. The condition may be primary (idiopathic) or secondary. Primary canine acanthosis nigricans is almost exclusively a disease of dachshund dogs. The striking breed predilection and early age at onset strongly suggest that this type of canine acanthosis nigricans is a genodermatosis (1). Secondary canine acanthosis nigricans is associated with underlying disorders that include friction or intertrigo, secondary cutaneous infection, endocrinopathy, and hypersensitivity conditions. Any breed may be affected with the secondary form.

3. Epidermal nevi are developmental defects in the skin which may be hereditary in nature. Most epidermal nevi and all melanocytic nevi are associated with increased pigmentation. These lesions are often cosmetic, although secondary infections may occur, and some may be pruritic or inflammatory.