Transmission or Cause: Chronic exposure to ultraviolet light causes damage to skin cells.
Affected Animals: Occurs in animals with prolonged sun exposure to areas of thinly-haired, white skin. More common in older cats with white ears/facial areas and in dog breeds such as white boxers and bull terriers, pit bulls, and dalmatians.
Clinical signs: Initially, affected skin is reddened, scaly, and may lose hair. With continued sun exposure thickened skin, red bumps, plaques, and nodules develop which may become ulcerated and crusty. In cats, commonly affected areas include eartips, nose, lips, and eyelids. In dogs, commonly affected areas include the stomach, inner thighs, armpits, and the top of the nose. Secondary bacterial infections may occur.
Diagnosis: Diagnosis is made by characteristic appearance and location of the skin lesions, by ruling out infectious causes of red rashes such as bacterial, parasitic, or fungal infection with appropriate screening tests, and ultimately by skin biopsy which reveals damaged skin cells, sometimes with early precancerous changes.
Prognosis: The prognosis is variable. In early cases, avoidance of further sun exposure can halt progression of disease. In more advanced cases, especially with continued sun exposure, many affected animals will develop aggressive skin cancer such as squamous cell carcinoma. The sun damaged skin is also more at risk of developing other tumors such as hemangioma or hemangiosarcoma.
Treatment: The most important treatment is complete avoidance of sun exposure ideally by confinement indoors, especially between 9am-5pm. If some sun exposure is unavoidable, applying a waterproof sunscreen with SPF > 25 to exposed areas may be helpful (but is usually groomed off rapidly by cats). Additionally, specially made sunsuits are available for dogs: http://www.designerdogwear.com/Treatment of secondary infections is important, and anti-inflammatory medications may be used to decrease skin inflammation. Oral retinoids (synthetic vitamin A derivatives) may help heal non-cancerous lesions. Surgical or laser excision of localized lesions may also be helpful. Once skin cancer has developed, the treatment of choice is aggressive surgical removal of the cancer +/- radiation therapy.
Prevention: Prevention of solar dermatitis is achieved only by prevention of excessive sun exposure to animals with white skin.