Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA
Solar dermatitis (photodermatitis, actinic dermatitis) is a broad term referring to acute or chronic dermatitis resulting from exposure to ultraviolet (UV) light. High-energy, shorter-wavelength light (UVB, 280–320 nm) is responsible for acute phototoxic reactions (sunburn), while longer wavelengths (UVA, 320–400 nm) penetrate deeply into the skin and are responsible for the permanent damage associated with chronic solar dermatitis.
Solar dermatitis is typically seen on lightly or nonpigmented skin with minimal or no hair cover. While affected animals typically spend considerable time outdoors, chronic solar dermatitis may even be seen in indoor-only animals, as much UVA light is not stopped by standard window glass. In dogs, affected areas typically include the planum nasale, dorsal and rostral muzzle, ventrum and ventrolateral thorax, especially in "sunbathers" that like to lay on their sides or backs. In cats, affected areas typically include the pinnae, preauricular skin, eyelid margins and planum nasale. Involvement of the bridge of the nose and rostral muzzle may also be seen.
Acute phototoxicity is characterized by erythematous, thickened and painful skin. Severe cases may develop vesicles, erosions, or frank necrosis with the later development of post-inflammatory scaling. Chronic solar dermatitis is characterized by palpably thickened skin, erythema, erosions, ulcerations, scarring, comedones (often grossly dilated and filled with pastelike debris). Secondary bacterial infection and neoplastic transformation (squamous cell carcinoma, hemangioma/hemangiosarcoma) are common.
Acute Solar Dermatitis
Cases of acute solar dermatitis are expected to be self-limiting if further solar exposure can be avoided. Specific therapy is similar to that used for any minor burn.
The discomfort associated with this condition may be considerable, and short-term analgesic therapy with nonsteroidal anti-inflammatory drugs (NSAIDs) or tramadol is indicated. Very inflamed cases may benefit from prednisone (1.1 mg/kg once daily for 7–10 days) in conjunction with an appropriate analgesic agent, such as tramadol.
Cool baths may help to decrease some of the discomfort. If erosion or ulceration is present, use of an antiseptic shampoo or spray will help to prevent the development of secondary infection. Topical glucocorticoids (1% hydrocortisone spray or gel; hydrocortisone-containing shampoos) may help to decrease discomfort and inflammation.
If infection is already present, systemic antibiotic therapy (such as cephalexin 22 mg/kg twice daily, cefpodoxime 5–10 mg/kg once daily or amoxicillin/clavulanate at 22 mg/kg twice daily) is appropriate.
Chronic Solar Dermatitis
Therapy of chronic dermatitis is more challenging, as much of the damage associated with chronic solar exposure is permanent.
Irreversible damage and dilation of hair follicles may be seen in the affected area, and patients may develop follicular impaction, furunculosis, cellulitis and severe secondary bacterial infection as a result. Long-term courses of antibiotics (6–8 weeks or more, ideally selected by antibiotic sensitivity testing) may be necessary to resolve these infections. These patients will benefit from frequent baths with antiseptic agents such as chlorhexidine to help minimize the frequency of subsequent bacterial infections. Nonetheless, recurring bacterial folliculitis may become a long-term issue for many patients.
Lesions of canine solar dermatitis have been demonstrated to have increased product ion of COX- 2, and some cases will respond at least partially to COX-2 inhibitors. Firocoxib (5 mg/kg once daily) has been demonstrated to decrease the erythema, dermal thickening and other lesions in dogs. Other COX-2 inhibitors may also be of benefit, including topical 3% diclofenac gel (applied twice daily). The response is typically slow - some response may be noted by 60 days, but gradual improvement may continue over the next several months.
Natural or synthetic retinoids may be helpful for some lesions. Vitamin A can be given to dogs at 800–1000 U/kg once daily with a fatty meal. Alternately, the synthetic retinoid acitretin can be given to dogs at 0.5–1 mg/kg once daily and to cats 5–10 mg per cat once daily. Patients should be monitored for the development of keratoconjunctivitis sicca and hepatotoxicity. Neither vitamin A nor synthetic retinoids should be administered to pregnant females, as the teratogenic potential of these compounds is high. Topical retinoids such as tretinoin may also be helpful. However, these medications are photosensitizing and topical sunscreens should also be applied.
Topical application of imiquimod may decrease the severity of focal solar dermatitis or early squamous cell carcinoma lesions, especially in cats. The product is to be applied two to three times weekly until the lesions are resolved. Significant inflammation may be seen at the application site, at least initially.
Limitation of Sun Exposure
The most critical factor in the treatment of solar dermatitis is limitation of additional sun exposure. Solar radiation is at its highest from 9 am to 3 pm. Ideally, patients should be kept indoors and away from open windows or doors during that time. In addition, clients should be encouraged to minimize the time that their pet spends sunbathing by windows. Although window glass blocks most UVB rays (minimizing the risk of acute sunburn), much of the UVA light (involved in chronic solar dermatitis) will pass through the glass.
If patients cannot be kept indoors, some benefit may be derived from keeping them in the shade. However, UVA light readily reflects from many surfaces. Sunscreen can be used to protect vulnerable areas. Products designed for dogs and cats are ideal, but may be difficult to find. Waterproof, high SPF (30+) products should be used. Sunscreens incorporating zinc should be avoided in dogs and cats, as ingestion may produce zinc toxicity. Sunscreens incorporating salicylates should be avoided in cats. Best results may be seen when the sunscreen is applied shortly before sun exposure. Reapplication may be necessary in a few hours if prolonged exposure is expected.
If sunscreen application is not possible, the use of UV protective dog garments (k9topcoat.com, fluppies.com. au, designerdogwear.com, others) may help protect the trunk and legs. Patients wearing these garments should be monitored to be certain that they do not overheat.
Early lesions may respond to beta carotene supplementation. Dogs may receive 30 mg/kg and cats may receive 30 mg per cat. The medication is initially administered twice daily for 30 days, and once daily thereafter. Other therapies have been proposed (vitamin E, vitamin C, tacrolimus, topical 5 fluorouracil, pentoxifylline), but to the author's knowledge no research has been conducted to demonstrate the benefit of these medications. Tattooing has been suggested to protect against solar exposure. However, the ink is deposited in the dermis, leaving the epidermis and most superficial dermis exposed. For this reason, tattooing is unlikely to be of benefit and may actually make matters worse by concealing the progression of disease. Severe lesions or lesions that have undergone neoplastic transformation may be treated by surgical excision, cryotherapy, laser therapy or strontium irradiation.
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