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Photosensitization is the incapacity of the body to react to sunlight normally and typically manifests as swelling and inflammation of the skin.

WHAT: More than just a sunburn or contact dermatitis, photosensitization is a reaction between the sunrays and the skin of horses that contains a specific chemical or compound. Classic examples of  “photoproducts” that result in primary photosensitization are ingestion of plants such as St. John’s wort, buckwheat, smartweed, clover, and perennial ryegrass, as well as medications including phenothiazine, thiazides, sulfonamides (trimethoprim sulfa), and tetracycline.

WHY: After ingestion or administration, the plant or drug products circulate through the body and reach the skin. When the sun’s ultraviolet rays contact the skin, they react with the molecules in a complex photochemical reaction. Such a reaction causes intense inflammation and death of skin cells where the sun and photochemicals meet.

WHERE: The most commonly affected parts of the horse’s body are the unpigmented, or white-skinned, areas. Here, the dermatitis is most obvious, usually characterized by redness, inflammation, and hair loss. Researchers believe the compounds react more readily to sunrays in these unpigmented zones than in dark areas.

WHEN: Any time a horse is exposed to the sun’s ultraviolet rays and the plant or chemical compounds are circulating in the horse’s body, they are at risk.

Further, “secondary photosensitization” occurs in the face of liver disease. Horses with severe liver disease are unable to normally metabolize plant by-products and therefore accumulate in the blood and react with the sun in unpigmented areas of the skin. A classic example of secondary photosensitization is due to phylloerythrin, a bacterial breakdown product of chlorophyll. A healthy liver can remove phylloerythrin from the bloodstream, but a diseased liver cannot, resulting in the accumulation of phylloerythrin in the blood.

“In the human literature, there is some evidence that omega-3 fatty acids can minimize the deleterious effects of photosensitization,” explains Kathleen Crandell, a Kentucky Equine Research (KER) nutritionist. “One study* published in Experimental Dermatology explored the possibility of systemic photoprotection through naturally sourced nutrients such as omega-3 polyunsaturated fatty acids. That study concluded that omega-3 fatty acids show potential for protecting the skin from ultraviolet injury, both acute and chronic.”

EO•3, a marine-derived source of omega-3 fatty acids developed by KER, strengthens immune function and reduced allergic reactivity. EO•3 is available in the U.S. and Australia.

Crandell also notes that vitamin E has been used in humans with photosensitization. Consider Nano•E, a natural nanodispersed vitamin E formulation produced by KER.

Liver disease in horses can be managed through diet. Consult with a KER nutritionist for more information.

*Pilkington, S.M., R.E. Watson, A. Nicolaou, et al. 2011. Omega-3 polyunsaturated fatty acids: photoprotective macronutrients. Experimental Dermatology 20(7):537-543.  


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