Knowledge is key to safety; Plants that poison horses
Because antidotes are rare, plant-induced diseases in horses can be irreversible and sometimes lethal.
Knowledge of which plants are toxic and when horses are likely to be poisoned will help reduce losses and ensure animal health and safety.
This series will introduce some common plants that poison horses in North America, describe poisoning and the subsequent plant-induced disease and outline current recommendations for treatment and management practices to avoid exposure.Horses are relatively selective grazers and generally are poisoned less frequently than other livestock, but there are exceptions.
Some poisonous plants are palatable to horses and exposed horses readily eat them. Others may be eaten by some animals even though they are unpalatable to the rest of the herd.
Because some horses may actively seek and eat toxic plants, it has been suggested they become addicted to certain toxic plants. There is little experimental support for addiction, but individual horses do develop strong feed preferences. Such animals certainly are at greater risk of poisoning.
As a rule, both preferences and palatability are different when poisonous plants are dried and included in stored feed. Most horses readily accept toxic plants included in hay or processed feed. In herd situations, competition for food can enhance poisoning, as animals hurry to eat what they can or fight to keep lesser-status animals from eating.
Besides processing dietary carbohydrates, lipids, amino acids, vitamins and micronutrients, the liver also is the primary organ that filters and detoxifies ingested toxins. It has limited responses to injury and many toxic, immunologic and infectious diseases result in similar hepatic lesions. This is especially true of chronic disease that nearly always results in cirrhosis (fibrosis, necrosis with nodules of regenerative nodular hyperplasia). This makes obtaining a definitive diagnosis difficult or impossible.
Hepatotoxic plants are a special challenge, because lower doses generally result in delayed onset. Often months or years are required for animals to develop clinical signs of liver failure, making documentation difficult. The following hepatotoxic plants are the ones that most commonly poison horses in North America:
Most horses will not eat free-standing pyrrolizidine alkaloid (PA)-containing plants. Poisoning generally occurs when PA's contaminate hay or prepared feeds. PA poisoning in horses generally causes severe, irreversible liver disease.