Photosensitization in Horses

Photosensitization is a painful skin condition where a horse's skin becomes overly reactive to ultraviolet light. It most often shows up on lightly pigmented skin or areas covered by white hair, including the muzzle, eyelids, ears, and lower legs. The skin can become red, swollen, crusted, and tender very quickly after sun exposure.

This is not the same thing as a routine sunburn. With photosensitization, light-reactive compounds are present in the body and are activated when sunlight reaches the skin. That reaction damages skin cells and can lead to oozing, peeling, and secondary infection if the area is badly irritated.

Horses can develop primary photosensitization, where the trigger comes directly from a plant, chemical, or medication, or secondary (hepatogenous) photosensitization, where liver disease prevents normal breakdown of compounds that then build up in the bloodstream. Secondary photosensitization is often the more concerning form because it may signal significant liver injury.

Many horses recover well when the cause is found early and sunlight exposure is reduced. The outlook is more variable when liver disease is severe or ongoing, so it is important to involve your vet rather than assuming the problem is only skin deep.

Photosensitization happens when sunlight interacts with light-reactive compounds in or on the skin. In primary photosensitization, the horse is exposed to a photodynamic substance directly. Reported triggers include certain plants such as St. John's wort and buckwheat, and some medications, including sulfonamides and tetracyclines. In these cases, the liver may be normal.

In secondary or hepatogenous photosensitization, the problem starts with liver dysfunction. When the liver cannot properly clear breakdown products such as phylloerythrin from chlorophyll metabolism, those compounds circulate in the blood and react in sun-exposed skin. This form is considered more common in horses than primary photosensitization.

Liver injury can be linked to toxic plants or contaminated forage. Reported examples include alsike clover, pyrrolizidine alkaloid-containing plants such as ragwort, blue-green algae exposure, and some mold-related forage problems. Viral hepatitis and other liver disorders can also contribute.

Risk is often highest in horses with white markings, pink skin, sparse hair coverage, or long hours of daytime turnout. Still, darker horses are not fully protected. If the underlying trigger remains in the pasture, hay, or medication plan, the skin may flare again even after the first lesions begin to heal.

Your vet will start with a physical exam and a close look at the skin pattern. Photosensitization often affects lightly pigmented or white-haired areas and can look different from simple sunburn because lesions may extend into haired white areas, not only bare pink skin. Your vet will also ask about turnout schedule, recent pasture changes, hay source, supplements, and medication history.

Bloodwork is often an important next step, especially a chemistry panel that evaluates liver values. If liver disease is suspected, your vet may recommend additional testing such as bile acids, ultrasound, or in some cases liver biopsy. These tests help separate a skin-limited problem from a more serious internal one.

Skin samples may be recommended if lesions are severe, unusual, or not responding as expected. A biopsy can help rule out other causes of crusting and ulceration. Your vet may also suggest reviewing pasture plants and forage sources, because identifying and removing the trigger is a major part of care.

Diagnosis is usually based on the combination of lesion pattern, sun exposure history, possible toxin or medication exposure, and lab findings. That full picture matters because treatment and prognosis can look very different for a horse with a temporary plant exposure versus one with ongoing liver damage.

Prevention starts with limiting exposure to known triggers. Walk pastures regularly, avoid overgrazing, and inspect hay for unusual weeds or poor-quality plant material. If your area has a history of toxic plants such as St. John's wort, alsike clover, or ragwort, ask your vet or local extension resources for help with identification and pasture management.

Sun protection also matters, especially for horses with white markings or pink skin. Many pet parents reduce risk by turning horses out overnight, using shade during the day, and adding UV-protective fly masks or sheets. Physical barriers are often more reliable than topical products alone.

Review medications with your vet if your horse has had a prior episode. Some drugs can increase photosensitivity, and your vet can help decide whether timing, dose, or alternatives should be considered. If a horse has known liver disease, ongoing monitoring may be part of prevention because flare-ups can happen when liver function worsens.

The biggest preventive step is early action. If you notice redness or irritation on white-haired areas after turnout, bring your horse out of the sun and contact your vet before the skin becomes deeply damaged. Fast changes in management can make a meaningful difference in comfort and recovery.