Sunburn vs Photosensitization in Horses: How To Tell the Difference

By Dr Duncan Houston

Skin reactions in horses are easy to underestimate at first.

A pink nose with peeling skin may just be simple sunburn. But when the reaction is more severe, appears on other white-skinned areas, or is paired with signs of illness, the problem may be photosensitization instead. That matters because photosensitization is not just a skin issue. In some horses, it points to plant exposure or underlying liver disease. (Merck Veterinary Manual)

This is where owners get caught out. The lesions can look similar early on, but the meaning is very different. Sunburn is usually a direct UV injury to exposed pale skin. Photosensitization happens when light-reactive compounds in the body make the skin overreact to sunlight, often because of plant toxins or impaired liver clearance of phylloerythrin, a chlorophyll breakdown product. (Merck Veterinary Manual)

If your horse has sore, red, crusted, or peeling white skin, especially outside the face, this is not something to brush off as “just a bit sunburnt.”

Quick Answer

Simple sunburn in horses usually affects unpigmented skin with heavy sun exposure, especially the muzzle, around the eyes, and other lightly haired facial areas. Photosensitization can look similar at first, but it is often more severe and may affect broader nonpigmented areas, sometimes because of plant-derived photodynamic compounds or liver disease that allows phylloerythrin to build up in the blood. (Merck Veterinary Manual)

Quick Decision Guide

Skin reaction limited mainly to the pink nose, muzzle, or around the eyes after strong sun exposure → simple sunburn is more likely. (Merck Veterinary Manual)

Painful, crusting, oozing, or sloughing lesions on white-skinned areas beyond the face, especially shoulders, legs, or body patches → photosensitization should be considered. This is an inference based on the fact that photosensitization commonly affects exposed, nonpigmented skin and can become more extensive than ordinary facial sunburn. (Merck Veterinary Manual)

Skin lesions plus jaundice, depression, poor appetite, or other signs of illness → liver-linked photosensitization becomes more concerning. (Merck Veterinary Manual)

Recent grazing changes, suspect plants, blue-green algae exposure, or possible fungal/toxic forage issues → primary or hepatogenous photosensitization should move higher up the list. (Merck Veterinary Manual)

What Sunburn Usually Looks Like

Sunburn is a direct ultraviolet injury.

In horses, it tends to affect lightly pigmented or hairless areas with more sun exposure, especially the muzzle, lips, and areas around the eyes. The skin usually becomes red, tender, dry, and later may peel or scab. It is painful, but it is usually a local skin problem rather than a sign of internal disease. This distinction is an inference based on the Merck/MSD descriptions of photosensitization affecting broader nonpigmented areas and being linked to photodynamic compounds or liver dysfunction. (Merck Veterinary Manual)

What This Usually Turns Out To Be

When owners think a horse has sunburn, the real situation is usually one of these:

• ordinary UV injury on pink facial skin

• a more severe light reaction that is actually photosensitization

• skin damage made worse by continued sunlight after the first lesions appeared

• irritation on top of another underlying problem

The mistake I see most often is assuming all peeling pink skin is harmless sunburn.

What Photosensitization Actually Is

Photosensitization happens when photoactive compounds circulate in the body and then react in the skin when exposed to ultraviolet light. This leads to skin damage through reactive oxygen species and membrane injury. Lesions are most common on unpigmented skin with UV exposure. (Merck Veterinary Manual)

In horses, the forms most often discussed are:

• Primary photosensitization, caused by ingestion of a photodynamic compound

• Secondary or hepatogenous photosensitization, caused by liver dysfunction that prevents proper metabolism of photodynamic substances such as phylloerythrin (Merck Veterinary Manual)

This is why photosensitization deserves more respect than routine sunburn. It may be the visible tip of a larger toxic or hepatic problem.

Why Liver Disease Can Cause Skin Reactions

One of the liver’s jobs is clearing phylloerythrin from the bloodstream. Phylloerythrin is produced from breakdown of chlorophyll in plants eaten by the horse. If the liver cannot process and remove it properly, phylloerythrin accumulates in the blood, reaches the skin, and reacts with ultraviolet light, damaging especially light-skinned areas. (MSD Veterinary Manual)

What Vets Care About Most

What matters most here is not just what the skin looks like. It is whether the skin lesions are happening in isolation or as part of a bigger pattern involving:

• jaundice

• poor appetite

• depression

• abnormal liver enzymes

• recent plant or forage exposure

That is the line between “treat the skin” and “investigate the horse.”

What Photosensitization Looks Like

Photosensitization lesions can start with redness and pain, but they often escalate further than simple sunburn. Merck describes erythema followed by edema, and with ongoing light exposure lesions may progress to vesicles, bullae, serum exudation, ulceration, scab formation, necrosis, and skin sloughing. Photophobia and agitation in sunlight are also common. (Merck Veterinary Manual)

This often shows up on white-haired, nonpigmented, or hairless skin. The face is commonly involved, but lesions can also affect other light-skinned body regions. If concentrations of photoactive toxins or phylloerythrin are high and sunlight is intense, even pigmented skin may be affected. (Merck Veterinary Manual)

Decision Checkpoint

If the lesions are severe, involve multiple white-skinned areas, or look wet, swollen, and sloughing rather than just dry and sunburnt, photosensitization should move much higher on the list.

Plants and Other Causes That Matter

Most photosensitizing compounds in animals are plant derived. Merck notes a wide range of plant-associated agents, including furocoumarin-containing plants in the Apiaceae and Rutaceae families, as well as a range of suspected forages under certain environmental conditions. The horse-owner MSD page also notes that hepatogenous photosensitivity in horses is commonly linked to grazing plants toxic to the liver, blue-green algae, or fungal toxins. (Merck Veterinary Manual)

This means the practical causes to think about are:

• plants with direct photodynamic compounds

• hepatotoxic plants

• toxic algae exposure

• mycotoxin or fungal-associated forage problems

Your pasture, hay, and recent feed changes matter a lot here.

Severity Framework

This framework is based on Merck/MSD descriptions of lesion progression and the added significance of concurrent liver-disease signs. (Merck Veterinary Manual)

What Needs To Be Ruled Out

Not every crusted white patch is photosensitization.

Important rule-outs can include:

• simple sunburn

• contact irritation

• trauma

• insect-related dermatitis

• fungal or bacterial skin disease

• immune-mediated skin disease

But when lesions are unusually severe, extensive, or paired with systemic signs, liver-associated photosensitization must be taken seriously. Merck specifically notes that hepatogenous cases are supported by liver enzyme changes, bilirubin changes, and gross or histologic liver disease. (Merck Veterinary Manual)

What Should You Do Right Now?

1. Get the horse out of direct sunlight immediately.

2. Check whether the lesions are limited to the face or involve broader nonpigmented areas.

3. Think about any recent pasture, hay, or feed changes.

4. Watch for jaundice, dullness, poor appetite, or other signs of illness.

5. Arrange veterinary assessment if lesions are severe, widespread, or paired with possible liver signs.

Merck’s treatment guidance for photosensitization emphasizes full shade or housing, with grazing only during darkness while the condition persists. (Merck Veterinary Manual)

Simple checkpoint:

localized pink-nose reaction after sun → more likely sunburn

broader painful lesions on white areas, especially with illness signs → think photosensitization

Common Mistakes Owners Make

Common mistakes include:

• assuming all light-related skin damage is simple sunburn

• continuing turnout in full sun after lesions appear

• focusing only on creams and ignoring possible liver disease

• not reviewing pasture and forage exposure

• waiting too long when lesions spread beyond the face

The biggest mistake is treating a possible internal-disease clue as only a skin problem.

When Is This an Emergency?

Seek prompt veterinary care if your horse has:

• severe swelling or sloughing skin

• extensive lesions outside the face

• marked pain or photophobia

• jaundice

• depression

• poor appetite

• evidence of liver disease or toxicosis

Merck notes that prognosis is generally good for primary photosensitization but poor for hepatogenous photosensitization and porphyria, and that severe stress and skin necrosis can be debilitating and even fatal. (Merck Veterinary Manual)

Prevention

Prevention depends on the cause.

For straightforward sunburn prevention, limiting UV exposure on pink skin and using physical protection is sensible. For photosensitization prevention, the bigger job is reducing exposure to relevant plants, hepatotoxins, toxic algae, and poor-quality forage, while identifying liver problems early. Merck/MSD also emphasize shade and avoiding ongoing sunlight exposure as core management once the condition appears. (Merck Veterinary Manual)

Frequently Asked Questions

How can I tell sunburn from photosensitization in a horse?
Sunburn is usually more localized to exposed pink skin, especially the face. Photosensitization is often more severe and may affect broader white-skinned areas, especially when paired with other signs. (Merck Veterinary Manual)

Does photosensitization always mean liver disease?
No. It can be primary from photodynamic compounds or secondary from liver dysfunction. In horses, hepatogenous photosensitivity is common, but not the only form. (Merck Veterinary Manual)

What is phylloerythrin?
It is a chlorophyll breakdown product that can build up in the blood when the liver is not clearing it properly, then react with UV light in the skin. (MSD Veterinary Manual)

What is the first thing to do if I suspect photosensitization?
Get the horse into full shade or indoors and arrange veterinary assessment, especially if lesions are severe or there are signs of illness. (Merck Veterinary Manual)

Can photosensitization be serious?
Yes. Primary cases can do well, but hepatogenous cases can be much more serious because they may reflect underlying liver disease. (Merck Veterinary Manual)

Final Thoughts

Simple sunburn and photosensitization can look similar early on, but they are not the same problem.

Sunburn is usually a local UV injury. Photosensitization is often a sign that something more important is happening, especially if lesions extend beyond the face or the horse seems unwell. That is the key clinical line.

If the skin reaction is broader, more severe, or out of proportion to ordinary sun exposure, do not assume it is harmless.

If you want help thinking through whether your horse’s skin reaction looks more like sunburn, photosensitization, plant exposure, or possible liver-linked disease, ASK A VET™ can help you work through the next step clearly and practically.