Common Name
Horsetails, scouring rush
Botanic Name
Equisetum arvense
Plant Family
Equisetaceae (horsetail family)
Weeds of moist, sandy soils in fields, along roadsides and banks of rivers, dams, etc. Six species occur throughout North America , generally preferring cooler, moist climates.
Worldwide, and throughout North America especially. There are at least 15 species of Equisetum.
Animals Affected
Horses, cattle, sheep
Toxic Principle
Thiaminase is the suspected toxin. Also contains aconitic acid and polustrine, and silicates. Plant is rarely eaten except when dried in hay. All species of Equisetum should be considered potentailly toxic to animals until proven otherwise.
Herbaceous, perennial, leafless plants with hollow stems that readily separate at the nodes. The leaves are reduced to papery scales with black tips that surround the stems at each node. The stems are cylindrical, ridged and rough to the touch owing to the high silicate content. There are 2 types of stem. Fertile stems are unbranched, and are tipped by a cone-like structure containing spores. Infertile stems have multiple whorled branches at the nodes. The plant reproduces from a deeply buried rhizome and from terminal spore bearing cones.
Weight loss, and diarrhea.
Horses suspected of horsetail poisoning should be immediately taken off the hay or pasture containing the Equisetum and fed a nutritious diet. Treatment with thiamine hydrochloride (1-2 mg/kg subcutaneously for several days) is beneficial in restoring thiamine levels to normal. Feeding grain as part of the diet affords a protective effect against the thiaminase.
Nervous System
Weakness and incoordination the of hind legs progressing to recumbencing. Animals continue to eat relatively well. Once horses are down and cannot get up, the prognosis is poor because of muscle degeneration. Horses may appear blind and have difficulty in seeing. Central nervous system depression develops in the more severely poisoned animal.
Blood pyruvate levels may be elevated. Response to thiamine therapy.
Special Notes
Most poisoning occurs when horsetail is incorporated in hay fed to livestock. Since the plant can remain green in the winter months it may be more readily eaten by horses and livestock. The term 'scouring rush' is apparently due to the fact that the coarse stems were used to effectively scour dirty cooking pots!!
Infertile stems of Equisetum arvense.