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Photosensitization
Certain substances make the skin abnormally sensitive to sunlight. When they circulate in the blood, the sun's rays trigger a real burn where the skin is poorly protected: muzzle, ears, eyelids, udder, vulva, folds above the hooves, and all...
What is it?
Certain substances make the skin abnormally sensitive to sunlight. When they circulate in the blood, the sun's rays trigger a real burn where the skin is poorly protected: muzzle, ears, eyelids, udder, vulva, folds above the hooves, and all white or clipped areas.
Animals with pigmented skin are spared in the same location, giving the characteristic appearance of lesions limited to white spots.
Two mechanisms exist. In the first, the substance comes directly from an ingested plant or an administered product. In the second, more serious, the liver is damaged and no longer eliminates a pigment resulting from the digestion of the grass: this pigment accumulates and makes the skin sensitive.
Key point
Skin lesions accompanied by yellow mucous membranes indicate liver damage and not a simple sunburn: it is a serious situation which often affects the whole lot.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The animals become agitated, seek shade, shake their heads, rub themselves.
Swelling of the ears, eyelids and muzzle; the ears become heavy and drooping.
Red, hot skin, very painful to the touch on light areas.
Then, oozing, thick crusts, then peeling of the skin in large patches; the tips of the ears may become necrotic and fall off.
Lesions of the udder which make suckling painful: the lambs are no longer left in the udder.
Lameness when the pad above the hooves is affected.
Reduced appetite, animals lying in the shade and no longer grazing.
In forms linked to the liver, yellow mucous membranes and marked depression are added to the skin picture.
Several animals from the same batch are often affected at the same time, which points to a common dietary cause.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Direct cause: ingestion of plants containing photosensitizing substances, such as St. John's wort or buckwheat, or administration of certain medications and products that sensitize the skin.
Hepatic cause, more common and more serious: the liver is damaged and is no longer able to eliminate a pigment resulting from the degradation of chlorophyll in the grass. This pigment accumulates in the blood and makes the skin sensitive to the sun.
Liver damage comes from toxic plants consumed on pasture, from mold present on stubble and fodder, or from poisoning, particularly from excess copper in sheep.
Strong sunlight is always necessary: the disease appears in clear weather, and the lesions are more marked in freshly shorn animals.
White animals, with light faces or depigmented areas.
Recent mowing, which exposes skin that has not seen the sun.
Grazing on plots invaded by photosensitizing or liver-toxic plants.
Grazing stubble and regrowth in hot, humid weather is conducive to mold.
High heat and intense sunshine, a common situation in the Maghreb in spring and summer.
Recent administration of a product without veterinary advice.
History of excess copper in livestock.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian examines the lesions and their distribution, which is very suggestive, and determines whether it is a direct form or liver damage.
A blood test assesses the functioning of the liver: this is the key test, because it guides the prognosis and the action to be taken for the entire herd.
It is necessary to inspect the plot and the forage, look for the plants involved and the molds, and make a list of all the products administered recently.
The veterinarian rules out other skin diseases: scabies, contagious ecthyma, dermatophilosis, ordinary bites and burns.
The other animals in the lot should be examined, even those that are not yet showing anything.
Treatment
The first action is immediate and costs nothing: removing the animals from the sun. Bring them into a building or under shelter during the day, change plots, and only take them out at dusk.
Immediately remove the suspected source: plot, moldy fodder, administered product.
The veterinarian manages pain and inflammation, prevents or treats wound infections, and treats skin lesions. The products and their quantities are subject to its prescription.
When the liver is involved, it puts in place appropriate support and assesses the chances of recovery.
Comfort care is very important: water and fodder within reach, clean and dry litter, protection against flies that settle on wounds, help with breastfeeding when the udder is affected.
Never apply a corrosive product, tar or motor oil to burnt skin: this is a widespread practice which aggravates the lesions.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Provide shade on all plots, natural or artificial, and allow animals free access during hot hours.
Graze early in the morning and at the end of the day during periods of strong sunlight.
Inspect the plots and eliminate plants known to be photosensitizing or toxic to the liver; avoid grazing invaded areas.
Do not feed moldy fodder and avoid grazing mold-laden stubble and regrowth in hot, humid weather.
Avoid putting freshly shorn animals out in direct sunlight; plan mowing and sheltering accordingly.
Never administer any medication or product without veterinary advice: some make the skin sensitive to the sun.
Strictly respect the rules for copper supplementation in sheep, as excess is a cause of liver damage.
Possible complications
Wound infection, aggravated by flies and maggot laying in hot weather.
Loss of tips of ears or large areas of skin.
Mastitis and cessation of breastfeeding when the udder is affected.
Marked weight loss in animals that stop grazing.
In forms linked to the liver, lasting damage to this organ, persistent jaundice and death.
Systematic recurrence until the cause is identified and removed.
When to see a veterinarian
Your pets' ears, eyelids or muzzle swell and become painful.
Scabs or peeling skin only appear on white or shaved areas.
Several animals from the same batch are affected at the same time: look for a food cause and call.
The mucous membranes or the white of the eye are yellow: this is a sign of liver damage, consult without delay.
A ewe refuses to suckle because of lesions on the udder.
Lesions appear after a change of plot, mowing or the administration of a product: report it, it is decisive information.
Prognosis
Good in direct forms: once the source is removed and the animals are placed in the shade, the skin heals in a few weeks, even if the appearance remains marked.
Reserved for dark in forms linked to the liver: the skin is only the visible part of a deeper attack, and the evolution depends on the severity of the liver damage.
Recurrences are the rule if the plot, fodder or product responsible are not identified and removed.
Frequently asked questions
Why are only my white sheep affected?
Because skin pigment protects against the sun's rays. The white, depigmented or freshly shaved areas do not have this protection: they are the ones that burn, which gives the very characteristic distribution of the lesions.
Is it contagious to the rest of the herd?
No. It is not a disease that is transmitted from one animal to another. If several animals are affected at the same time, it is because they share the same plot, the same fodder or the same product.
Can I apply ointment or oil to the wounds?
Do not apply anything without advice. Traditional practices based on tar, oil or corrosive products aggravate already deep burns. Put the animals in the shade, protect them from flies and call the veterinarian.
Should I take a blood test when I only see a skin problem?
Yes, it's useful. It allows us to know if the liver is involved, which completely changes the prognosis and the course of action for the rest of the herd exposed to the same cause.
References
ANSES — Agence nationale de sécurité sanitaire (France)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 09 August 2026
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