Which plant is known for thiaminase activity causing neurological signs in horses?

• A. azalea (rhododendron)
• B. perilla (perilla)
• C. bracken fern (pteridium)
• D. lily of the valley (convallaria)

Answer: (C)

Bracken fern stands out because it contains thiaminase, an enzyme that destroys thiamine (vitamin B1). Thiamine is essential for brain energy metabolism; when it's depleted, neurons can’t get the glucose they need, leading to neurological dysfunction. In horses that ingest bracken for a period, this manifests as instability and incoordination described as staggers, with signs like ataxia, tremors, hind‑limb weakness, and altered micturition in some cases.

Other plants listed have different toxic constituents and produce signs not tied to thiamine deficiency, so they don’t cause the same thiaminase‑driven neurologic picture in horses. If bracken exposure is suspected, stopping intake and providing thiamine supplementation can help mitigate the deficiency and improve outcome with supportive care.