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Photosensitisation is a condition where pink or lightly pigmented skin reacts abnormally to sunlight.


The non-pigmented areas of skin on the face and lower limbs are most commonly affected. They become swollen, inflamed, itchy and very sore. Serum exudes onto the surface of the skin and a secondary bacterial infection often develops. In sever cases the skin dies and begins to slough.


Photosensitisation occurs when a horse eats certain plants or has liver disease. Some plants, for example St. John's Wart and some types of clover, contain photodynamic agents which are absorbed in the horse's gut. When absorbed they reach the horse's skin and here they absorb energy from the sunlight and this damages the surrounding skin cells.

The chlorophyll in plants is broken down by bacteria in the horse's gut and a product called phylloerythrin is produced. This is removed from circulation by the liver and excreted in bile, but if a horse has liver disease the excretion of bile is reduced and some phylloerythrin stays in the horse's system. When it reaches the skin it can cause photosensitisation.


Photosensitisation is diagnosed on the basis of the symptoms. Ensure you check the horse's field for the presence of any poisonous plants and a blood test should be done to check the liver function.


The horse must be stabled until the condition has been treated. Thoroughly soak any hay that is fed to keep the dust levels to a minimum.

The affected areas must be cleaned regularly, even if the skin is very sore. It should be done gently with lukewarm water, an antibacterial soap and cotton wool. Ensure the area is dried thoroughly and apply an antibiotic ointment which will help to reduce inflammation and prevent infection.

The vet may administer antibiotics, antihistamines or corticosteroids but the treatment will depend on the severity of the reaction and the cause of the condition.

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