Photosensitivity
Photosensitivity is a photodynamic skin reaction — sun-exposed, usually unpigmented skin that burns, swells, or sloughs after a plant, a drug, or liver disease leaves a photodynamic agent in the skin. It is not hives (an allergic wheal reaction), not rain rot (bacterial scabs), and not proud flesh (overgrown wound tissue). A photograph cannot diagnose photosensitivity. A still cannot tell primary (plant or drug) from hepatogenous (liver) photosensitivity and cannot diagnose liver disease. Diagnosis is a vet exam, often bloodwork. The skin scan is a first look at the patch, not a verdict that your horse has photosensitivity, and there is no photosensitivity scanner.
Symptoms to watch for
Red, swollen, or painful skin on unpigmented areas — a blaze, muzzle, white sock, or pink patch — after sun
Blistering, weeping, or sloughing of white or pink skin that was exposed to sunlight
A horse that seeks shade, is head-shy, or resists a bridle or boots on white-skinned areas
Crusts or raw patches that follow the white markings, not the drip line of a wet rug
Dullness, off feed, or yellow gums or eyes — a reason to call the vet, not a photo verdict
A change in size or surface of the patch is a reason to book a vet, not a photo verdict
Common causes
- A photodynamic reaction — a plant, drug, or liver-held pigment in the skin that reacts with UV, not an allergy and not a bacterial scab
- Primary photosensitivity from photodynamic plants or drugs — the trigger is a clue, not a diagnosis
- Hepatogenous (liver) photosensitivity, where the liver cannot clear chlorophyll breakdown products — a still cannot confirm or rule out liver disease
- Unpigmented or thinly haired skin that lets UV reach the dermis — location is a clue, not a diagnosis
- Not hives (raised wheals), not rain rot (bacterial paintbrush scabs), and not proud flesh (overgrown wound tissue) — those can look different and still need a vet
Prevention
- Watch white or pink skin after sun, and book a vet rather than waiting for a photo guess
- Note plants, new drugs, or a dull or off-feed horse so you can tell the vet
- Shade and a UV rug can protect exposed white skin while you wait for the vet — they are not a diagnosis
- Do not treat a photo, a sunburned patch, or a scan result as a diagnosis
- Skin photos can help you and your vet track the patch over time — they do not confirm or rule out photosensitivity, and they cannot diagnose liver disease
What to do
- 1
Book a vet visit for sun-exposed white or pink skin that is red, swollen, blistered, or sloughing. Diagnosis is a vet exam, often bloodwork — not a photograph.
- 2
Photograph the patch in good light and keep it on the horse's record so you and the vet can see change over time.
- 3
Move the horse into shade and do not start a cream, steroid, or liver tonic from a photo guess.
- 4
Use a skin scan only as a first look at the patch to share, not as a photosensitivity test. There is no photosensitivity scanner.
- 5
If photosensitivity is confirmed, follow the plan your vet prescribes — that may include bloodwork to check the liver, finding a plant or drug trigger, shade, and wound care for sloughed skin. Do not start a treatment protocol from a photo.
- 6
Keep the vet on the same page. Share what you see; do not treat a photo as a verdict.
Pick up the phone if…
Schedule a vet visit if unpigmented or white skin is red, swollen, blistered, or sloughing after sun. Call sooner if the horse is dull, off feed, jaundiced, or the patches are spreading or painful. A photograph cannot diagnose photosensitivity. A still cannot tell primary (plant or drug) from hepatogenous (liver) photosensitivity and cannot diagnose liver disease. Diagnosis is a vet exam, often bloodwork. Photosensitivity is a photodynamic / sun / liver reaction — not hives, not rain rot, and not proud flesh. The skin scan is a first look at the patch, not a verdict, and there is no photosensitivity scanner.
Related topics
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Educational content only. Not a substitute for veterinary advice.