Early treatment of large intravenous doses of thiamin by your veterinarian often can save a horse with bracken poisoning. Like bracken fern, horsetails and scouring rushes, sometimes found in hay, also contain thiaminase toxin. These plants grow up to three feet high and are found in damp places all across the United States. Unlike bracken fern, these plants don't cause appetite loss. In early spring, before lush grass comes in, horses often consume sprouting weeds, some of which are toxic.
Cockleburs are a prime example, causing liver disease and appetite loss.
Several kinds of trees also are poisonous to horses. Their leaves and bark contain amygdalin, a material that converts to cyanic or prursic acid as leaves are crushed or wilt, which causes cyanide poisoning when ingested. Death can be sudden, so immediate treatment by your veterinarian is critical. He will probably inject a combination of sodium thiosulfate and sodium nitrate to rescue the hemoglobin from the cyanide. Some horses who are found dead following storms and are presumed to have been killed by lightning are sometimes actually victims of cherry poisoning.