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Photosensitivity in Cattle Grazing Brassica Crops
Abstract and Figures
Fast-growing Brassica forage crops, comprising turnip, rape, rutabaga, and kale varieties or interspecies crosses, are important in the provision of high-quality, easily digestible animal feed in many countries. The feeding of Brassica is associated with a number of potential problems, including photosensitization. This photosensitivity ranges from mild to severe. This article reports data on the implicated Brassica cultivars, as well as clinical observations, serum chemistry findings, skin biopsy and liver biopsy histopathology, gross necropsy and histopathological observations of spontaneous cases of Brassica (in particular turnip) photosensitivity in dairy cattle, and treatment and prevention strategies. In cattle, Brassica photosensitization is associated with increased activities of γ-glutamyl transferase and glutamate dehydrogenase, and raised phytoporphyrin (phylloerythrin) concentrations in serum. Thus, it is classified as a hepatogenous, or secondary, photosensitization. Histopathological lesions in the skin and liver of affected animals and bile duct changes, distinctly different from those seen in facial eczema (sporidesmin toxicosis), are described for the first time. In contrast to the situation with many cases of chronic facial eczema, the biliary and fibrotic changes appear to regenerate and not become relentlessly progressive. The toxin(s) responsible for the hepato-and cholangiotoxicity in cattle grazing Brassica is unknown. On the basis of a brief review of the literature on Brassica secondary compounds, and work done in rats, it appears possible that toxicity may be caused by degradation products of glucosinolates, in particular the nitrile or isothiocyanate derivatives.
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