Horsetails (Equisetum hyemale
) are non-woody, erect, long-lived, fern-like perennials composed of more than 30 species in the genus. Equisetum hyemale is native to the northern hemisphere including Europe, North America and Asia. It is a highly aggressive plant, and considered to be a trouble-some weed in most regions. Once established, E. hyemale can be very difficult to remove due to its extensive spreading rhizome system that produces numerous shoots and tubers. E. hyemale can grow in a variety of different habitats, however it thrives in wet, poorly drained areas of fields and grasslands, wet meadows, adjacent water bodies (streams, creeks, rivers, etc.) and other sites with high water tables as well as along roadsides, railroad tracks and beaches. It can even grow in up to 4 inches of standing water. It is most frequently found growing in acidic and wet soils. E. hyemale does not produce any flowers or seeds as it reproduces by spores, horizontal rhizomes and tubers.
Equisetum spp contains multiple chemical compounds, such as silicates, aconitic acid, palmitic acid, nicotine, 3-methyloxypyridine, equisitine, palustrine, dimethylsulfone and thiaminase. However, thiaminase is the chemical of primary concern for horses. Thiaminase is an enzyme that destroys thiamine, an important B vitamin that is needed for proper brain function. Although Equisetum spp are found in pastures, which is a concern, the bigger concern is horsetails in hay, as it remains toxic even when dried.
Symptoms of poisoning in horses' develop slowly, and begin with unthrifty appearance, weight loss without loss in appetite, diarrhea, and slight incoordination. Without treatment, signs will progress to more extreme--leading to balance problems and loss of control of muscle movement, eventually leading to death in approximately 1-2 weeks.