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

The hay looks good. It smells clean. Your horse eats it without hesitation. And yet something is wrong: mud fever 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 most underestimated causes of intestinal 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 to cushion 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 produced as a byproduct of fungal metabolism, mostly under certain conditions: moisture, warmth, poor ventilation, mechanical damage to the feed.
The insidious thing is that the fungus itself does not have to be active for the toxins to cause damage. Mycotoxins are heat-stable, acid-stable and chemically very stable. They survive the drying of hay, the pressing into pellets, and even some heating processes. Once contaminated, it 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 moist 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, 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 treacherous: studies show that commercial horse feed from the German market contains several mycotoxins in almost all samples examined. The individual concentrations were usually below the critical limit values - but the combination of several toxins can amplify effects that would not have occurred individually.

What mycotoxins do in the horse's gut

When feed containing mycotoxins enters the horse's digestive tract, a silent damage begins, which rarely manifests as acute poisoning, but as a creeping burden.

1. Damage to the intestinal mucosa
2. Mycotoxins, especially trichothecenes such as T-2 and DON, directly attack the intestinal mucosa. They inhibit protein synthesis in the mucosal cells, which reduces the regeneration capacity of the intestinal epithelium. The intestinal barrier becomes more permeable - a condition known as "leaky gut". This disturbed barrier allows bacteria, toxins and undigested food components to enter the bloodstream and trigger systemic inflammatory reactions.2. Destabilization of the microbiome
   The intestinal microbiome reacts sensitively 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 - exactly what we described in Part 1 and Part 2 of this series as the starting point for many horse ailments.
3. Stress on the liver and immune system
   Mycotoxins that are not bound in the intestine enter the bloodstream via 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 examined outbreaks of liver disease in horse herds and found mycotoxin contamination in hay in a significant proportion of affected farms - even where the hay appeared outwardly unremarkable.
4. Chronic low-level exposure - the real problem
   Most horses do not suffer acute mycotoxin poisoning. What they suffer from 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 that 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 discoloration or sticky spots in the hay bale •       Hay from wet harvest years or poorly ventilated storage •       Caution: Good appearance does not rule out mycotoxin contamination

 

How Zeolite Works Against Mycotoxins

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

What Zeolite Can Do – and What It Cannot

• Can: Adsorb aflatoxin B1 and certain trichothecenes in the intestine
• Can: Reduce toxin load on the intestinal mucosa
• Can: Buffer inflammatory reactions in the intestinal area
• Can: Stabilize the intestinal environment and indirectly protect the microbiome
• Cannot: Replace poor 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 – this is the vet'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

• Bring in hay as dry as possible at harvest: target value under 15% water content
• Store well-ventilated – no tightly stacked bales without air circulation
• If problematic hay is suspected: order a mycotoxin test (laboratories offer hay analyses)
• Immediately sort out visibly moldy or very dusty hay

Adjust Feeding Management

• Reduce concentrate feed amounts: grains and concentrate mixtures are common sources of mycotoxins
• Change feed batches if symptoms appear – even if the hay looks fine visually
• Keep feeding areas clean: moisture and feed residues promote mold growth in the feed trough

Gut Support with Zeolite

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

For exact dosage: Feeding Zeolite to Horses – Dosage, Application & Tips from practice on this blog.
Check out Zeolite Products for Horses in our shop.

Key Takeaways from This Article

• Mycotoxins are invisible, heat-stable, and widespread – even in visually good hay
• Chronic low-dose exposure is the real problem – not acute poisoning
• Mycotoxins gradually damage the intestinal mucosa, microbiome, and liver
• Zeolite can adsorb certain mycotoxins in the intestinal tract and weaken their effect
• Hay quality is the most important variable – 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 detect mycotoxins in hay myself?

Visible mold and severe 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 sure way.

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

Yes. A good appearance does not rule out 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 a 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 permanent feed supplement. It is particularly useful during periods of higher mycotoxin exposure: autumn and winter (current hay from the harvest year), spring (transitional feed), and under known damp storage conditions.

★ What should I do if I suspect acute poisoning?

Immediately change feed and consult the 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 conserved roughage for mycotoxins, phytoestrogens, and pesticides throughout the year. Proves that mycotoxin contamination also occurs 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 farms where the hay appeared outwardly flawless. 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 (incl. clinoptilolite) as a feed additive. Summarizes studies on mycotoxin binding, discusses in-vitro and in-vivo evidence, and clearly states the limitations: effectiveness is toxin-specific and not universal. https://pmc.ncbi.nlm.nih.gov/articles/PMC7106472/

 

More from the Series: Horse Gut & Detoxification

All parts of the series: •        Part 1: The Horse's Gut – How It Really Works •        Part 2: Signs Your Horse Has Gut Problems – 12 Warning Signs •        Part 3: Mycotoxins in the Horse's Gut – The Silent Risk in Hay ← you are here •        Part 4: Zeolite and the Horse's Gut – What Animal Research and Practice Jointly Confirm •        Part 5: Gut Cleanse for Horses – Step-by-Step Program with Zeolite

 

Next in the series →

Part 4: Zeolite and the Microbiome – What the Research Says

Read more

→ Contaminants in Horse Feed
→ Zeolite - everything you need to know about feeding horses: Q&A and guide