Sunlight has beneficial effects for horses, including the manufacture of vitamin D by the skin, relief of muscle and tendon stiffness or soreness and possibly even improved immunity.
But horses with pink-skinned areas may suffer sunburn if overexposed and could be at higher risk for equine skin cancer over the years.
Zinc-oxide cream, the white goop lifeguards use on their noses, is one of the most effective sunscreens available. It's safe for horses and helps heal any skin irritation or blistering. You can generally also use any sunscreen product on the market for people, such as those containing PABA, on your horse. However, some horses can have allergic/sensitivity reactions to these products or to fragrances in them.
Since the reactions themselves are also red and angry-looking, like a sunburn, if you respond to the reactions by putting even more sunscreen on the horse, you can end up with a severe reaction with open, oozing skin. Some sunscreen-sensitivity reactions, though, occur only after the horse has been exposed to the sun and are the result of a reaction between the sunscreen agent and the sun.
To test the horse for sensitivity, apply the product at night to a sensitive area like the back of one pastern, and check it in the morning. If there's no redness or swelling, use the product during the day on only one pink-skinned area. If the spot with the sunscreen applied actually looks redder at the end of the day than the areas that didn't have it, your horse is sensitive to that sunscreen. Don't use it on him.
A particularly severe type of sun reaction is photosensitivity, where the skin becomes red, swollen, and cracks open. Crusts will develop, and loss of the skin can occur. These reactions can easily develop infections and are extremely painful. The reactions only occur in pink-skinned areas. When horses have the pink skin/white hair on their legs, photosensitivity reactions may be confused with scratches/"dew poisoning" or chigger infestations. Several drugs and plants can also cause photosensitivity.
Possible Photosensitizing Agents
The following ingredients or products can cause a photosensitivity reaction in your horse:
• Tetracycline antibiotics
• Sulfa antibiotics
• Nonsteroidal anti-inflammatory drugs, especially ketoprofen
• Chlorpromazine tranquilizers
• St. John's Wort
• Klamath weed
• Rape pasture
• Goat weed
• Spring parsley
• Alsike clover