Facial Eczema in Horses

Facial eczema in horses is a photosensitization syndrome, not a routine itchy skin allergy. In this condition, the skin becomes abnormally sensitive to ultraviolet light after the horse is exposed to a photodynamic compound. In classic facial eczema, the trigger is sporidesmin, a toxin produced by the fungus Pithomyces chartarum growing on dead pasture litter. That toxin damages the liver and bile ducts, which then allows photoreactive compounds to build up in the body.

Once that happens, sunlight can injure exposed skin very quickly. The most affected areas are usually lightly pigmented or thin-haired regions such as the muzzle, lips, eyelids, ears, and white facial markings. Horses may look sunburned at first, but the problem can progress to swelling, oozing, crusting, and painful skin sloughing.

The word "facial" can be misleading. While the face is commonly involved, other unpigmented areas can also be affected. Some horses mainly show skin lesions, while others also have signs of underlying liver disease, which can make the condition more serious and lengthen recovery.

This is a condition where early veterinary guidance matters. Prompt shade, pasture changes, and supportive care can limit additional skin injury while your vet works out how much liver involvement is present.

The classic cause of facial eczema is sporidesmin toxicity. Sporidesmin is a mycotoxin produced by the fungus Pithomyces chartarum, which grows on dead plant material in pasture. After a horse eats contaminated pasture, the toxin is absorbed and concentrated in the liver and biliary system, where it can damage bile ducts and reduce normal bile flow.

That liver injury matters because chlorophyll from plants is normally broken down into compounds that are cleared through the liver. When the liver cannot process those compounds well, phylloerythrin can build up in the bloodstream and skin. Sunlight then activates it, causing inflammation and tissue damage in exposed, lightly pigmented skin.

In horses, facial eczema is one form of secondary photosensitization, meaning the skin problem happens because of liver dysfunction rather than because the skin itself is the primary issue. Other liver diseases, certain plants, and some medications can also cause photosensitization, so your vet may need to sort through several possibilities before deciding the pasture fungus is the most likely trigger.

Risk tends to be higher when horses graze pasture with a lot of dead litter and environmental conditions favor fungal growth. A horse can also look normal for days before signs appear, because there is often a delay between toxin exposure and visible skin lesions.

Your vet usually starts with a history and physical exam. Helpful details include when the lesions started, whether they are limited to white or thin-haired skin, what pasture the horse has been grazing, whether other horses are affected, and whether any new feeds, supplements, or medications were introduced. The pattern of lesions can strongly suggest photosensitization, but it does not prove the cause by itself.

Bloodwork is often the next step. A CBC and chemistry panel, along with liver-focused testing such as bile acids, can help show whether the liver is inflamed or not functioning normally. Horses with toxin-related liver injury may have elevated liver enzymes and other abnormalities that support the diagnosis.

Depending on the case, your vet may also recommend ultrasound of the liver, skin cytology or culture if infection is suspected, or a biopsy in less clear cases. These tests help separate facial eczema from sunburn, contact irritation, vasculitis, autoimmune skin disease, plant-associated photosensitization, and other causes of crusting facial lesions.

There is no single home test that can confirm facial eczema in a horse. Diagnosis is usually based on the combination of clinical signs, pasture exposure risk, and evidence of liver involvement. That is why early veterinary evaluation is so useful, especially before lesions become severe.

Prevention focuses on reducing exposure to the toxin and reducing UV injury. If facial eczema is a known regional risk where you live, talk with your vet about local pasture conditions, seasonal patterns, and whether your horse should be moved off suspect pasture during higher-risk periods. Dead pasture litter and fungal growth are part of the classic exposure pattern, so pasture management matters.

For horses that have had photosensitization before, practical prevention may include shade access, daytime stall rest, UV-protective fly masks, and turnout during lower-UV hours while your vet evaluates the situation. These steps do not fix liver injury, but they can reduce additional skin damage.

Pasture review is also important. Your vet may suggest looking for other possible triggers such as hepatotoxic plants, recent feed changes, or medications that could contribute to photosensitivity. In some regions and species, zinc strategies are used for sporidesmin prevention, but horses are not managed exactly like sheep or cattle, so do not start supplements or preventive products without veterinary guidance.

If your horse develops even mild redness on white skin after turnout, act early. Bringing the horse into shade and arranging a veterinary exam can be the difference between a manageable skin flare and a much more painful, prolonged recovery.