Sunburn and Photosensitivity in HorsesBy Kentucky Equine Research Staff · October 21, 2004
Just as humans can be burned by the sun's ultraviolet rays, light-colored horses may suffer from sunburn. Even horses with dark coats can be vulnerable to sunburn if they have white markings on their faces or legs.
Sunburn is seen most often in spring and early summer when longer periods of sunlight coincide with shedding of winter hair. Skin around the eyes, top of the tail, muzzle, and ear tips may become reddened and may swell, blister, or peel in severe cases.
Ways to prevent sunburn include:
• Keep horses stalled during the hours of most intense sun, and avoid turning horses out without access to shade.
• Use a child-safe human sunblock preparation on areas that are likely to burn. Be careful not to get sunblocking products in the horse's eyes. Apply to a small area first to check for skin reactions before applying to a large area of the horse's skin.
• Reapply sunblock regularly. Horses that are grazing may need frequent applications because creams will be wiped off on tall grass. Some products come in a range of bright colors, which will make it easier to see when it's time to put on more cream. A number of shampoos, fly sprays, and coat conditioning products include a sunblock but may not contain enough screening to be effective. Check labels for sun protection factor (SPF) claims. The higher the SPF number, the better the protection.
• Look for a fly mask with extensions that cover the ears and most of the muzzle. For light skinned horses with thin coats, consider turning out in a light cotton sheet or summer fly sheet that offers UV protection.
Danger in the Grass Common pasture plants may trigger sunlight hypersensitivity in horses, making them much more likely to develop serious skin damage after sun exposure. Primary sensitization occurs when sunlight reacts with plant toxins circulating in capillaries near the skin's surface.
Ingestion of other plants, as well as some molds and mycotoxins, causes liver damage, one sign of which is increased skin reaction to sunlight; this is known as secondary sensitization. Some plants can cause both types of reaction.
Alsike clover, a wild strain of clover with white or pinkish blossoms, is one of the most common culprits. It grows well in wet weather, spreading quickly through fields that do not have vigorous stands of pasture grasses. When only a few plants are present, they may be removed by hand or by selective spraying. If grazing areas are heavily infested with these plants, complete renovation of the pasture may be the only way to remove undesirable vegetation.
Other plants that cause photosensitization are St. John'sWort, bishop's weed, spring parsley, and wild buckwheat. St. John'sWort is particularly difficult to eliminate, as it spreads both by seed and by underground rhizome. Its resistance to standard herbicides has led to the development of effective biological control methods including the use of beetles, moths, and midges that feed on the plant's foliage and flowers.
In some cases, signs of photosensitivity may not show up for several weeks after a horse has eaten alsike clover or another dangerous plant. Because the toxins are not destroyed by drying, hay made from any of these plants may cause a reaction long after pastures have stopped growing.
Horses that have been treated with tetracycline and some other drugs may also develop hypersensitivity to sunlight. Sunburn caused by plant ingestion or veterinary treatment is likely to be much more extensive and severe than simply reddened skin.
Resources to the Rescue Agricultural extension agents have the knowledge to identify dangerous plants in pastures or in hay. These experts can also advise on the most effective way to rid fields of unwanted vegetation.
Tack shops and equine catalogs offer a selection of sunblock products, fly masks, and turn-out sheets to protect horses against insects and excessive exposure to sunlight. Horse owners should get veterinary advice if their horses develop serious reactions to sun exposure.
Treatment may be needed to relieve pain and prevent infection. Especially in a horse that has never shown much sensitivity to sunlight, a sudden or severe skin reaction may be a sign of liver disease or damage, and early intervention by a veterinarian is critical in returning the horse to health.