Human carcinogenesis and bracken fern: a review of the evidence.

The complex taxon embraced in the Pteridium genus, popularly known as bracken fern and notorious weeds in many parts of the world, is one of the few vascular plants known to induce cancer naturally in animals. It has been known for long to be acutely toxic to livestock and sublethal chronic oral feeding of bracken fronds leads to cancerous lesions in the urinary bladder, or bovine enzootic haematuria (BEH) and ileum of cattle. Bracken poisoning has been attributed chiefly to ptaquiloside, a norsesqui-terpene which is also a potent carcinogen inducing various malignancies in laboratory animals. It is capable of alkylating uncoiled DNAbases at key proto-oncogenes of selected organs. Some human populations also eat young bracken shoots and epidemiological studies in Japan and Brazil have shown a close association between bracken consumption and cancers of the upper alimentary tract. In addition, other studies reveal that the mere presence of bracken swards represents a greater risk to die of gastric adenocarcinoma for people who live more than 20 years in such areas or are exposed in childhood. This work reviews the bracken-cancer connections established by in vitro and in vivo experiments and epidemiological studies in various parts of the world, and provides insights into the possible bridges for bracken carcinogens to reach the human diet. Also, specific points where more research is needed are highlighted.