The Danger of Mycotoxins


Did you know there are toxic substances tucked away in your horse's feed and forage? You can't see them, and you can't eradicate them, but chances are these compounds, called mycotoxins, are present. 

Specific molds and fungi produce mycotoxins in soils, grains, and forages when environmental conditions are favorable. Once produced, they are generally very stable and will persist for a long time during storage. Horses that consume grains and forages contaminated by mycotoxins can suffer from a variety of health issues. As horse owners we should be particularly concerned, says Max Hawkins, PhD, nutritionist with Alltech's mycotoxin management team, because mycotoxins inhibit protein synthesis, which negatively impacts the animal's physiology and ability to function and repair tissues. 

The molds that produce mycotoxins are visible on contaminated feed. However, the mycotoxins can still be present after the mold dies or falls off the feed. These molds can be classified as either field fungi, which grow on plants while they are still rooted in the ground, or storage fungi, which develop after plants are harvested and stored. Field fungi require high moisture conditions (20-21% moisture), while storage molds can grow at lower moisture levels (13-18%). Forage and feed producers and property managers should monitor both moisture and temperature levels carefully so they can be aware of the potential for mycotoxin-producing molds to form. Horse owners should learn to recognize the signs of mycotoxin exposure so they can have their veterinarian out to assess the horse, diagnose the problem, and start a recovery plan. 

How Harmful are They?

It is near-impossible to find grain, pasture, and hay that is completely mold- and mycotoxin-free. In a 2010 study, German researchers Liesener et al. found at least one type of mycotoxin in each of 62 samples of commercial horse feeds, with many samples having more than one. However, the levels present were well below dangerous. So instead of taking on the impossible task of eliminating mycotoxins, owners should focus their efforts on minimizing their horses' exposure to these elements. 

Fortunately, harmful levels of some mycotoxins are rarely seen. In addition, most horses will avoid moldy (and potentially mycotoxin-containing) forages because they are not palatable. The molds and mycotoxins found in grains, however, often do not affect palatability, so nutritionists and veterinarians find these the most concerning. It is also important to realize, says Hawkins, that a feed containing low levels of several different mycotoxins might be just as detrimental to a horse's health and performance as a single mycotoxin at a 'toxic' level. 

Some problems stemming from mycotoxin consumption are acute (immediate), while others are chronic (long-term). It all depends on the type and amount of mycotoxin the horse ingests. Other factors that affect toxin response include age, workload, stress level, nutritional status, and immune status. A healthy adult show horse or broodmare that consumes a diet with low to moderate mycotoxin levels over a long time might exhibit clinical signs such as decreased athletic performance and/or breeding capability with no other observable issues. Other general signs include appetite loss, weight loss, unthriftiness, respiratory issues, increased susceptibility to infectious diseases (poor immune function), and poor growth rate. 




Allowable Limit


Aspergillus flavus

20 ppb (parts per billion)

Deoxynivalenol and Nivalenol (DON or Vomitoxins)

Fusarium graminearum, F. colmorum

2 ppm (parts per million)

Ergot Alkaloids

Neotyphodium spp

.3-.5 ppm


Fusarium moniliforme (verticillioides)

1 ppm

Lolitrem B

Neotyphodium lolii

2 ppm

Ochratoxin A

Penicillium verrucosum

1 ppm (limit in swine)

Ochratoxin A

Apergillus ochraceus

1 ppm (limit in swine)


Rhizoctonia leguminicola

10 ppm

T2 and HTZ

Fusarium langsethiae, F. poae, F. sporotrichioides

.5 ppm


Fusarium spp

.5 ppm

Types and Signs of Mycotoxins

Aflatoxins, produced by Aspergillus strains, thrive in hot and dry climates. Producers and property owners should monitor and test soybeans, oilseeds, and corn and other cereal grains grown in hot environments, especially if the plants are stressed during the growing season (e.g., due to drought). These crops can become contaminated both in the field and during storage. 

The target organ of aflatoxin is the liver, says Hawkins. 'This toxin can cause ataxia (incoordination), tremors, elevated temperature, anorexia, weight loss, icterus (yellowing of eyes and skin), hemorrhages and bloody feces, and brown urine. More recently aflatoxin has been associated with recurrent airway obstruction (RAO, or heaves) when inhaled, which would be more of an issue with older and stabled horses rather than horses on pasture.' 

Ochratoxins are produced by Aspergillus and Penicillium species, mainly due to improper storage. They have been found in oats, barley, corn, and wheat grown in both temperate and tropical regions. This class of mycotoxin targets the kidneys and can cause kidney failure. Clinical signs, says Hawkins, include anorexia, poor growth rate, poor performance, immune suppression, and excessive urine output and increased water consumption due to kidney problems.

Trichothecenes, which include deoxynivalenol (DON, or vomitoxin), are field mycotoxins produced by a variety of molds, with Fusarium species being the most common. Corn and wheat products, including wheat straw, can contain DON. It appears to be produced after temperatures fluctuate between hot and cold. Horses usually refuse to consume contaminated feeds and begin to lose body condition. Other signs include immune system suppression, decreased growth, poor performance, gut irritation and diarrhea, changes in blood chemistry that indicate liver damage, and colic. Hawkins calls DON a 'flag' mycotoxin, meaning it usually indicates the presence of other mycotoxins.

Zearalenone is often found in plants