A wide variety of feed toxins specifically attack the liver. Usual symptoms of liver damage such as jaundice (or, icterus) and elevated liver enzymes in the blood may be present, but usually the first symptoms noticed are severe damage to the skin on exposure to sunlight. Liver damage can become fairly advanced before stock owners notice their animals are behaving as if they are ill, so blood may not be drawn or the color of membranes assessed in the early stages. But when the liver has been damaged, chlorophyll and related plant pigments cannot be completely catabolized, and some of the intermediate breakdown products build up in the blood. These fragments capture energy from sunlight and release it in such a way that overlying skin tissue is damaged and may develop into serious sores, sloughing off and leaving the animal open to infection. This secondary photosensitiza-tion is observed after liver damage caused by the ingestion of pyrrolizadine alkaloids from groundsel (Senecio), alsike clover (in horses), and Lantana camara. The latter plant, an escaped ornamental, has been responsible for the deaths of thousands of Ethiopian cattle.
Primary photosensitization can also occur without liver damage when plants such as buckwheat and St. Johnswort (or, Klammath weed) are consumed in sunny climates, since they contain compounds that trap light energy and cause skin damage in the presence of an intact liver.
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