Bracken fern toxicity

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Bracken Fern Toxicity

Braken Fern Poisoning

Bracken fern (Pteridium aquilinum) is a perennial fern that is distinguished by it's large, broadly triangular fronds that are divided into three main parts, with each part bipinnately subdivided. Bracken fern is usually found in open wounds on sandy and gravelly soil, dry pastures and meadows, and abandoned fields.

Toxic components
Bracken fern contains high amounts of the enzyme thiaminase, which is present in all parts of the plant.
Thiaminase inhibits absorption of the necessary vitamin thiamin (B1), pyrimidine, and thiazole. The amount of thiaminase present in the plant varies depending on the time of year and is at its maximum amount in the summer. Bracken fern remains toxic even when it's withered or dried, such as when it is accidentally baled into hay. In fact, if it is stored, the resulting toxic effects often increase. Bracken fern is palatable to some horses, who will seek it out even when other forages are available. Late summer is generally when most poisonings occur in horses, when other forage sources may be scarce or dried out from drought periods.

Clinical Signs
Horses poisoned by bracken fern may initially show signs of a "tucked up" appearance, unsteady gait, change in behavior (such as acting nervous or timid), and/or congested mucous membrances. As the condition progresses, horses may stand abnormally, with their legs spread apart, and if asked to walk, will do so with a staggering gait.


Progressive loss of coordination
Decreased appetite
Weight loss
Muscle tremors
Death (within 2 to 10 days)


  • History
  • Clinical signs
  • Physical exam
  • Blood count

While waiting for your veterinarian

  • Remove the horse from bracken-infested pastures or remove hay, if suspected of contamination by bracken fern.



Large doses of thiamin over the course of a week or two can aid in the recovery of horses whose bracken consumption is discovered before the neurological signs become severe.


  • Make yourself aware of the weeds and plant species that can be invasive in pastures and/or poisonous to horses.
  • Take periodic walks around pastures to check for the presence of potentially poisonous plants
  • Check that hay does not contain dried up poisonous plants
  • If you borrow or hire farm machinery ensure it is clean prior to arriving on your property, the same goes for lending of your own equipment.
  • Quarantine new animals in a separate paddock the first 10 days to 2 weeks after arrival. Weed seeds can be passed through an animal's digestive tract.



Scientific Research

General Overviews

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Risk Factors

  • Braken ferns growing in pastures where horses are turned out
  • Poor quality hay