Canine solar dermatitis, or chronic sun damage to the skin, is a common dermatologic disorder in hot, sunny climates. It can also affect animals that live in high altitudes or that spend a great deal of time outside, even in temperate zones.
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.
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
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.
Reduce sun exposure
The main treatment recommendation for solar dermatitis is restricting sun exposure by keeping the dog indoors during the day, especially between 9 a.m. and 3 p.m., which is considered the most intense UV radiation time.2 If some sun exposure is unavoidable, then frequent (twice a day11) topical application of a waterproof, high-SPF sunscreen (a product with an SPF > 15 absorbs more than 92% of UVB rays12 ) that is labeled as safe for babies and that protects against UVA and UVB rays is indicated.2 Having the dog wear a T shirt may help decrease sun exposure2; however, it is often impossible to cover all at-risk areas of the skin. A dog sun suit is available at www.designerdogwear.com, or clients may be able to sew a sun suit for their pets by using sun-blocking fabric available for people.
Beta-carotene or acetretin
To decrease sun damage, beta-carotene (30 mg orally b.i.d. for 30 days then 30 mg/day for life) in combination with anti-inflammatory doses of oral glucocorticoids may be effective in early cases.2,13 Skin damage may also be reduced by administering oral retinoids (synthetic vitamin A analogues, which are more potent and less toxic than vitamin A2,14 ) such as acitretin at a dosage of 0.5 to 1 mg/kg orally every 24 hours.2 The potential side effects of retinoids include keratoconjunctivitis sicca, mucocutaneous lesions, vomiting, diarrhea, musculoskeletal abnormalities, triglyceride elevations, and hepatotoxicity,2,11 so careful monitoring is necessary. Additionally, retinoids are highly teratogenic.2 A positive clinical response to retinoids should be noted within four to six weeks. At that point, the frequency of medication can be reduced to an alternate-day basis.11
Because of the expense of retinoids, oral vitamin A has been used anecdotally for canine solar dermatitis, but controlled clinical trials and published studies elucidating doses or side effects are not available. The dosage of oral vitamin A in dogs should not exceed 400 IU/kg/day, and patients should be monitored for the same potential side effects as listed for retinoids.2
Actinic keratosis in people may be helped by using a topical immunomodulator such as imiquimod (Aldara—3M Pharmaceuticals), but studies in dogs are lacking. Imiquimod's mechanism of action involves inducing local antitumor and antiviral immune responses by stimulating lymphocytes, dendritic cells, and macrophages. In people, imiquimod cream is applied to affected areas two to three times a week for four to 16 weeks.15-18 According to the Aldara package insert, the size of the treated area should be no larger than 2 x 2 in, and the cream should not be used near the eyes, lips, or nostrils. The cream is left on the skin for eight hours before being washed off with mild soap and water. Side effects include localized redness, crusting, burning, and pruritus at the site of application. In dogs with more extensive lesions of solar dermatitis, the expense of Aldara (about $245 for a box of 12 individual 250-mg single-use packets) may preclude its use.
Monitor for disease progression
Kimberly S. Coyner, DVM, DACVD
Dermatology Clinic for Animals
5231 W. Charleston Blvd.
Las Vegas, NV 89146
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