Photosensitization in horses is easy to detect and easy to treat. What may not be so easy is identifying the cause.
Dr. Jeremy Powell, University of Arkansas Extension veterinarian, said getting some history from the owner can help pinpoint the cause of the ailment. “Sometimes they’ll point out that their horse had ingested something that could lead to photosensitization,” Jeremy said. Among the potential culprits are commonly used antibiotics like tetracycline or sulfonamides, and forages found in pastures; that would include certain Clovers, Buckwheat, St. John’s wort and perennial Ryegrass.
Jeremy said he doesn’t see the disorder commonly, but it does show up intermittently. “Sometimes, the animals can slough a little bit of that skin,” he said, “but usually what has happened is they’ve lost hair around that area, and it’ll be inflamed; it may be hyperpigmented or reddened.” The first step of treatment is to get the animal into an area that’s permanently shaded, like a stable or stall, then comes addressing the wound.
Jeremy said, “Skin typically heals fast, and you just use basic wound management to treat it; you can put topical ointments on to help it heal.” Treatment can become more complicated if the animal has been away from the owner for several days and returns with severe skin necrosis, particularly around a sensitive area like the eye. It’s also important to identify the source of the toxicity, and be sure it’s no longer available to the animal; if it’s a plant, the field may have to be mowed or treated with an herbicide.
Dr. Philip Johnson, professor and instructional leader of equine medicine and surgery with the University of Missouri’s College of Veterinary Medicine and Surgery, said photosensitization can also be caused by a liver disease. In emailed correspondence, Philip said, “In the absence of liver disease, photosensitization is referred to as ‘primary’. When it arises in the context of liver disease, photosensitization is referred to as ‘secondary’.” He said most cases tend to be seen and heard about in the fall, which he says may be attributed to “the dehydration of the dying plant, and concentration of potential photosensitizing toxins within the plant.”
Philip said photosensitization only occurs in horses that are lacking skin pigment to some extent; while it is most evident in Paint horses, it is never seen in gray horses.
In severe cases where skin blistering occurs, antimicrobial treatment may be needed; some horses may exhibit pain, which may require treatment with a nonsteroidal anti-inflammatory drug (NSAID).
However, primary photosensitization is not usually dangerous, and Philip said, “Awareness is helpful for improved outcome; as soon as horse owners see signs, remove the patient to the barn” – that is, get the horse out of the sunlight. In addition, horse owners can prevent the disease by monitoring pastures for known photosensitizing plants and weeds, with increased vigilance in the fall. And he added, “Knowledge of plant species that can either cause photosensitization or liver disease would be a good strategy to promote avoidance.”
