A horse with pink skin that blisters and peels may be having a painful reaction to sun exposure – but it may not be sunburn, as many would assume. Instead of overexposure to ultraviolet rays, this reaction takes place internally — photosensitivity occurs when horses with large white patches of skin graze on Saint John's Wort or alsike clover. The photodynamic compound in the plants circulates in the blood. When the compound comes close to pink skin and is exposed to the sun's UV rays, a chemical reaction occurs that damages tissues.
Secondary photosensitivity occurs when a horse's liver is damaged and unable to filter the toxin from the bloodstream, causing the same reaction.
In photosensitive horses, the skin on any white area of their body can blister, including on their face, legs or body. After the blisters, tight scabs will form. These scabs eventually peel away in a lengthy, painful process. Though horses can be afflicted with all sorts of skin disorders, photosensitivity is an exceptionally painful one.
If crusts appear on the horse's skin, it's imperative to identify if they are only located on white markings. If the crusts cross a border into darker hair, photosensitivity is most likely not the culprit. If there's any question, a veterinarian should be called for the best course of treatment.
Affected horses are often removed from fields that may contain phototoxic plants and kept indoors until the skin heals.
Read more at EQUUS.
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