Diagnosis and treatment of solar dermatitis in dogs

Skin damage from the sun in dogs can be easily mistaken for other skin diseases. Make sure you and your clients are aware of the signs and are doing everything possible to prevent this potentially serious condition.
Aug 01, 2007

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.

1. Severe solar dermatitis with scarring and furunculosis on the medial hock of a pit bull.
Unfortunately, solar dermatitis can sometimes mimic other skin diseases such as allergies or pyoderma and, thus, go unrecognized and untreated until irreversible damage or sun-induced skin cancers have developed. This article reviews the pathogenesis, clinical signs, diagnosis, and treatment of solar dermatitis.


2. Solar dermatitis causing erythematous, scaly hypotrichotic bullae on the dorsolateral trunk of a pit bull. Note the pigmented areas are normal. In this case, this dog's lesions mimicked those of seasonal allergies, as they occurred seasonally in the warm months and initially seemed to improve with glucocorticoids and antibiotics, but occurred more severely each summer. The key to the diagnosis was the lack of marked pruritus and the distribution of the lesions, sparing pigmented areas. A biopsy confirmed solar dermatitis.
The solar spectrum is composed of about 40% visible light rays (400 to 700 nm), 50% infrared light rays (700 to 20,000 nm), and 9% ultraviolet (UV) light rays (100 to 400 nm).1 UVA rays (320 to 400 nm) penetrate more deeply into the skin than UVB rays do, and UVA rays are associated with photosensitivity reactions.2,3 Prolonged exposure to shorter wavelength UVB solar rays (290 to 320 nm) causes phototoxicity (sunburn), directly damaging keratinocytes and causing superficial skin blood vessel dilatation and leakage. Sun damage of epithelial structures subsequently leads to increases in inflammatory cytokines, prostaglandins, and leukotrienes as well as toxic oxygen intermediates that perpetuate and amplify tissue injury.2,4-6 Prolonged and repeated sun damage leads to keratinocyte proliferation, mutagenesis, atypia, and premalignant actinic keratoses, which can progress to invasive squamous cell carcinoma.2

3. Solar dermatitis causing erythema, scaling, crusts, and hypotrichosis on the lateral trunk of a pit bull. A contributing factor to the truncal hypotrichosis was iatrogenically induced hyperadrenocorticism after 1.5 years of daily administration of 20 to 40 mg prednisone for erroneously diagnosed allergic dermatitis.
Melanin present in pigmented skin absorbs UV rays, which in most cases helps prevent deeper UV light penetration and actinic damage.1 Black skin absorbs about 45% more solar radiation than white skin does.1 UV light contributes little in terms of thermal or heat burden, whereas visible radiation can penetrate into the skin and create thermal injury and necrosis.1 Sunlight-induced burns and necrosis affecting black spots in Dalmatians have been reported,7 but this uncommon type of sun-induced thermal burn has a different pathogenesis than that of canine solar dermatitis.


4. More subtle solar-induced lesions of patchy erythema, alopecia, and fibrosis with comedones and scattered small bullae on the dorsolateral trunk of an American bulldog.
Solar dermatitis most commonly affects the white-haired and nonpigmented skin of short-coated breeds such as pit bull terriers, Staffordshire bull terriers, bull terriers, boxers, Dalmatians, American bulldogs, and whippets, but any dog with white or lightly pigmented hair and skin is at risk. Sun damage usually occurs on nonpigmented thinly haired areas such as the flank, inguinal and axillary areas, and the dorsal nose, but it can occur on the dorsal and lateral trunk and lateral limbs as well as other areas (Figures 1-4).2 In dogs that prefer to lie on one side of their body, lesions may be worse on the more chronically sun-exposed side.