Photosensitization in Sheep: Facial Eczema, Sun Damage & Skin Lesions

Photosensitization is a skin reaction that happens when light-activated compounds in the body react with sunlight and damage exposed skin. In sheep, this often shows up on the face, ears, eyelids, and muzzle because those areas have less wool coverage and more sun exposure. Although many pet parents describe it as a bad sunburn, it is usually more than ordinary sun damage.

There are two broad patterns. Primary photosensitization happens when a sheep eats or contacts a plant that contains a photodynamic compound. Secondary, or hepatogenous, photosensitization happens when the liver is damaged and can no longer clear phylloerythrin, a breakdown product of chlorophyll. Facial eczema is a classic hepatogenous form linked to sporidesmin toxin from Pithomyces chartarum growing on dead pasture litter.

Affected sheep may become very uncomfortable in bright light and frantically seek shade. Mild cases can improve with prompt protection from sunlight and supportive care. More severe cases can involve deep skin injury, jaundice, poor appetite, weight loss, and even death if liver damage is significant.

Because several toxic plants, pasture conditions, and liver disorders can look similar, your vet is the right person to sort out the cause and help you choose a care plan that fits your flock and budget.

Photosensitization in sheep has several possible causes, and the distinction matters because treatment and prevention may differ. Primary photosensitization happens when a sheep eats or contacts plants that contain light-sensitizing compounds. Reported plant triggers include St. John’s wort, buckwheat, some brassicas, clovers, alfalfa-related problems, and certain nightshade species. White sheep and sheep with pink skin around the eyes and mouth are often more visibly affected.

Secondary photosensitization is tied to liver damage. In healthy sheep, the liver clears phylloerythrin, a photodynamic compound formed during chlorophyll breakdown. If the liver or bile ducts are injured, phylloerythrin builds up in the blood and sunlight then damages exposed skin. This is why skin lesions can be the visible tip of a deeper liver problem.

In sheep, one of the best-known secondary causes is facial eczema, also called sporidesmin toxicosis or pithomycotoxicosis. It is caused by sporidesmin toxin from the fungus Pithomyces chartarum, which grows on dead pasture litter, especially during warm, humid periods after rain. The toxin damages the liver and bile ducts, and signs of photosensitization often appear about 10 to 14 days after exposure.

Other contributors can include toxic weeds, pasture mismanagement, overgrazing, and environmental conditions that increase fungal growth or force sheep to eat risky plants. Your vet may also consider other liver diseases, inherited susceptibility, and less common toxic exposures if the pasture history does not clearly fit.

Your vet usually starts with the pattern of lesions, the flock history, and a close look at pasture, hay, and recent weather. Timing matters. Primary plant-related cases may show up within a few days of exposure, while secondary cases linked to liver injury can appear later, sometimes even after sheep have been moved off the original pasture.

A physical exam helps your vet assess how much of the problem is skin injury versus whole-body illness. They may look for jaundice, dehydration, weight loss, eye pain, fever, or signs that grazing has become difficult. If facial eczema is suspected, your vet will also think about pasture litter, warm humid conditions, and whether other animals in the group are affected.

Bloodwork is often useful, especially when liver damage is a concern. Elevated liver enzymes can support hepatogenous photosensitization, and more advanced cases may need additional testing to assess liver function, hydration, and overall stability. In some situations, your vet may recommend skin sampling, liver biopsy, or pasture and feed investigation to help confirm the cause.

Diagnosis is often a combination of history, lesions, and ruling out look-alike problems such as ordinary sunburn, contact dermatitis, ecthyma, trauma, fly irritation, or other toxicities. That is why taking photos, saving plant samples, and noting when signs first appeared can be very helpful for your vet.

Prevention starts with pasture awareness. Walk grazing areas regularly and look for toxic weeds, heavy dead litter, overgrazed sections, and recent changes in forage quality. If your flock includes white-faced sheep or animals with lightly pigmented skin, watch them especially closely during bright sunny periods because lesions are often easier to trigger and easier to see.

For facial eczema risk, pasture management matters. Sporidesmin-producing fungus grows on dead plant material and tends to be favored by warm, humid weather after rain. Avoid forcing sheep onto risky, litter-heavy pasture, and work with your vet or local extension resources if your region has a known seasonal pattern. In some systems, zinc-based prevention may be discussed before exposure, but the dose, timing, and suitability should always be guided by your vet.

Good grazing management can also lower risk from plant-related photosensitization. Avoid overgrazing, rotate pastures, and control weeds before sheep are pushed to eat plants they would normally avoid. Be cautious after drought, frost, or rapid pasture changes, since plant toxicity can shift with growth stage and stress.

Finally, act early. If one sheep starts seeking shade or develops swollen ears, red skin, or facial crusting, move the group to shade and review the pasture right away. Fast intervention can reduce suffering, limit flock spread, and help your vet identify the source before the evidence disappears.