Mycotoxins are toxic metabolites produced by molds (fungi). Ingesting these toxins can cause a serious brain disease and can lead to death. Horses are particularly sensitive to mycotoxins, and clinical signs can range from digestive upset and hormone disruptions to neurological conditions to death.

Mycotoxins might be found on cereal grains commonly used as ingredients in horse feed such as corn, oats, or barley.

In this article adapted from the Equine Disease Quarterly, we learn more about mycotoxins and horses.

Finding Mycotoxins

Aflatoxin is a mycotoxin produced mainly by the molds Aspergillus flavus and Aspergillus parasiticus. These are generally considered to be tropical or semi-tropical fungi thriving under conditions of high temperature and humidity. Four chemical forms of aflatoxin can be measured simultaneously by many commercial laboratories.

Horses, like many other species, are very sensitive to feed-borne aflatoxins. Aflatoxins mainly target the liver, causing necrosis and potentially cancer.

More common on a global basis, however, are several hundred identified Fusarium mycotoxins. The study of Fusarium mycotoxicoses is considerably more complex than the study of aflatoxicosis.

(Editor’s note: When horses are exposed to fumonisins, it can result in equine leukoencephalomalacia or ELEM. That issue is characterized by depression, ataxia, abnormal behavior, head pressing, and often death.)

Fusarium fungi thrive in soils from areas with temperate climates. These areas include North America, much of Europe and Asia, and South America.

The most common Fusarium mycotoxins include DON (also known by the chemical name deoxynivalenol or the common name vomitoxin), zearalenone, fumonisin, and fusaric acid.

Moldy Corn Poisoning

Horses are particularly sensitive to feed-borne fumonisin, which can cause mortalities due to equine leukoencephalomalacia. This brain disease is also more commonly known as “moldy corn poisoning.”

The most significant current controversy regarding mycotoxins in feeds is the concept of conjugated or “masked” mycotoxins. These are mycotoxins that are produced by molds on feedstuffs pre-harvest, then are chemically modified by the plants invaded by the molds. These modified mycotoxins are thought to be toxic, but are non-detectable by analytical techniques currently used by most commercial laboratories.

Utilizing research methods, scientists are actively studying these compounds in order to understand their importance.

Increasing Problems

The frequency of mycotoxin contamination of feedstuffs might be increasing, in part, to unfavorable growing conditions caused by extreme weather.

(Editor’s note: In 2023 in Missouri, for example, a corn survey found high fumonisin levels in 122 corn samples collected from 86 corn-producing counties.)

Final Words

Improved analytical techniques for mycotoxin detection are needed to minimize exposure of horses to contaminated feeds and forages in the future. Feed manufacturers routinely test grains for the presence of mycotoxins before using them to help ensure safe feed.

Further Reading

The original article was written for the University of Kentucky Gluck Equine Center’s Equine Disease Quarterly by Dr. Trevor K. Smith of the Department of Animal and Poultry Science, University of Guelph, Ontario, Canada. The Equine Disease Quarterly is funded by underwriters at Lloyd’s London brokers and their Kentucky agents.