Thursday, December 12, 2002
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"Sunburn" in Horses May Be Due to More Than the Sun
by Kevin H. Kline, PhD, University of Illinois, Dept of Animal Science
A relatively common complaint among owners of fair-skinned horses is reddened and peeling skin during sunny weather. The most often affected area is around the eyes, nose and mouth of horses having white face markings, but other areas with white hair coat and underlying pink skin including the legs may be affected. While this dermatitis is often simply attributed to sunburn, ingestion of plant toxins of various origins is often the underlying cause of this condition known as photosensitization. Certain plants cause the sun-mediated skin damage through direct interaction of sunlight and plant toxins circulating through capillaries in the sensitive light-skinned regions. This effect is known as primary photosensitization. Some plants may cause dermatitis through a secondary toxic effect pursuant to liver damage by plant chemicals, and some plants pose the risk of both types of photosensitization.
Equine specialists at the University of Illinois have received numerous telephone calls related to this problem. The most common cause identified in the majority of these cases has been the ingestion of wild white clover, also known as Alsike clover (Trifolium hybridum). This clover with white or pinkish white flowers can grow profusely in moist conditions and overrun pastures that do not have vigorous stands of pasture grasses. This undesirable clover for horses is sometimes included in pasture seed mixes, either intentionally or unintentionally, and looks similar to the large white clover known as Ladino (Trifolium repens) that is thought to pose fewer health problems than Alsike.
Horse owners that have fair-skinned horses showing signs of photosensitization should remove the horse both from the sun exposure and the source of the plant toxins. The degree of dermatitis that a white-skinned horse may display is related to the amount of plant toxin that is ingested, the amount of sunlight exposure, and individual tolerance to the toxins. While most cases of Alsike clover photosensitization are not life threatening, rare cases of severe debilitation or death of horses have been reported, probably due to the secondary form of the disorder caused by liver damage. Problems with Alsike clover toxicity generally peak in the late spring and early summer when moisture is abundant in the pastures, and lessen during dryer weather in mid and late summer. The problem often recurs during early fall when moderating temperatures and moisture levels allow the shallow-rooted clover to reestablish itself.
Horse owners that observe signs of photosensitization in their horses should check their pastures for excessive growth of Alsike clover, but should also keep in mind that other plants and some molds may also predispose horses to "sunburn dermatitis". For example, a horse that has ingested St. Johns Wort, wild buckwheat or certain mycotoxins including Sporodesmin may present symptoms similar to one that has ingested Alsike. Pasture renovation using more desirable species of forages may be necessary to avoid recurring problems with plant toxicity. Information regarding pasture establishment and renovation may be obtained from the University of Illinois Extension office nearest you.
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