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PHOTOSENSITISATION This condition develops when the skin becomes sensitised to certain wavelengths of sunlight by the presence of specific photodynamic agents within the tissue. Photodynamic substances absorb energy from light, and the cells in the exposed tissues are severely damaged as a result. There are two major types of photosensitisation; primary, in which the photodynamic substance is absorbed intact from the alimentary tract; secondary, in which the photosensitisation is secondary to a disease of the liver. A third form of photosensitisation can result from contact of the skin with certain types of plant sap. Some plants, such as St. John*s wort (Hypericum perforatum), buckwheat (Polygonum fagopyrum) and wild carrot (Cynopterus spp), contain photodynamic substances, which cause primary photosensitisation. Some chemical compounds are also photoactive. Secondary photosensitisation follows the effects of the mycotoxin produced by the fungus Pithomyces chartarum or ragwort poisoning. CLINICAL SIGNS It is the non-pigmented thin-skinned areas of the body with poor hair cover which are affected. Lesions are often seen on the head and ears, along the neck and back, and if the animal is lying down the udder and teats may be affected. Affected animals are quite distressed and inappetent. Initially, there is severe swelling and oedema of the white parts of the head and ears, the latter being very pendulous due to their increase in weight. The eyelids, face and lips also become swollen. There appears to be severe irritation and affected animals frequently rub themselves on objects in an attempt to alleviate the irritation with further self-inflicted trauma. Serous exudation through the skin occurs and this dries to form yellow crusts or scabs. This acute inflammation is followed by death and sloughing of the affected skin, often leaving the ears dried, twisted and withered. Regeneration of the skin may take several weeks. Removal of the animal to shaded, cool, and well ventilated housing will prevent further damage to the skin, treatment to prevent fly strike and antibiotics to control secondary infection may be needed. If the animal has a genetic predisposition to the disease, or suffering from liver failure then the prognosis is poor. |
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