Photosensitization in the Horse

 

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

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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.”


Primary Photosensitization


The photosensitizing agent (usually a plant or a drug) is eaten or injected and travels to the skin or contacts the skin, explains Rees. “Examples of plants that cause primary photosensitization include St. John’s Wort, buckwheat, burr trefoil, smartweed, and perennial rye grass,” she says. “Examples of drugs that cause photosensitization include phenothiazine, thiazides, rose bengal, acriflavines, methylene blue, sulfonamides (trimethoprim sulfa), and tetracycline

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Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey’s Guide to Raising Horses and Storey’s Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at https://insidestorey.blogspot.com that comes out on Tuesdays.

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