Experimental photosensitization and toxicity in sheep produced by Tetradymia glabrata

Factors affecting the phototoxic response of sheep to Tetradymia glabrata were investigated. Photosensitization, which occurs regularly in the field situation, was unpredictable and difficult to produce experimentally. In attempting to understand this unpredictability we concluded that the plant does not lose its toxicity after field collection either through volatility of the toxin, plant enzymatic degradation of the toxin or dehydration. The chlorophyll source, from which the suspected phototoxic agent phylloerythrin is derived, did not appear to be important. The single case of photosensitization produced gave no clues as to why it occurred. Some predisposing factors important to the production of tetradymia related photosensitization in sheep are unknown. In disagreement with previous reports the hepatotoxic response of sheep to Tetradymia glabrata was extremely variable. Also plants in comparable stages of growth, but from different locations, varied in their toxicity to sheep. Young budding plants are most toxic, however, toxicity of the plant remains through maturity.