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.
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.
1. Severe solar dermatitis with scarring and furunculosis on the medial hock of a pit bull.
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
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.
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.
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.
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.
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.