Horsetail, Marestail, Horserush, or Snake Grass
Horsetail, marestail, horserush, or snake grass are the common names given to members of the genus Equisetum, of which E. arvense is the most common. Currently, horses are rarely poisoned by horsetail, but reports 30 to 40 years ago indicate that the plants were a problem if contained in hay fed to horses. Hay containing 20% horsetail fed to horses for over 2 weeks caused neurologic signs. Successful treatment of the affected horses with thiamin hydrochloride (vitamin B1) indicated that the primary toxin in horsetail is an enzyme (thiaminase) that destroys this vitamin. Although a variety of other substances have been identified-including silica, various alkaloids, and organic acids-the primary toxic effects appear to be related to horsetail's antithiamin properties.
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Horsetails are perennial rush-like plants with characteristic jointed hollow stems arising from an extensive underground root system. The plant appears leafless but has small scale-like leaves with black tips that surround the stem at each node. The plants reproduce by spores produced in a cone-like structure at the ends of the stems. Horsetails also produce sterile much-branched stems. Found throughout North America, the plants prefer wet soils and tend to form dense stands.
Affected horses show signs of weakness, depression, and incoordination especially of the hindquarters. Other signs that have been associated with horsetail poisoning include diarrhea, constipation, muscle tremors, and corneal opacity. A diagnosis of horsetail poisoning is generally based upon the presence of significant quantities of the plant (20% or more) in the hay being eaten, and the animal's response to the administration of large doses of thiamin (vitamin B1).
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