Toxic Weed: Bracken Fern

Bracken fern (Pteridum aquilinum) or brake fern, eagle fern.  A perennial fern with triangular leaves that can reach two to three feet high. Grows in woodlands and moist open areas.  It is found coast to coast, except for desert climates of the Southwest. So, be careful on the trails.

The danger: Bracken fern contains thiaminase, which inhibits absorption of thiamin, which is vitamin B1. Thiamin is necessary to nerve function, and deficiencies can lead to neurological impairment. The relative toxicity of individual leaves is low–horses must consume hundreds of pounds to experience ill effects. However, bracken fern is unique among the toxic plants in that some horses seem to develop a taste for it and will seek it out even when other forages are available.  Signs: Signs are neural dysfunctions resulting from vitamin B1 deficiency and can include depression, incoordination and blindness.  Treatment: 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 are severe.