Photosensitization in Pigs: Light-Sensitive Skin Reactions Explained

Photosensitization is not the same thing as an ordinary sunburn. It happens when a light-reactive chemical, called a photodynamic agent, is present in the body and sunlight activates it in the skin. That reaction damages skin cells and can cause significant pain, swelling, oozing, crusting, and skin loss.

In pigs, this tends to show up on areas with less protection from pigment or hair, especially the ears, face, back, and other exposed skin. Some pigs develop only mild redness and discomfort. Others can develop severe inflammation, cracked skin, or tissue necrosis if exposure continues.

There are different forms of photosensitization. Primary photosensitization happens when a pig is exposed to a photodynamic substance directly, such as certain plants, chemicals, or medications. Secondary, or hepatogenous, photosensitization happens when liver disease prevents normal removal of phylloerythrin, a pigment formed during chlorophyll breakdown. In production animals, the secondary form is considered the most common overall.

Because the skin lesions can look dramatic and some causes involve liver injury, this is a condition worth discussing with your vet early. Fast shade, wound protection, and finding the underlying cause can make a big difference in recovery.

Photosensitization in pigs has several possible causes. In primary photosensitization, the pig is exposed to a substance that becomes activated by sunlight while circulating in the body. Veterinary references describe plant-derived photodynamic compounds as common causes across animal species. Examples reported in livestock include compounds from St. John's wort and buckwheat, and some medications can also trigger this reaction. Merck also notes that phenothiazine anthelmintics have been reported to cause primary photosensitization in swine.

In secondary or hepatogenous photosensitization, the skin problem starts with liver dysfunction. When the liver or bile flow is impaired, the body cannot clear phylloerythrin normally. That pigment then builds up in the bloodstream and reacts in the skin when sunlight hits it. This form is especially important because the skin lesions may be the visible clue to a deeper liver problem.

Potential liver-related triggers in pigs can include toxic plants, mycotoxins, or other causes of hepatic injury. Merck notes that aflatoxins and fumonisins can cause liver injury in swine, and photosensitivity has been reported in pigs exposed to alsike clover. In real life, your vet will also think about look-alike problems such as ordinary sunburn, chemical irritation, mange, bacterial skin infection, or trauma.

The timing can vary. Some pigs show signs within a couple of days of exposure to a photodynamic agent, while secondary cases tied to liver injury may appear later. That is one reason a careful history about feed, pasture, medications, and recent management changes matters so much.

Your vet usually starts with the pattern of the skin lesions and the history. Photosensitization is suspected when a pig has painful redness, swelling, or crusting on sun-exposed skin, especially after a recent change in pasture, hay, feed, medication, or deworming history. The distribution of lesions matters because exposed, lightly pigmented, or thin-haired areas are often affected first.

A physical exam helps your vet decide whether this looks more like photosensitization, ordinary sunburn, infection, parasites, or another skin disorder. If liver disease is possible, your vet may recommend bloodwork to look for changes in liver enzymes and bilirubin. Merck notes that increased sorbitol dehydrogenase, gamma-glutamyl transferase, alkaline phosphatase, and direct bilirubin can support hepatogenous photosensitization.

In some cases, your vet may suggest additional testing such as chemistry panels, CBC, feed review, pasture inspection, or biopsy of damaged skin. Definitive confirmation of abnormal porphyrin handling can involve measuring porphyrins in blood, urine, and feces, although that is not always needed in routine field cases.

Diagnosis is often a combination of pattern recognition and ruling out other causes. That is why photosensitization should be treated as both a skin problem and a possible clue to a toxin exposure or liver issue.

Prevention starts with reducing both sun exposure and exposure to triggers. Make sure pigs have reliable shade, especially during strong midday sun. If a pig has pale skin, sparse hair, healing skin, or a history of sun-related problems, that protection becomes even more important.

Review pasture plants, hay sources, and feed quality regularly. Some photodynamic plants can cause primary photosensitization, and some plants or mycotoxins can damage the liver and set up secondary photosensitization. Good feed storage matters because mold growth and mycotoxins can contribute to liver injury in swine.

Use medications thoughtfully and keep a record of what was given and when. If your pig develops skin lesions after a new drug, dewormer, or pasture change, tell your vet right away. That history can help narrow the cause much faster.

If one pig is affected, look at the whole group and the environment. Remove suspected feed or pasture exposures, increase shade access, and ask your vet whether other pigs should be checked. Early action is often the best way to prevent a mild skin reaction from becoming a painful herd problem.