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The Polypodiaceae includes the bracken fern Pteridium aquilinum, which is worldwide in distribution and is established as acutely and chronically toxic to livestock and laboratory animals. The plant contains the enzyme thiaminase, which results in anorexia and ataxia in horses, bright blindness in sheep due to retinal neuroepithelium degeneration, and depression of bone marrow in cattle (Cheeke, 1998). The plant also produces carcinoma of the upper alimentary tract and urinary bladder, caused by ptaquiloside (Cheek, 1998). A number of other structurally related illudanetype sesquiterpene glucosides have been isolated and identified (Nagao et al., 1989). There is evidence that carcinoma can result in calves from ingestion of the toxin through the milk of cows grazing bracken fern (Smith and Seawright, 1995). Epidemiological studies in Japan and Brazil have suggested a close association between bracken frond (fiddleheads) consumption and cancers of the upper alimentary tract (Alonso-Amelot and Avendano, 2001; Brown et al., 1999), but there is evidence that fiddleheads processed by salting can be eaten safely; other routes of exposure may therefore be a factor (Hirono et al., 1972).

The Ranunculaceae includes Aconitum (aconite, wolfsbane, monkshood) and Delphinium (larkspur) species that are acutely toxic to livestock (Kingsbury, 1964). Dried roots of aconite were historically used as an external application for treatment of pain, such as from rheumatism. The plant has been used to poison baits for pest animals, for execution of criminals, and for homicidal purposes. Accidental deaths have been caused by mistaking the root for that of horseradish (Kingsbury, 1964). The dried root can contain up to 1.5 percent by weight of alkaloids with the primary toxic constituent being the diester diterpenoid alkaloid aconitine, comprising approximately 30 percent of the total (Dewick, 2002). Aconitine is acutely toxic, with an oral LD50 in mice of 1 mg/kg, and has been used to produce heart arrhythmia in experimental animals (O’Neil et al., 2001). The root of A. ferox (Indian aconite) is extremely toxic and very small quantities can produce fatal cardiac depression (Klasek et al., 1972). The major alkaloid constituent is pseudoaconitine (Klasek et al., 1972). The extreme toxicity of Aconitum species indicates that use of any parts of the plant or preparations thereof should be a matter of extreme concern.

Delphinium (larkspur) species also contain norditerpene alkaloids and have produced large-scale losses of cattle, especially when grazed in the early growth and flower/seed stage (Cheeke, 1998). Death is often rapid, preceded by staggering gait, recumbancy, muscle twitching, and rapid and irregular pulse (Cheeke, 1998). Delphinium species contain over 40 different diterpenoid alkaloids with highly variable compositions and concentrations (Cheeke, 1998). A major constituent is deltaline, generally in excess of 50 percent of the total alkaloid content, with an intravenous LD50

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