Bracken fern is an herb that is perennial. It can grow leaves as large as 3-4 feet in good soil. It is usually found in large patches because it has a horizontal root system, allowing it to easily spread.
It is found throughout the United States, in its preferred habitat of dry, open woodlands.
The entire plant is toxic, though the rhizomes are the most toxic part. Rhizomes are horizontal branches, usually found underground, that allow the plant to spread and start new shoots.
A horse has to consume pretty large amounts of bracken before poisoning will occur. He has to consume 3-5% of his body weight for at least a month to induce symptoms.
Thankfully horses usually avoid this plant, but will eat it when no other forage is available, and some horses even develop a taste for it.
This plant is toxic because it contains large amounts of thiaminase, which breaks down thiamine which is an important B vitamin.
When this plant is eaten, the horse no longer has adequate levels of thiamine, causing thiamine deficiency...therefore the symptoms of this fern's poisoning are similar to thiamine-deficiency:
This is one of the few poisonings that has an easy cure in horses. In cases of bracken poisoning, your veterinarian will administer thiamine intravenously, usually at the rate of 5 mg/kg body weight. The injection will usually then be given in the muscle for the few days following. In severe cases, nursing care may be needed, as well as systemic antibiotics.