Mykotoxine im Pferdedarm – das stille Risiko im Heu (3/5)

Mycotoxins in the Horse's Gut – The Silent Risk in Hay (3/5)

April 20, 2026 Steinkraft GmbH

The hay looks good. It smells clean. Your horse eats it without hesitation. And yet something is wrong: diarrhea that won't go away. A coat that doesn't shine. Recurring digestive problems with no clear cause.
Mycotoxins are one of the most common and underestimated causes of gut problems in horses – and they are invisible to the naked eye.
In this part of the series, we take a close look: What are mycotoxins, how do they get into feed, what do they do in the horse's gut – and how can zeolite help mitigate this silent risk.

What are mycotoxins – and where do they come from?

Mycotoxins are toxins produced by certain molds. The name comes from Greek: mykes = fungus, toxikon = poison. They are formed as a byproduct of fungal metabolism, usually under specific conditions: moisture, warmth, poor ventilation, mechanical damage to the feed.
The insidious thing is that the fungus itself doesn't have to be active for the toxins to cause damage. Mycotoxins are heat-stable, acid-stable, and chemically very resistant. They survive the drying of hay, pressing into pellets, and even some heating processes. What is once contaminated remains contaminated.
And they are far more widespread than most horse owners realize. An Austrian study from 2023 examined pasture grass and conserved roughage for mycotoxins and found that contamination also occurs in inconspicuous, fresh growth – not just in visibly moldy feed.

The most important mycotoxins in horses

There are over 400 known mycotoxins. The following are particularly relevant for horses:

Mycotoxin	Producer Fungus	Main Source	Possible Consequences in Horses
Aflatoxin B1	Aspergillus	Concentrates, corn, peanuts	Liver damage, immunosuppression
Deoxynivalenol (DON)	Fusarium	Grains, hay in damp conditions	Feed refusal, intestinal irritation, vomiting (rare)
Zearalenone (ZEA)	Fusarium	Hay, grains, pasture	Hormonal disorders, cycle problems in mares
Fumonisin B1	Fusarium	Corn, concentrates	Liver damage, equine leukoencephalomalacia (ELEM)
T-2 / HT-2 Toxin	Fusarium	Hay, oats, damp storage	Mucosal damage, immunodeficiency
Ochratoxin A (OTA)	Aspergillus, Penicillium	Grains, silage	Kidney damage, immunosuppression

Multiple contamination is particularly insidious: studies show that commercial horse feed from the German market contains several mycotoxins simultaneously in almost all tested samples. Individual concentrations were usually below critical limits – but the combination of several toxins can amplify effects that would not have occurred individually.

What Mycotoxins Do to the Horse's Gut

When mycotoxin-containing feed enters the horse's digestive tract, a silent damage begins, rarely showing as acute poisoning, but rather as a creeping burden.

Damage to the intestinal lining
Mycotoxins, especially trichothecenes like T-2 and DON, directly attack the intestinal lining. They inhibit protein synthesis in mucosal cells, which reduces the regeneration capacity of the intestinal epithelium. The intestinal barrier becomes more permeable – a condition known as "leaky gut." Through this disturbed barrier, bacteria, toxins, and undigested food components can enter the bloodstream and trigger systemic inflammatory reactions.2. Destabilization of the microbiome
The gut microbiome is sensitive to mycotoxins. The toxic substances shift the balance of bacterial populations: beneficial, fibrolytic bacteria decline, while toxin-tolerant germs gain the upper hand. The result is dysbiosis – precisely what we described in Part 1 and Part 2 of this series as the starting point for many horse ailments.
Strain on liver and immune system
Mycotoxins that are not bound in the gut enter the bloodstream through the intestinal wall and are broken down in the liver – an energy-intensive process that places a constant strain on the liver. A 2022 study in the Journal of Veterinary Internal Medicine investigated outbreaks of liver disease in horse populations and found mycotoxin contamination in hay in a significant proportion of affected farms – even where the hay appeared normal externally.
Chronic low-level exposure – the real problem
Most horses do not suffer acute mycotoxin poisoning. What they suffer is chronic low-dose exposure: daily, with every handful of hay, for months. The consequences are not dramatically visible – but they accumulate. Dull coat, sluggish digestion, increased susceptibility to colic, an immune system that doesn't get up to speed. Symptoms we described in Part 2 as warning signs.

How do I recognize problematic hay?

• Visible mold: clear exclusion criterion – never feed such hay

• Dustiness: high spore load, even without visible fungus

• Musty, sour, or unusual smell

• Heat development in the bale: sign of active fermentation

• Dark discolorations or sticky spots in the hay bale

• Hay from damp harvest years or poorly ventilated storage

• Caution: Good appearance does not rule out mycotoxin contamination

How Zeolite Works Against Mycotoxins

This is where the unique property of clinoptilolite zeolite comes into play: its porous lattice structure, which acts like a molecular sieve.
Zeolite can adsorb certain mycotoxins in the digestive tract – this means they bind to the surface of the mineral and are thus transported through the intestine in a bound state, without being absorbed. The body does not come into contact with them.
It is important to understand the selectivity: clinoptilolite primarily binds smaller, polar mycotoxins such as aflatoxins and trichothecenes. Larger, more non-polar toxins like zearalenone are less effectively bound. Zeolite is not a universal adsorbent, but it is an effective building block in protecting the gut.
A review of zeolite and other mineral toxin binders (PMC 2020) confirms that clinoptilolite shows a demonstrable reduction in the toxic effects of aflatoxins and other mycotoxins in studies on livestock populations – both in vitro and in animal trials.

What Zeolite Can – and Cannot – Do

Can: Adsorb Aflatoxin B1 and certain trichothecenes in the gut
Can: Reduce toxin load on the intestinal mucosa
Can: Buffer inflammatory reactions in the intestinal tract
Can: Stabilize the gut environment and indirectly protect the microbiome
Cannot: Replace bad hay – the source of the problem must be addressed
Cannot: Bind all mycotoxin types equally effectively (e.g., zearalenone less effectively)
Cannot: Cure an already manifest poisoning – that is the veterinarian's job.

Practical Measures: What You Can Do

The best strategy against mycotoxins is a combination of prevention (hay quality), reduction of exposure (storage), and targeted gut support.

Check and Ensure Hay Quality

Harvest hay as dry as possible: Guideline under 15% water content
Store in a well-ventilated area – no tightly stacked bales without air circulation
If problematic hay is suspected: Commission a mycotoxin test (laboratories offer hay analyses)
Immediately sort out visibly moldy or very dusty hay

Adjust Feeding Management

Reduce concentrate feed quantities: Grains and concentrate mixtures are common sources of mycotoxins
Change feed batches if symptoms occur – even if the hay appears fine
Keep feeding areas clean: Moisture and feed residues promote fungal growth in the feed trough

Gut Support with Zeolite

Give zeolite as a daily feed supplement: binds toxins already in the digestive tract before they are absorbed
Particularly useful in autumn and winter when hay from damp harvests is fed
In combination with sufficient roughage and exercise, zeolite acts as part of a holistic gut protection strategy

For precise dosage: Feeding zeolite to horses – dosage, application & tips from practice on this blog.
View zeolite products for horses in our shop.

Key Takeaways from This Article

Mycotoxins are invisible, heat-stable, and widespread – even in hay that looks good
Chronic low-dose exposure is the real problem – not acute poisoning
Mycotoxins silently damage the intestinal mucosa, microbiome, and liver
Zeolite can adsorb certain mycotoxins in the digestive tract and reduce their effect
Hay quality is the most important factor – zeolite supplements it, but does not replace it
In part 4, we will take a closer look at the research on zeolite and the microbiome

Frequently Asked Questions

★ Can I identify mycotoxins in hay myself?

Visible mold and heavy dust can be recognized – but many mycotoxin contaminations are invisible. If you have suspicions, feed laboratories offer hay analyses for common mycotoxins. This is the only reliable way.

★ My horse has had loose stools for weeks and the hay looks good. Could mycotoxin be the cause?

Yes. Good appearance does not exclude mycotoxin contamination. If other causes have been ruled out, changing hay and trying a toxin binder (e.g., with zeolite) is a sensible next step – always in consultation with your veterinarian.

★ Is zeolite the same as other toxin binders?

No. There are various mineral adsorbents: bentonite, montmorillonite, HSCAS, and clinoptilolite zeolite. They differ in pore structure, binding capacity, and selectivity. Zeolite is not the strongest adsorbent for all mycotoxins, but it has a favorable safety profile and is well-proven for long-term use in horses.

★ Can I give zeolite all year round?

Yes. Zeolite can be given as a continuous feed supplement. It is particularly useful during periods of higher mycotoxin exposure: autumn and winter (hay from the current harvest year), spring (transition feed), and in known damp storage conditions.

★ What should I do if I suspect acute poisoning?

Immediately change feed and consult a veterinarian. Acute mycotoxin poisonings – recognizable by sudden neurological symptoms, severe weight loss, or liver failure – are veterinary emergencies.

Sources:

[1] Changes in the nutrient profile and the load of mycotoxins, phytoestrogens, and pesticides in horse pastures during spring and summer in Austria

Son, V. et al. · 2023 · Journal of Equine Veterinary Science

Austrian field study that examined pasture grass and preserved roughage throughout the year for mycotoxins, phytoestrogens, and pesticides. Proves that mycotoxin contaminations occur even in fresh, inconspicuous growth and in hay without visible mold.

https://pubmed.ncbi.nlm.nih.gov/37925115/

[2] Association between forage mycotoxins and liver disease in horses

Durham, A. E. · 2022 · Journal of Veterinary Internal Medicine, peer-reviewed

Clinical study linking outbreaks of liver disease in horse populations to mycotoxin contamination in hay – even in operations where the hay appeared outwardly fine. Examined 54 different mycotoxins using LC-MS/MS analysis.

https://pmc.ncbi.nlm.nih.gov/articles/PMC9308415/

[3] The role of natural and synthetic zeolites as feed additives on the prevention and/or treatment of certain farm animal diseases: A review

Multiple authors · 2020 · Animals (MDPI), peer-reviewed, Open Access

Comprehensive review of zeolite (including clinoptilolite) as a feed additive. Summarizes studies on mycotoxin binding, discusses in vitro and in vivo evidence, and clearly states the limitations: efficacy is toxin-specific and not universal.

https://pmc.ncbi.nlm.nih.gov/articles/PMC7106472/