Pteridium aquilinum is highly palatable, but toxic, to animals, both invertebrates and vertebrates. Several modes of action have been studied:
The fronds produce a large number of ecdysomes, which are hormones that cause uncontrolled molting in insect larvae and thus disrupt normal maturation processes.
The fronds, are cyanogenic. They contain a substance known as prunasin that is converted into the poison, hydrogen cyanide, in response to tissue damage (such as from insect predation).
The plants, particularly the rhizomes and young fronds, produce type I thiaminase, an enzyme that breaks down thiamine and thus causes vitamin B deficiency. The effects are cumulative over time and even hay contaminated with bracken can produce symptoms. The name bracken staggers has been applied to this problem in horses, but a variety of neurological symptoms occur and also anorexia, hemorrhaging, conjunctivitis, fever, muscular tremors and spasms, accelerated heartbeat, and seizures. Some of the symptoms are thought to represent more than one interacting causal agent. Mortality can be high.
The plants are rich in tannins, which bind to proteins, enzymes, and other cellular products, doing all sorts of harm in the bodies of organisms that ingest the tissues of bracken. Cooking tends to break down tannins.
The plants are carcinogenic, containing a substance known as ptaquiloside, as well as other norsesquiterpene glucosides. Regions in which humans ingest large quantities of bracken have long been known to have elevated rates of stomach and esophageal cancers, and various gastrointestinal and urinary cancers also have been observed in livestock. Related symptoms in some animals include retinal degeneration (bright blindness of sheep) and acute hemorrhagic disease (tied to degeneration of the bone marrow). Laboratory experiments on animals confirmed the link between Pteridium and these symptoms. When bracken is eaten by cattle, ptaquiloside apparently also can be transmitted through the milk, potentially leading to stomach and esophageal cancers. Even the spores are carcinogenic.
Bracken also can contribute secondarily to human health hazards. Areas with dense stands of bracken can develop a thick layer of decaying thatch from old fronds and a humid microhabitat develops under the canopy of living fronds. Such a habitat apparently promotes higher densities of ticks that are carriers of Lyme Disease.
Additionally, bracken has been shown to be allelopathic (producing compounds that inhibit the germination/growth of other plants), although the results of laboratory studies to demonstrate this have varied. Source documents: Hodge (1973), I. A. Evans (1976, 1986), W. C. Evans (1976, 1986), Gliessman (1976), Hannam (1986), Hudson (1986), Ferguson and Boyd (1988), Hirono (1990), Villalobos-Salazar et al. (1990, 1995), Brown (10995), Hopkins (1995), Ouden (1995), Potter and Pitman (1995), Alonso-Amelot et al. (2000), Simán et al. (2000), Smith et al. (2000), Burrows and Tyrl (2001), Moran (2004).