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
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
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:
Pyrrolizidine alkaloid-containing plants: (Senecio
jacobaea- ragwort, stinking willie, tansy ragwort (Photo 1); S. integerrimus- lamb's tongue groundsel; S. longilobus- wolly groundsel, treadleaf groundsel; S. ridellii – riddell's groundsel; S. spartioides – broom groundsel; S. vulgaris – common groundsel; Crotalaria sagittalis – arrow crotalaria, rattle box, wild pea; C. spectabilis – showy crotalaria; C. retusa – rattlebox; Amsinckia intermedia – tarweed, fingerweed, fiddleneck, fireweed; Echium vulgare – blue thistle, blue devil, viper's bugloss; E. plantagineum – Patterson's curse, Salvation Jane; Heliotropium europaeum – Heliotrope; Cynoglossum officinale – houndstongue (Figure 2).
Photo 1: Senecio jacobaea (ragwort, stinking willie, tansy ragwort) is a noxious weed that is a native of the British Isles.
It was inadvertently introduced into Western Europe, South Africa, Australia, New Zealand and North America. It is common
in the northwestern states. Tansy ragwort is a tall (0.5m-1.5m), erect plant that is unbranched except at the inflorescence.
In the spring it forms a tall, flowering stalk with composite heads that have terminal, flat-topped clusters about 1 cm tall
and individual 1 cm long yellow rays. Each plant produces thousands of wind-dispersed seeds. Leaves are variably sized up
to 22 cm long and 10 cm wide, with two or three deep pinnate divisions. It commonly invades well-drained pastures, forests
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.