Photosensitisation appears to be similar to sunburn but is in fact caused by harmful chemicals circulating in the blood which cause a reaction to sunlight in areas of skin which are not protected by pigment. It thus occurs on the white or pale parts of skin and may be caused by certain harmful plants in the diet or when it gets liver disease.

When a horse has a liver problem it cannot process food as effectively as it does normally does and this results in too much of a chemical in the circulation which is derived from