Toxic substances in the food of animals: Some recent findings of australian poisonous plant investigations

Some of the more interesting and significant recent Australian studies of plant toxins are those dealing with furanosesquiterpenoid essential oils, irritant diterpene esters, thiaminase I, trihydroxyindolizidine, carboxyatractyloside and oxalates. Furanosesquiterpenes, found mainly in Myoporum spp. , require activation by microsomal mixed‐function oxygenases in order to produce injury in the tissues in which they accumulate. One such compound isolated from M. deserti , deisopropylngaione, causes necrotizing lesions in the liver, kidney and lung. The native pteridophyte Marsilea drummondii contains thiaminase I at such a high level of activity that it can cause clinical thiamine deficiency in adult sheep which graze it at times of the year when it is growing profusely. Potent diterpene esters (which have been found to be potent cocarcinogens in mice) are to be found in thymelaeaceous plants such as Pimelea spp. One such compound, simplexin, also causes marked contraction of vascular smooth muscle, giving rise to the cardiopulmonary and hepatic circulatory disorder in cattle, known as St. George Disease. Carboxyatractyloside has recently been found to be the toxin in Xanthium spp. The compound is a hypoglycaemic agent, an inhibitor of mitochondrial oxidative phosphorylation and causes acute liver necrosis. Swainsona spp. contain the a ‐mannosidase inhibitor trihydroxyindolizidine. Consumption of the plants by animals results in the typical lysosomal storage disease a ‐mannosidosis. Oxalate or acid oxalate ions in plants normally cause toxicity per se in grazing animals, but when present in low levels in various native and introduced grasses eaten by horses, they can produce negative calcium balance and secondary nutritional hyperparathyroidism within several months.