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in humans (Hirschhorn, 1983). Betel quid chewing is an etiologic factor for oral cancer, and arecoline is suspected to contribute to the pathogenesis of cancer by producing mucosal inflammation and growth of oral epithelial cells (Keeler and Tu, 1983).

The Aristolochiaceae has a number of Aristolochia species that contain the nephrotoxic nitrophenanthrenes aristolochic acids, such as A. serpentina (snakeroot). These plants have been used in traditional medicines, especially in Chinese and Indian herbal remedies, which are consumed only for short periods. In European use, where the plant was consumed over several weeks, there is good evidence based on misidentification that they are responsible for acute kidney failure and possibly urothelial carcinoma (Cosyns et al., 1998; Violon, 1997).

The Asclepidaceae is known to contain a considerable number of Asclepias species (milkweeds) that are acutely toxic to livestock and domestic fowl (Cheeke, 1998; James et al., 1992). As little as 0.05 percent of an animal’s weight of green A. labriformis can result in death; most species are toxic at 0.25 to 1 percent (Kingsbury, 1964). Poisoning is characterized by symptoms of weakness, staggering, seizures, and coma, appearing within a few hours, followed by death within 1 or 2 days (Everist, 1981; James et al., 1992). The toxins are usually cardiac glycosides (cardenolides) structurally related to digitoxigenin (Cheeke, 1998).

The Asteraceae encompass a significant number of genera that contain hepatotoxic unsaturated pyrrolizidine alkaloids. Among these are Eupatorium species (gravelroot) and Tussilago farfara (coltsfoot) (Keeler and Tu, 1983). Others such as Packera candidissima (hierba de milagro) (Bah et al., 1994) and Senecio longilobus (gordolobo yerba) are also hepatotoxic, with the latter having been documented as causing severe hepatic fibrosis and death in infants and children (Cheeke, 1998; Stillman et al., 1977) Senecio species have a particular propensity to accumulate high levels of the alkaloids (up to 18 percent dry-weight basis in S. riddellii), and are a frequent cause of poisoning in livestock (Kingsbury, 1964; Molyneux and Johnson, 1984). Seeds of Senecio and Heliotropium spp have caused large-scale poisonings of humans in southern Africa, central Asia, and India (Cheeke, 1998; Colegate and Dorling, 1994; Kellerman et al., 1988). The alkaloids can also be sequestered in milk, eggs, and honey (Colegate and Dorling, 1994; Keeler et al., 1978). Pyrrolizidine alkaloid toxicity is characterized primarily by progressive hepatic cirrhosis (veno-occlusive disease) (Everist, 1981; Garland and Barr, 1998). The alkaloids have been demonstrated to be genotoxic and mutagenic, and cause cancer in rats, but evidence is insufficient to establish carcinogenicity in humans (Garland and Barr, 1998;

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