by Sally Colby
If your horse is a pinto, paint or perhaps a cremello or bald face, chances are good that you’re concerned about sun damage.
Dr. Rose Nolen-Walston, DVM and board certified internal medicine specialist at the University of Pennsylvania School of Veterinary Medicine, says that while many of her colleagues see a range of fairly common sun-related issues in the field, her position at Penn Vet means that she sees the worst of the worst. “I see the scarier ones,” she said. “Animals that have photosensitization, either from a problem with their liver or a drug reaction. My colleagues in the field are pretty good at picking these up, and often they’ll have run some tests and already know there’s a liver problem by the time they send them to me.”
Nolen-Walston explains the difference between sunburn and photosensitivity. “The sun emits UV light and UV radiation and skin is sensitive to that,” she said, explaining sunburn. “Unpigmented skin is most likely to be damaged. Photosensitization is different. The horse’s skin — either because of a problem with its liver and a buildup of abnormal pigments in the skin, or sometimes a drug reaction or a genetic problem — becomes really sensitive to the rays of the sun, and the pigments in the skin actually magnify the burning effects of the sun.”
Although many horse owners are aware that their horse is either prone to sunburn or has photosensitivity, convincing a horse to keep a fly mask and/or sheet can be a problem. The fly mask should be well-made and fit the horse properly without touching its eyes. A breakaway halter can be placed on top of a fly mask to keep it on and in extremely challenging cases, the mask can be hand-sewn to the halter to prevent the horse from removing it. Sunscreen fly masks are quite helpful and probably the best option for most horse owners whose horses are on pasture for part of the day. Masks that claim to block UV rays should include the percentage of UV protection on the packaging.
Keeping a UV fly sheet on a horse can also be challenging. Nolen-Walston says there’s no tried and true method to keep a sheet on a horse that’s determined to remove it, but purchasing a sheet from a company that guarantees a replacement is a good solution for some horse owners.
With or without a mask or a sheet, horses should be provided with shelter from the sun. Horses tend to use a shelter if there’s ample space for safe sheltering for all horses in a group and without bullying by one or two horses. Shelters should be as fly-free as possible and easy to clean. If a horse has pink skin and spends most of their time on pasture, they should be protected with at least a face mask all summer and into fall. Preventing sunburn on the lips can be achieved with a thick layer of zinc oxide, but a grazing horse will lose much of it during normal grazing and eating activity.
Nolen-Walston says that several plants cause photosensitivity, including one clover variety that sometimes appears in pastures. Alsike clover, which is similar in appearance to other clovers, has three leaves and smooth stems but doesn’t have the white crescents typical of other clovers. Alsike clover tends to be taller and more stemmy than other clovers and horses tend to avoid it unless they’re left with no other grazing options. Nolen-Walston suggests having a county extension agent visit the farm if you suspect alsike clover.
A serious case of overexposure to the sun begins with sunburn on the unpigmented skin of the nose, ears and sometimes on white legs. When it becomes worse, sunburn also occurs on pigmented skin. “Plants don’t cause photosensitization by themselves,” said Nolen-Walston. “They cause liver damage.” Some of the plants that can lead to liver damage and subsequent phototoxicity include St. Johnswort, ragwort, comfrey, kale, bermudagrass, buttercup and lantana.
Some drugs, including oxytetracycline, which is used more frequently in summer to treat Potomac Horse Fever, and sulfonamides such as SMZ can cause photosensitivity. “The more common thing we see is secondary photosensitization,” said Nolen-Walston. “This is simply due to liver failure. When the horse eats grass or any other green-leafed plant, they ingest a lot of chlorophyll. The gut switches that chlorophyll into a metabolite called phylloerythrin. This phylloerythrin goes through the blood and the liver should clean it up and spit it out and dump it into the intestines where it’s excreted without any problems. But when the liver starts to fail, that phylloerythrin builds up in the skin and causes phototoxicity — hypersensitivity to sunlight.”
What about horses with bald faces and blue eyes? These horses don’t always have issues with sunburn, but if they have white or pink tissue on the eyelids, they are likely at risk. “If we see horses with pigmentation over some of the eyelid and pink eyelids over the rest,” said Nolen-Watson, “we only see UV light-associated cancers form in the pink areas. That’s how well the pigmented skin protects from these cancers.”
Owners of gray horses often worry about melanoma, but Nolen-Walston says there’s not a lot to be concerned about regarding sun exposure. “As far as I know, melanomas in horses don’t have any association with the sun,” she said. “They are associated with the gene that causes graying of the hair coat. Horses are born bay, black or chestnut, and if they have the graying gene, they lose the melanin in the hair follicles and become gray over time. A horse that has the gray gene also has the gene for melanoma. Most of the time they get big ugly lumps under the tail or under the throatlatch, or sometimes under the eyes. But they don’t get the kind of melanomas that humans get that metastasize through the body, and it isn’t nearly as scary.”
If a horse is suffering from sunburn, they may act quite uncomfortable, so bute or banamine is appropriate. “But like any drug that’s strong enough to do good, it’s strong enough to do harm,” said Nolen-Walston. “If you’re going to use bute or banamine, double check with your veterinarian to make sure your horse is healthy enough for bute or banamine because they can worsen gastric ulcers and are bad for kidneys.” Other healing measures for a case of sunburn include aloe vera gel on the muzzle followed by a moisturizer. A cooling bath is also helpful and the horse should be kept inside until any peeling and blistering is healed.
