Photosensitization in the Horse / by Allen Gutowski

Photosensitization is a serious skin condition characterized by "sunburned," crusty skin that dies and sloughs away. It is usually caused by a reaction to something the horse has eaten, but the skin problem does not appear until the animal is exposed to sunlight. Christine Rees, DVM, Dipl. ACVD (dermatology), assistant professor of dermatology in the college of veterinary medicine at Texas A&M University, says three factors contribute to the development of photosensitization: Presence of a photoactivating substance in the skin, exposure to ultraviolet light, and lack of skin pigment (which enables more UV light to penetrate the skin).

"There are three classifications of photosensitization: Primary photosensitization, hepatogenous photosensitization (due to liver impairment), and photosensitization due to abnormal pigment production," says Rees. "The latter, called porphyria, is genetic, very rare, and seen more often in cattle than in horses." 


Whether it's primary or secondary photosensitization, it's important to get the animal out of the sun. "The length of time necessary will depend on whether it's a primary or secondary problem," explains Talcott. "The animal needs basic supportive care for the lesions. This typically consists of some type of bathing to clean the lesions or use of a topical ointment to ease the pain and inflammation, depending on the location and severity."

Cold water (hydrotherapy) can help reduce swelling on the lower legs. Steroids can help relieve the inflammation.