There are a number of plant species that are toxic or poisonous to horses.
Plant toxicity in horses is generally thought to be uncommon because these plants are usually unpalatable, and horses avoid eating an amount sufficient to cause a problem. Only on rare occasion does a horse consume a sufficient quantity of a toxic plant to cause obvious illness or death.
But low grade poisoning may be more common than we realize, especially in horses kept on small acreages and dry lots.
There are literally thousands of species of toxic plants. Examples are ivy (which can cause skin and gastrointestinal irritation), horse chesnut (which can cause a horse to have a peculiar gait), Oleander (cardiac abnormalities, colic, sweating, difficulty breathing, muscle tremors, sudden death), Wild Mustard (which can cause salivation and gastrointestinal irritation, colic and diarrhea), Monkshood (which can cause gastrointestinal and cardiac distress, salivation, weakness and sudden death), various nightshade varieties (which can cause salivation, lacrimation, diarrhea, trembling, weakness, paralysis and cardiac distress).
Purslane and many other plants contains oxalates that can cause kidney failure. Hounds Tongue, Common Groundsel (Senecio), Tansy Ragwort, Fiddleneck and Rattle Box contain pyrrolizidine alkaloids, which cause liver disease.
These are only a few of the wide variety of plants that can cause toxicity and sometimes death in horses. For more information about a specific plant or weed variety, type it in to the search bar or visit the linked Outside Resources – which contain extensive lists, photos and information on individual plant species.
In most cases, evidence for ingestion of the plant must be found. Plant material might be found in the stomach at postmortem. Evidence of consumption in the pasture might be found.
In some cases, a diagnosis can be presumed based on tell-tale signs. For example, Yellow Star Thistle toxicity a/k/a Chewing Disease, causes a classic abnormal chewing movement of the mouth.
Depends on the particular plant species ingested. If ingestion is thought to be very recent, there may be reason to try to remove stomach contents. This is rarely of much value as the stomach empties rapidly.
For particular poisonings, hospitalization with intensive care and IV fluids may be needed. In rare cases, there is an antidote for particular poisonings.
Other Diagnoses Considered
Prognosis & Relevant Factors
The prognosis ranges widely depending on the type of plant and amount ingested. Sometimes simply removing access to the plant and close monitoring of the horse is all that is needed. Sometimes, the damage is irreversible once signs of illness appear. Sometimes there is no opportunity to save a horse; you may simply find them dead.
I Might ObserveRelated Observations
QUESTIONS TO ASK MY VET
Helpful Outside ResourcesCredible Equine Health Information on the Internet