Toxicity of Equisetum to Horses

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Table of Contents

1. Introduction
2. Clinical Signs
3. Treatment
4. Identifying Equisetum
5. Control
6. References

Introduction

Present and persistent since the Paleozoic era (250 to 540 million years ago), the plants of the genus Equisetum, commonly known as horsetail, are considered to be living fossils. These widespread, perennial, fern-like plants are found in most temperate areas of the world. They can have detrimental effects on horses if consumed in large quantities (e.g., 4 to 5 lbs per day for a 1000-lb horse, for 1-2 weeks).

The presence of Equisetum in pasture is not a primary concern. However, the ingestion of contaminated hay can result in poisoning. Consumption at pasture is usually limited by the plant's high silicate content and the abundance of other palatable forage options. If cut, dried and mixed in with hay in moderate levels (20% Equisetum, or more, of the horse's dry-matter intake (1)), intoxication will more than likely occur from anywhere between 1 to 4 weeks. There have been many investigations into the probable cause of toxicity. Equisetum contains compounds such as silics, aconitic acid, plamitic acid, nicotine, 3-methyloxypyridine, equisitine, palustrine, dimethylsulfone and thiaminase (1). Thiaminase is the primary source of symptoms in horses (2). It is an anti-thiamine enzyme which inhibits the production of vitamin B1. Vitamin B1 is responsible for extracting energy from carbohydrates, fats and proteins (3). However, there has been no obvious relation to low thiamine and the nervous signs observed (3).

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Clinical Signs

Symptoms of Equisetum poisoning are seen primarily in young, rapidly-growing horses but cases of poisoning have also been reported in cows and sheep. The development of symptoms of Equisetum poisoning initiates slowly. The first signs may be a general scruffy physical appearance, weight loss (without a particular loss of appetite), diarrhea and slight uncoordinated movements. If not treated, the disease will progress to a point where the horse will show a loss of muscular control, staggering gaits and extreme balance issues. The horse is prone to become uneasy and nervous due to the inability to control muscle movement, it may lie down and not be able to get up and seizure, but ultimately will die from sheer exhaustion within approximately 1-2 weeks (2, 4).

Treatment

Early identification of the symptoms is critical in the treatment of Equisetum poisoning. If caught early, the source of Equisetum can be removed and a full recovery can be expected. Initially, laxatives may be administered; then, a treatment of thiamine (vitamin B1), 5 mg/kg body weight, intravenously for 3 hours, followed by intramuscular injections for several days to replenish vitamin B1 levels (3). The best prevention is ensuring Equisetum is not present in hay. By educating yourself on what normal, good-quality hay looks like, identifying foreign and potentially dangerous materials will be easier.

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Identifying Equisetum

Equisetum species are often found on sandy and gravely wet soils, such as in marshes, wet meadows, and the banks of streams, lakes and ponds. Equisetum arvense (Figures 1 and 2), also known as field horsetail, is the most common and variable species of horsetail. Typically, E. arvense can be identified early in the season by the presence of its fertile stem that is 6 inches tall (more or less), brown/beige in colour and has nodes that are encompassed by long, dark, clasping, leaf sheaths (5). Atop of the stem is a small, scaled cone, which holds hundreds of thousands of miniscule spores (Figure 2). The green, hollow, sterile stem, as seen in Figure 1, is usually spotted in late spring and can reach up to 18 inches tall with numerous, thin, needle-like branches whorled around each node (5). The plants have modified stems called rhizomes that grow laterally underground and produce numerous above-ground stems.

Control

The plant's persistent, rhizomatous, rooting system, and its ability to produce mass amounts of spores from the fertile stem, make eradication of Equisetum difficult. Draining the excess water from marshy areas will help to decrease populations. Cutting the identified fertile stems in the early spring before spore distribution is an optimal way of decreasing future populations. Increasing the fertility of the soil and improving drainage will encourage cultivated grasses and legumes to grow and out-compete Equisetum. In addition, increasing grass and legume concentration will lower the Equisetum portion of the dry-matter intake. Herbicides such as 2, 4-D and MCPA have been found to have control of the vegetative growth of Equisetum species when applied to the vegetative portions (6). When applying any type of herbicide, it is important to read dilution and application instructions before use.

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References

1. Kingsbury JM. Poisonous Plants of the United States and Canada. Englewood Cliffs, New Jersey: Prentice-Hall, Inc., 1964:114-118.
2. Knight AP, Walter RG. A Guide to Plant Poisoning of Animals in North America. Jackson, Wyoming: Teton NewMedia, 2001:224-225.
3. Radostits OM, Gay CC, Blood DC, Hinchcliff KW. Veterinary Medicine, 9th ed. Edinburgh: WB Saunders Company Ltd., 2000:1556-1558.
4. Burrows GE, Tyrl RJ. Toxic Plants of North America. Ames, Iowa: Iowa State Press, 2001:434-438.
5. Cobb B. A Field Guide to the Ferns. Boston: Houghton Mifflin Co., 1963:194-213.
6. Ministry of Agriculture, Food and Rural Affairs. Guide to Weed Control 2006-2007, Publication 75.
   
   
   

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