Research by equine specialists at the University of Illinois shows that what horse owners often think is sunburn, is actually dermatitis due to photosensitization which is caused by toxins from plants leading to sun-mediated skin damage as the plant toxins circulate through capillaries in sensitive light-skinned areas on the horse such as around the eyes, nose, mouth and other areas with shite hair coat and pink skin.
Causes of sunburn in horses
What horse owners often think is sunburn, is actually dermatitis due to 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.
The most common cause identified in the majority of these cases has been the ingestion of wild white clover, also known as Alsike clover. 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 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 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.
Aa 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.