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PHOTOSENSITISATION (Facial eczema; Yellowes; Head Grit) Photosensitisation is caused by the exposure to sunlight of sheep which have a photodynamic substance present in their tissues. Photodynamic substances absorb energy from light, and the cells in the exposed tissues are severely damaged as a result. As with cattle, 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. Some plants, such as St. John *s Wort (Hypericum perforatum), buckwheat (Polygonum fagopyrum) and wild carrot (Cynopterus spp), contain photodynamic substances, which cause primary photosensatisation. Some chemical compounds are also photoactive. Secondary photosensitisation follows the pathological effects of the mycotoxin produced by the fungus Pithomyces chartarum or ragwort poisoning. An inherited congenital photosensitivity as a result of an inherited defect has been described in Corriedale and Southdown lambs.PREVALENCE This is unknown but can be quite high in lambs in certain flocks, often upland or bill flocks. It is seen mainly in late May, June and early July. CLINICAL SIGNS As with cattle, it is the non-pigmented thin-skinned areas of the body which are affected. Thus, lesions are mainly seen on the head and ears. Normally, a thick and/or pigmented fleece will protect the body but lesions can occur along the dorsal midline parting of wool on the back. 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 lambs 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. Affected lambs usually regain their appetite and recover to a certain extent but do not gain weight as quickly as non-affected lambs. In Norway photosensitisation in lambs has been attributed to Bog asphodel which is quite common on many Scottish hills: photosensitisation also occurs in sheep on rape. In New Zealand facial eczema is a serious problem in sheep grazing rye grass pastures contaminated by the fungus Pithomyces chartarum. |
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