Photosensitization in Cows

Photosensitization in cows is a sunlight-triggered skin injury that happens when a photodynamic compound is present in the body and reacts with ultraviolet light. The result can look like severe sunburn, but it is usually more painful and can progress to swelling, oozing, crusting, ulceration, and skin sloughing. White or lightly pigmented skin is most vulnerable, although severe cases can affect darker skin too.

In cattle, photosensitization is often divided into primary, secondary (hepatogenous), and less commonly inherited forms. Primary photosensitization happens when a cow eats or is exposed to a phototoxic substance directly. Secondary photosensitization is more common in production animals and develops when liver disease prevents normal clearance of phylloerythrin, a chlorophyll by-product that then builds up in the body.

This condition matters because it is not only a skin problem. Some cows are also dealing with underlying liver damage, eye pain, reduced feed intake, weight loss, lower milk production, or nursing problems if teat skin is affected. Fast action can limit skin damage and help your vet identify whether the problem is a one-off exposure or part of a larger herd issue.

Photosensitization in cows has more than one possible cause, which is why a herd history matters so much. In primary photosensitization, the cow eats or contacts a phototoxic agent that reaches the skin and reacts with sunlight. Reported triggers in cattle include certain plants and forages, with risk varying by region, season, rainfall, plant growth stage, and feed quality.

In secondary or hepatogenous photosensitization, the bigger problem is the liver. When the liver or bile ducts are damaged, the cow cannot clear phylloerythrin normally. That compound comes from chlorophyll metabolism in the gut, so grazing cattle are especially exposed. Merck notes this is the most common form in production animals. Liver injury may be linked to hepatotoxic plants, blue-green algae, liver flukes in some regions, mold-related toxins, or other causes of cholestasis and hepatocellular damage.

Less common causes include inherited porphyrias and occasional idiopathic cases where the exact trigger is not identified. Outbreaks have also been reported with water-damaged alfalfa hay, moldy straw, orchard grass hay with foxtails, winter wheat, brassicas, and some clovers. Because the list of possible triggers is broad and regional, your vet may ask detailed questions about pasture changes, hay lots, fertilizer use, irrigation, recent storms, pond access, and whether only white-faced or white-uddered cattle are affected.

Diagnosis usually starts with a history and physical exam. Your vet will look at which animals are affected, what the lesions look like, how quickly they appeared, and whether they are concentrated on nonpigmented skin. Photosensitization is often suspected when cows show photophobia, erythema, edema, and painful lesions on sun-exposed white areas.

The next step is figuring out why it happened. Your vet may recommend bloodwork to look for liver injury or cholestasis, including changes in enzymes such as GGT, ALP, and SDH, along with bilirubin. If liver-related disease is suspected, additional testing may include fecal checks for parasites in appropriate regions, feed or plant review, water evaluation, or liver biopsy in selected cases.

Diagnosis is often a mix of clinical pattern recognition and ruling out look-alikes. Sunburn, contact dermatitis, chemical irritation, frost injury, ringworm, mange, and some viral or bacterial skin diseases can overlap. In unusual or inherited cases, porphyrin testing in blood, urine, or feces may help confirm porphyria. For herd outbreaks, your vet may also inspect pasture plants and stored feed so treatment and prevention can target the true source.

Prevention starts with reducing exposure to known triggers and catching early cases fast. Walk pastures regularly, especially after heavy rain, drought stress, fertilization, irrigation changes, or when introducing cattle to unfamiliar forage. Review hay quality, avoid moldy or water-damaged feed, and pay attention to ponds or tanks where blue-green algae may bloom. If your area has liver flukes or known toxic plants, build prevention plans with your vet and local extension resources.

Management changes can make a big difference. Provide reliable shade, especially for white-faced or lightly pigmented cattle during high UV periods. Rotational grazing, weed control, forage testing when needed, and keeping records of pasture moves can help identify patterns before a larger outbreak develops. If one animal shows suspicious lesions, moving the group off the suspect field early may prevent more cases.

For herds with a previous outbreak, ask your vet to help create a seasonal risk plan. That may include monitoring liver-related disease risks, checking water sources, reviewing mineral and feed programs, and deciding when diagnostic sampling is worthwhile. Prevention is often less costly than treating advanced skin injury, production losses, or a herd-wide exposure event.