Merck Manual

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Pet Owner Version

Photosensitization in Horses


George M. Barrington

, DVM, PhD, DACVIM-LAIM, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University

Reviewed/Revised Oct 2019 | Modified Oct 2022

Photosensitization is a condition in which skin becomes overly sensitive to ultraviolet light (sunlight). This condition is not sunburn, although the difference can be difficult to distinguish. Photosensitization occurs when certain compounds, which are activated by light, are present in skin that is exposed to ultraviolet (UV) light. The photodynamic compounds are energized by the UV light and cause chemical reactions that damage cells in the skin and lead to ulceration and fluid accumulation. Many chemicals, including some that are fungal and bacterial in origin, may act as photosensitizing agents. Affected areas are usually those that are lightly pigmented or that have little hair, such as the lips, eyelids, and tips of the ears.

Photosensitization is often classified according to the source of the photodynamic compound. Those seen most often in horses include systemic photosensitivity (also known as primary or type I photosensitivity) and hepatogenous photosensitivity (also known as secondary photosensitivity). Hepatogenous photosensitivity is more common in horses and often involves impaired liver function. If the liver is unable to metabolize certain chemicals normally, they accumulate in the blood. A common cause of hepatogenous photosensitivity is poisoning by grazing plants toxic to the liver or ingesting blue-green algae (cyanobacteria) or fungal toxins.

The signs associated with photosensitivity are similar regardless of the cause. Photosensitive animals are hypersensitive when exposed to sunlight and squirm in apparent discomfort. They scratch or rub lightly pigmented, exposed areas of skin such as the ears, eyelids, or muzzle. Bright sunlight can cause typical skin lesions (ranging from hives to redness and scaling of skin), even in black-coated animals. Redness develops rapidly and is soon followed by swelling. If exposure to light stops at this stage, the abnormalities soon resolve. When exposure is prolonged, fluid discharge, scab formation, and death of skin tissue can result.

Signs are easily recognized in advanced cases of photosensitivity but are similar to the effects of sunburn in early or mild cases. Reference to the specific diseases in which photosensitization is a sign may assist your veterinarian in diagnosing the underlying disease. Evaluation of liver enzymes and liver biopsies may be necessary to confirm the presence of liver disease. Examination of blood, feces, and urine for porphyrins may also be performed.

Treatment involves mostly soothing the signs. While photosensitivity continues, animals should be shaded fully or, preferably, housed and allowed out only at night. The severe stress of photosensitization and extensive death of skin tissue can be highly harmful, even deadly. Corticosteroid injections may be helpful in the early stages. Wounds to the skin should be kept clean to minimize secondary skin infections. Exposure to flies must be prevented because skin damaged during photosensitivity attracts flies and other insects. The skin lesions heal remarkably well, even after extensive damage. The outcome for an individual horse is related to the site and severity of the primary lesion and/or liver disease, and to the degree of healing.

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