Solar dermatitis (ie, sunburn) is caused by direct UV light injury to skin cells. Both UVA, which is least absorbed by the ozone layer and falls just above the range of visible light at 320-400 nm, and UVB (290-320 nm) cause skin damage; UVC (100-290 nm) is completely absorbed by the ozone layer and does not reach the skin.1
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The severity of solar dermatitis is related to the form, duration, and intensity of sun exposure. UVA penetrates the epidermis and causes the greatest damage at the basal keratinocyte layer and the dermis; this is the main cause of photoaging and wrinkling. UVB primarily penetrates the epidermis. This can produce reddening and inflammation and can promote development of cutaneous cancers.1 Acute exposure to UV light can damage keratinocytes, alter the skins immune system, and cause keratinocyte proliferation and mutation.2
Solar dermatitis frequently occurs on the dorsal muzzle (nasal) in dogs and cats, as well as in other nonpigmented regions (eg, eyelid margins, lip margins) and on the ventrum (truncal) in dogs. Nasal lesions begin as erythema and scaling at the junction of the nasal planum and dorsal muzzle and expand caudally as inflammation causes adjacent hair loss, which exposes the skin to additional UV light. Subsequent scarring and postinflammatory depigmentation can further dermatitis development (Figure 1), and lesions can wax and wane depending on seasonal sunlight fluctuations.