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., four to five pounds per day for a 1,000-pound horse, for one to two 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 drymatter intake¹), intoxication will more than likely occur from anywhere between one to four 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 thiaminase¹.

Thiaminase is the primary source of symptoms in horses². It is an anti-thiamine enzyme that inhibits the production of vitamin B1. Vitamin B1 is responsible for extracting energy from carbohydrates, fats and proteins³. However, there has been no obvious relation to low thiamine and the nervous signs observed³.

Clinical Signs

Clinical signs 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 might be a general scruffy physical appearance, weight loss (without a particular loss of appetite), diarrhea, and slightly 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 might lie down and not be able to get up, and seizure, but the horse ultimately will die from sheer exhaustion within approximately one to two weeks^{2, 4}