Toxicity is the capacity of a substance to cause injury, illness, or death in a living organism. In equine medicine the term is used both as a property of a substance (e.g., “the toxicity of red maple leaves”) and as a clinical description of a horse’s condition when a toxic dose has been absorbed. Dose is the critical variable: virtually any substance is harmful at a sufficiently high concentration, and many substances with known toxic potential — such as certain therapeutic dose versus toxic threshold-based medications — are therapeutically beneficial at controlled doses.
Plant toxicity represents the most common source of equine poisoning. Horses on unmanaged unmanaged grazing exposure may encounter red maple, black walnut shavings, yew, bracken fern, Sorghum spp., or various alkaloid-bearing plants. Toxicity thresholds vary: a horse consuming a handful of yew leaves can die within hours from cardiac arrest, while chronic low-level ingestion of pyrrolizidine alkaloids from plants such as ragwort causes cumulative hepatic toxicity that presents clinically only after significant liver damage has accumulated.
Mycotoxins — toxic metabolites of molds growing on hay, grain, or silage — represent a subtler but pervasive risk. Fumonisin, deoxynivalenol, and aflatoxin are among the mycotoxins documented to cause neurological, reproductive, or hepatic disease in horses consuming contaminated feed. Feed testing is the only reliable way to assess mycotoxin risk before symptoms appear.
Chemical toxicity from pesticides, herbicides, heavy metals, or improperly stored medications is less common but clinically significant. Any horse with unexplained acute neurological signs, hepatic disease, or sudden death should be evaluated for toxic exposure; consultation with the body condition history and a thorough review of diet and environment are essential first steps.