Mycotoxins in the Equine Environment

Most horses spend a significant portion of their days during the growing season grazing on pastures. Certain grasses are more prone to contamination with mycotoxins, including rye-grass, fescues and white clover. Ideal conditions for the endophytes to proliferate on these grasses are periods of hot, dry weather followed by rains. Toxicosis from these endophytes may present as “ryegrass staggers,” in which horses lose coordination and balance; the condition also causes abortion, headshaking, and collapse. Other conditions include “fescue poisoning,” which presents as loss of appetite, abortion and foaling problems; and “slobbers” (excessive salivation, tears, uncontrolled diarrhea and urination) induced by exposure to slaframine from contaminated white clover.

These conditions can also result from exposure to preserved forages prepared from contaminated grasses. It has been reported that approximately 15 percent of Canadian hay is contaminated with mycotoxins. Preservation of high-moisture hay (haylage) may be an excellent strategy to reduce mycotoxin exposure, as commercial inoculants used to facilitate the ensiling process in well-made haylage create conditions that are unfavorable for mold growth and can reduce mycotoxin exposure by about 25 percent. 

Grains and Commercial Feeds

Cereal grains, especially maize (corn), can be a ready source of mycotoxin contamination. Grains can become infected with mold either in the field or during storage. Common mycotoxins found on cereal grains include aflatoxin, fumonisins, tricothecenes and zearalenone. 

Aflatoxin toxicosis can result in death, growth suppression, cancer, liver damage, and inhibition of mineral absorption including iron, phosphorus and copper. Ponies fed diets high in aflatoxin (2 ppm) show significant liver damage, whereas much lower concentrations (0.3 ppm) have reportedly caused death in horses. 

Poisoning with fumonisins results in a neurological disease called equine leukoencephalomalacia. This disease presents as loss of coordination, depression, muscle tremors, and loss of swallowing reflex. 

Tricothecene toxicosis causes clinical signs including anorexia, weight loss, immune suppression, poor performance and colic.

Zearalenone is a common culprit in horses experiencing reproductive problems.  

Bedding

Bedding can be an important source of mycotoxin poisoning in horses, and straw is a particular risk because horses are often quite willing to consume it when not offered an alternate forage source throughout the day. A common practice of sprinkling water on straw bedding to reduce dust can accelerate mold growth and encourage subsequent mycotoxin formation.