Bracken fern is widely distributed in open woodlands with sandy soils and disturbed (clear cut or burned) areas throughout North and South America, Europe, Australia, and Asia. Ingestion of a large quantity of bracken fern results in signs of poisoning related to thiamine deficiency. The toxic effects appear to be cumulative and may require 1 to 3 months to develop, depending on the species of animal, quantity consumed, time of year, and other factors. Both leaves and roots may be toxic. Most severe poisonings are seen after periods of drought when food is scarce. However, the plant is toxic even when present as a contaminant in hay, and cases have occurred in stabled animals. Horses seem to be particularly susceptible.
In horses, signs of bracken-induced thiamine deficiency (bracken staggers) include loss of appetite, weight loss, lack of coordination, and a crouching stance with the back and neck arched and the feet placed wide apart. When the horse is forced to move, its muscles may tremble. In severe cases, the heartbeat is very rapid and irregular. Death (usually 2 to 10 days after onset) is preceded by convulsions, muscle spasms, and spasms of the back muscles that cause the head and lower limbs to bend backward and the trunk to arch forward. The rectal temperature is usually normal but may reach 104°F (40°C).
Plants other than bracken fern, such as horsetail and turnip, can also cause thiamine deficiency. In horses, the condition must be distinguished from other nervous system disorders, including rabies or poisoning from Crotalaria species or ragwort. Blood tests can confirm the diagnosis.
Treatment is highly effective if thiamine deficiency is diagnosed early. Injection of a thiamine solution followed by oral supplementation is suggested. Animals similarly exposed but not yet showing signs should also be treated with thiamine, because signs can develop days or weeks after the source of bracken has been removed.
Bracken is usually grazed when more suitable food is not available, although individual animals may develop a taste for the plant, particularly the young tender shoots and leaves. The problem most often shows up in early spring (tender bracken shoots) or late summer (poor pasture conditions). The disease has been prevented in horses by improving pasture management and fertilization or by alternating bracken-contaminated and non-contaminated pasture at 3-week intervals.
Bracken fern growth can be hindered by close grazing or trampling in alternate grazing pasture systems. In time, bracken can be eliminated from a pasture using this approach or by regular cutting of the mature plant or, if the land is suitable, by deep plowing. Herbicide treatment using asulam or glyphosate can be an effective method of control, especially if combined with cutting before treatment.