5. Solar dermatitis on the inguinal area of an American bulldog. After years of sun damage, a large squamous cell carcinoma
has formed.
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The duration and intensity of sun exposure influence the degree of skin damage. The initial signs of actinic damage are erythematous,
scaly lesions, which may be tender. With repeated sun exposure, actinic folliculitis, follicular cyst formation, and dermal
fibrosis occur.8 In dogs with pigmented areas on their skin, there is often sharp demarcation between areas of normal skin with protective
pigment and damaged nonpigmented skin (Figure 2).2 With chronic sun exposure, damaged areas become thickened and scarred with comedones, erosions, ulcers, crusts, and draining
tracts.2 Secondary bacterial pyoderma is common.9 Sun-induced skin tumors such as squamous cell carcinoma (Figure 5), hemangioma, and cutaneous hemangiosarcoma may occur. Although affected dogs may lick damaged areas, the pruritus associated
with solar dermatitis is usually otherwise minimal, unlike that in dogs with allergic dermatitis. However, some dogs can have
solar dermatitis and concurrent allergies.
DIAGNOSIS
Diagnosing solar dermatitis involves considering a patient's signalment and clinical signs and ruling out other causes of
scaly, erythematous dermatitis or folliculitis (e.g. bacterial, Demodex species, and dermatophyte infections). Lack of resolution of skin lesions with empiric therapy should raise your suspicion
of solar dermatitis and the need for further investigation.
6. Inguinal erythema, scarring, and furunculosis caused by chronic sun damage in the same dog as in Figure 1. The circled
areas indicate biopsy sites.
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Ultimately, skin biopsy and histology are used to diagnose solar dermatitis and solar-induced neoplasia. Depending on the
degree of secondary bacterial infection, systemic antibiotics may be indicated for two to three weeks before biopsy to ensure
that infection does not alter histologic interpretation. Additionally, clearing secondary skin infection also increases the
likelihood of correctly selecting solar-induced rather than infection-induced skin lesions for biopsy.10 However, if an obviously neoplastic skin mass is noted on physical examination, perform a biopsy immediately to expedite
treatment. Biopsy can be performed by administering lidocaine locally and obtaining multiple skin punch or excisional biopsy
samples of different lesions (Figure 6).
Since some of the histologic changes can be seen with other conditions such as bacterial folliculitis, one key to a definitive
histologic diagnosis is to include a complete history with the biopsy submission form, including signalment, degree of sun
exposure, distribution of lesions, clinical description of lesions, response or lack of response to prior therapies, and current
medications (including glucocorticoids) that could affect the histologic findings. Requesting a full histologic description
and seeking interpretation by a veterinary dermatopathologist are recommended.
In early cases of solar dermatitis, histologic examination shows variable degrees of perivascular dermatitis, folliculitis,
and dermal fibrosis or increased collagen accumulation or collagen damage. Solar elastosis (linear bands of degenerated basophilic
elastin accumulation arranged parallel to the skin surface) may be seen. In chronic cases, histologic examination may show
follicular cysts, pyogranulomatous inflammation, and precancerous actinic keratosis or neoplastic cells.2,9,10
TREATMENT
The best treatment for canine solar dermatitis is prevention. Educate owners of at-risk dogs about the need for sun avoidance
starting at a young age. Also tell owners of affected and at-risk dogs that oral and topical medications cannot replace sun
avoidance in treating and preventing solar dermatitis.
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