Photosensitisation

CAUSE: a condition in which skin becomes overly sensitive to UV light (sunlight). This condition is not the same as sunburn, although the difference can be difficult to distinguish. Photosensitisation occurs when photodynamic agents are deposited in the skin and are subsequently exposed to and absorb abnormal amounts of UV light. Unlike sunburn, horses get “burnt” when it’s not particularly sunny.

There are two common types in horses. The first occurs following the ingestion of preformed photodynamic agents which are present in certain plants such as St John’s wort, buckwheat and clover. The second can accompany advanced liver disease and occurs when a product called phylloerythrin accumulates in the skin. This is normally detoxified by the liver, but when the liver is diseased, this product enters the bloodstream and accumulates in the skin.

DIAGNOSIS: clinical signs are similar to sunburn. It is most commonly seen in light-coloured skin, on the white or pink parts of the muzzle area and back of the pasterns but can affect any area of pink skin on the body. Bloods can be taken to confirm liver involvement if your horse is showing other signs of liver disease or to rule out liver disease.

TREATMENT: treatment involves eliminating causative plants from the horse’s diet and treating for sunburn (see box, right). If there is severe liver involvement, prognosis is guarded and depends on the severity of the underlying liver disease which needs further investigations and treatment if possible.

PREVENTION: eliminate access to plants containing the photodynamic agent. For severe cases, treat the liver disease, if possible.