VPT 421: Veterinary Toxicology (2+0)

THIAMINASE CONTAINING PLANTS

• Source: Bracken fern (Pteridium aquilinum) and horsetail (Equisetum arvense)

Toxic principles

• Several toxic principles are recognized including
  • Thiaminase.
  • A factor that damages the bone marrow.
  • Ptaquiloside is the suspected carcinogen in brackenfern.
  • Cyanogenic glycoside (significance not clearly established).

Mechanism

• Toxicity in horses (and rats) is due to a thiaminase enzyme which destroys thiamine in the digestive tract before it can be absorbed), resulting in central nervous system dysfunction and damage.
• Thiamine deficiency results in impaired pyruvate utilization. Blood pyruvate levels rise. Animal suffers from abnormal energy metabolism due to inadequate ATP production.
• Toxicity in cattle is due to an unidentified toxic principle and not thiaminase. At least part of the problem is due to a carcinogen.

Species affected

• Cattle most commonly affected. Reports of poisoning in horses, swine, and sheep are less frequent.
  • Horses - Mainly neurologic.
  • Cattle - Mainly bone marrow damage.