Equisetum arvense L. - Common horsetail
FAMILY: Equisetaceae—the Horsetail Family
This family of fern allies contains only one genus—Equisetum. Plants are primitive, sporebearing, annual or perennial, rhizomatous herbs having vascular tissue.
PHENOLOGY: The fruiting period is April to July.
DISTRIBUTION: Equisetum arvense thrives in a broad range of habitats from moist to wet, or in moderately dry sandy soil; it may grow in fields, woods, on stream banks, or along roadsides.
PLANT CHARACTERISTICS: Horsetail is not a true flowering plant with sepals and petals. The reproductive structure is a sporophyll in a terminally, spikelike cone composed of shield-shaped spore-bearing structures. Other characteristics include, stem: hollow, one large central canal surrounded by smaller ones under each ridge of the stem; jointed; impregnated with silica; striated or grooved; rushlike; branches: whorled; leaves: marginally united into a sheath around each node. Plants bear nongreen reproductive structures in spring and green vegetation during the remainder of the growing season.
POISONOUS PARTS: All parts, green and dried, can be toxic. Hay containing 20% or more of E. arvense causes poisoning symptoms in horses in 2-5 weeks.
SYMPTOMS: Toxicosis is similar to bracken poisoning (see Pteridium) Apetite remains normal until near the end of the illness in Equisetum poisoning, whereas it is lost early in bracken poisoning. Ataxia, difficulty in turning, and the body wasting away followed by general weakening are early signs. In later stages animals may become constipated and the muscles rigid. Pulse rate increases and weakens, and the extremities become cold. The cornea of the eye may become opaque. Before death, the animal becomes calm and comatose. If poisoning is discovered early, the toxic plants removed from the diet, and proper nutrition given, animals can recover rapidly.
Horses are not infrequently affected by E. arvense. In advanced cases when a horse '"goes down" and cannot arise, the animal becomes nervous, making frantic attempts to stand. When a poisoned horse is exercised it will tremble and become muscularly exhausted. Cattle are not readily affected by E. arvense. In experiments with cattle, the only result was marked loss in the general condition of the animal over a forty-day period. Postmortem: gross lesions: none evident; histological lesions: diffuse lesions in the cerebral cortex such as polioencephalomalacia or cerebrocortical necrosis; bilaterally symmetrical zones of malacia involving various nuclei of the brain.
POISONOUS PRINCIPLES: The enzyme thiaminase is responsible for poisoning in non-ruminants. The previously suspected silica, aconitic acid, palmitic acid, nicotine, methoxypridine, equisitine, palustrine, and dimenthyl sulfide components are not of themselves toxic enough to produce poisoning. However, they may complicate the toxicosis. The toxic agent for ruminants is unknown and generally not fatal.
CONFUSED TAXA: No unrelated plants occurring in Pennsylvania look like species of the distinctive genus Equisetum. All members of this common genus should be considered poisonous.
SPECIES OF ANIMALS AFFECTED: Horses primarily are affected, with sheep to a lesser degree. Equisetosis is rarely fatal for cattle.
TREATMENT: (11a)(b); (26); massive doses of thiamine.
OF INTEREST: The stems, which contain silica crystals, are sometimes used to polish metal; hence the name scouring rush. Plants are occasionally used as an ornamental in moist places such as around ponds.