Hepatogenous Photosensitization in Cows: Skin Lesions Caused by Liver Disease

Hepatogenous photosensitization is a sun-triggered skin disease that develops because the liver is not processing waste normally. In cattle, the key compound is phylloerythrin, a pigment formed when rumen microbes break down chlorophyll from green plants. A healthy liver removes phylloerythrin through the bile. When the liver or bile ducts are damaged, that pigment builds up in the bloodstream and reaches the skin.

Once sunlight hits the skin, phylloerythrin reacts with light and damages tissues. That is why cows can develop painful lesions on white, lightly pigmented, or sparsely haired areas first. The face, ears, muzzle, eyelids, teats, udder, vulva, and topline are common sites.

This condition is sometimes called secondary photosensitization because the skin problem is secondary to liver disease. In grazing cattle, it may be linked to plant or fungal toxins that injure the liver, but your vet will also consider other causes of hepatobiliary disease.

The skin lesions can look dramatic, but the bigger concern is the liver injury underneath. Some cattle show obvious skin damage, while others in the same group may have significant liver changes before lesions become easy to see.

The immediate cause is failure of the liver or biliary system to clear phylloerythrin. In cattle, that usually means there has been enough liver or bile duct injury for this pigment to accumulate in the blood. Sunlight then activates the pigment in the skin, especially in nonpigmented areas.

A range of liver insults can trigger this process. Important causes include hepatotoxic plants, mycotoxins, and diseases that inflame or obstruct bile ducts. One classic example is facial eczema, in which the fungal toxin sporidesmin damages the liver and biliary tract, leading to hepatogenous photosensitization. Merck also notes other hepatotoxins in large animals, including some grasses and plants containing pyrrolizidine alkaloids, that can contribute to liver failure and sunlight sensitivity.

Pasture history matters. A sudden move to a new field, heavy growth of risky plants, dead plant litter supporting fungal growth, or a cluster of cases after a weather change can all raise suspicion. Cattle are considered among the more susceptible livestock species for pasture-associated photosensitization.

Not every sun-related skin lesion is hepatogenous. Your vet may also sort this condition from primary photosensitization, where the photodynamic agent comes directly from a plant or drug, and from ordinary sunburn, contact irritation, parasites, or infectious skin disease.

Diagnosis starts with a farm history and physical exam. Your vet will look at which animals are affected, what pastures or feeds they have been on, whether lesions are limited to sun-exposed pale skin, and whether there are signs of liver disease such as jaundice, poor appetite, or weight loss. Season and pasture conditions can be especially helpful in grazing outbreaks.

Bloodwork is often the next step. In cattle with liver injury, vets commonly check GGT, AST, bilirubin, bile acids, cholesterol, triglycerides, albumin, and clotting status. Merck notes that these values can rise with hepatobiliary damage associated with facial eczema and other hepatotoxicoses. Testing helps confirm that the skin problem is linked to liver dysfunction rather than being a skin-only disorder.

Your vet may also recommend pasture and feed investigation. That can include looking for suspect plants, reviewing hay or silage sources, and in some regions performing pasture spore counts when facial eczema is a concern. If the diagnosis is still unclear, additional options may include liver ultrasound, biopsy, or necropsy in a herd investigation.

Because several herd-level toxins can cause similar signs, diagnosis is often about putting the pieces together: lesion pattern, sunlight exposure, liver values, and evidence of a likely toxin or hepatobiliary disease source.

Prevention focuses on protecting the liver first. Walk pastures regularly, especially after seasonal changes, drought stress, heavy dead plant litter, or rapid regrowth. Remove cattle from fields with suspicious plants or conditions linked to fungal growth and review any new hay, silage, or byproducts if several animals become affected.

Good pasture management can lower risk. Avoid forcing hungry cattle onto questionable forage, rotate fields thoughtfully, and work with your local extension or your vet if you suspect toxic plants or regional problems such as facial eczema risk. In areas where sporidesmin is a concern, pasture monitoring and local guidance are especially important.

Provide access to shade for all cattle, but remember that shade alone does not prevent the disease if the liver is already being injured. It mainly reduces how severe the skin damage becomes while the underlying problem is addressed.

If one cow develops suspicious lesions, check the rest of the group early. Some cattle with hepatogenous photosensitization have significant liver injury before obvious skin lesions appear. Fast herd-level action can reduce losses, improve comfort, and help prevent repeat exposure.