Bracken is a type of fern that is found throughout the world, notably in areas with a more temperate climate. Horses will typically avoid eating bracken, but if the normal grazing grounds are lacking in edible plants, they will eat bracken fronds, becoming ill as a result. Fortunately, bracken fern toxicity is fairly rare in horses, since they need to eat very large quantities of it to become adversely affected.  The entire plant is considered toxic.

Symptoms

Most clinical signs of bracken fern toxicity are neurological signs and become progressively worse as the horse continues to consume the plant.  

• Nervousness
• Circling
• Poor coordination, staggering (“bracken staggers”)
• Muscle spasms
• Muscle tremors
• Recumbency (lying down)
• Blindness
• Convulsions
• Death 

Causes

A toxic reaction to bracken typically will occur after a sizable portion of the bracken plant has been ingested, or when a horse has been eating the plant over a period of time, typically 1-2 months. Thiamase, an enzyme found in the bracken fern, is known to be destructive to thiamine, or vitamin B1, which is an essential component of the metabolic processes in mammals. Deficiency of this key vitamin will lead to neurological problems like the symptoms listed above.

Toxicity of the bracken fern builds in the system cumulatively over the course of 1-3 months time, remaining in the body for some time after, even if the horse is removed from the source of the bracken fronds. Effective and prompt treatment is necessary, before the physical symptoms become severe.  

Diagnosis

There is no specific laboratory test for bracken fern toxicity. Clinical signs may lead your veterinarian to suspect bracken fern toxicity, especially if you live in an area rich in the plant, or know your horse has been eating it.

Treatment

Treatment for bracken fern poisoning is relatively straightforward once it has been diagnosed. Your veterinarian will put your horse on a regimen of thiamine supplementation over the course of several days until your horse shows signs of improvement.

Living and Management

One of the things in your favor is that horses generally do not choose to eat bracken fronds; they will only eat them when they have no other choice. In rare cases the horse may develop a taste for the plant (this affliction is similar to acorn poisoning).

Prevention

While it is all but impossible to completely eradicate this plant from your horse's environment, knowing how to recognize the leaves, keeping the growth to a minimum, and making sure that your horse has no reason to resort to eating them will go a long way toward avoiding a potentially deadly poisoning.