Bracken Fern
Bracken fern (Pteridium aquilinum) is found throughout the world and has been associated with poisoning in cattle, sheep, and horses whenever they are forced to eat the plant. Poisoning has been reported most frequently in cattle in England and Europe, and is relatively uncommon in horses. In the continental United States, bracken fern poisoning is most common in the northwest. Sensitive fern (Onoclea sensiblis) has also occasionally been reported to cause poisoning in horses.
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Bracken fern is a common perennial fern of woodlands and grows from an underground, dark brown, branching rhizome. The fronds emerge directly from the rhizome and may be up to 6.5 ft (2 m) in height. The overall leaf shape is triangular with each leaflet having curved-under edges. Spores are produced on the underside of the leaflets and appear as brown dots lining the leaf edge.
The toxic principle in bracken fern for horses is a vitamin B1 or thiamin-destroying enzyme (thiaminase) and thus causes a thiamin (vitamin B1) deficiency. Horses appear to have to consume a diet containing 30 to 50% bracken fern for at least 30 days before clinical signs appear. The rhizome is the most toxic, and the leaves are poisonous whether green or dried.
Unlike horses, bracken fern poisoning in cattle appears to be due to other undetermined factors that cause a fatal anemia due to bone marrow depression. A variety of compounds also have been identified in bracken fern that cause urinary bladder and intestinal tumors in cattle and other animals. Sheep develop retinal degeneration and blindness due to bracken fern poisoning. However, these effects have not been found to occur in bracken-fern-poisoned horses. Weight loss due to poor appetite, incoordination of the hindquarters, and a slow heart rate are the principal signs seen in horses. If horses continue eating bracken fern, progressive depression and rear leg weakness develop that may result in an inability to stand, coma, and eventually death.
Diagnosis of bracken fern poisoning is aided by the determination of low serum thiamin and increased pyruvate concentrations, absence of sleeping sickness, or severe liver damages, and evidence that the horse has eaten large quantities of bracken fern. Alleviation of clinical signs in 12 to 24 hours following thiamin administration is also a helpful means of confirming the diagnosis.
Affected horses should be given a bracken-fern-free diet and large doses of thiamin daily for 5 to 7 days. Recovery is usually complete in 2 to 3 days.
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