Do you know what's in your pasture other than grass? Foreign plants can colonize a pasture from remote sources of seed, sometimes conquering large flats in a single year. Fields that are poorly managed, previously flooded, overgrazed, or surrounded by overgrown fields may be prone to weed infestation. Horses grazing next to woodlands, roads, ornamental gardens, lawns, or orchards are at risk of eating a toxic plant. Pasture is the main source of toxic plants, but hay can be another. In fact, hay is suspected as a toxic source in cases where there is no pasture.
Fortunately, most horses won't eat toxic plants because they are distasteful. Furthermore, it is rare that a mouthful of a toxic plant will be dangerous. Exceptions include yew or water hemlock, which are quite deadly. Horses rarely have access to these plants.
Horses resort to eating toxic plants in times of drought when pastures are sparse. They also eat strange plants when denied forage, roughage, salt and minerals. Bored horses may also eat strange plants.
Nothing could be more important than to keep your pregnant mare away from toxic plants. The fetus is most vulnerable to exposure to toxins during the first trimester of pregnancy, when major organ development takes place. The classic example is consumption of skunk cabbage/hellebore during the first equine trimester, producing a cyclops foal. In short, keep your mares away from ANY unusual pasture or plants.
Treatment is rarely specific for individual plants. In general, your veterinarian will want to treat all suspected plant intoxications with mineral oil and/or activated charcoal to absorb the toxins, and intravenous fluids to speed the excretion of the toxic principle through the kidneys. Plant toxicities are discussed by the symptoms they produce.
Colic and Diarrhea
There are many plants that fall into this category. One common culprit is oak, which contains tannins, the toxic principle. Oak ingestion causes hard, dark feces and colic, later turning to bloody diarrhea, oral ulcers and signs of choke. Horses that wander into shrubs or woodlands, restricted from good quality pasture or hay, may ingest oak. Horses with bad teeth, given only complete pelleted feed, and young curious horses may taste leaves. Horses out on the trail may experiment with leaf ingestion if tied near a source. The summertime, when leaves are plentiful, is the most likely time to see oak toxicity.
Other plants that cause colic and diarrhea include:
Several diseases cause slobbering, frothing, and drooling, but when caused by toxic plants, the salivation arises from the mouth (indicating a problem with swallowing). Common plants that cause oral injury and slobbering include:
Liver Disease, Jaundice, Photodermatitis (Sun-Sensitivity)
A toxic exposure may be long gone before liver disease appears, but if a plant was responsible, it may still be in the pasture and is worth identifying. More than one horse affected with liver disease should definitely trigger a suspicion of plant toxicity. The plant toxin leaves a mark for the pathologist that distinguishes plant-induced from other causes of liver disease. This is good reason for a necropsy (autopsy) in horses with liver disease.
Some plants contain "photodynamic" substances that accumulate in the skin. At this location, they are easily excited by the sun, and release "radiant energy" (heat) that simply burns the skin. White hair affords little protection to sunlight, so burns occur in white regions only. St. John's wort and buckwheat contain these toxins.
Liver disease (jaundice signs) with secondary photosensitization
Other plants cause liver disease directly, and the accumulation of photodynamic chemicals as a secondary effect of liver failure. Examples of plants that contain toxins include:
Typical cases of plant poisonings include blindness, inability to grab and chew food, ataxia, convulsions and depression. Horses that exhibit asymmetric signs or spinal cord disease (as in EPM or West Nile virus infections) are not likely to have plant poisonings.
The most bizarre behavioral changes are caused by long-term ingestion of locoweeds. These horses are hyperexcitable, with a high stepping spastic gait, head bobbing, coupled with severe weakness.