Mycotoxins in Horse Diets – more than just Staggers by James Hart of Equine Herbals

The effects of mycotoxins on production animals and humans are well known but until recently less work had been done to look at how horses are affected. Horses may in fact be more susceptible to mycotoxins than ruminants because they are hind gut fermenters. Mycotoxins are more likely to be absorbed in the fore-gut and the gastric juices to do little to deactivate them so they pass from the stomach into the small intestine, where they can affect the gut lining or be absorbed into the bloodstream. In ruminants the robust digestive microflora in the rumen degrade and inactivate many mycotoxins into non toxic substances prior to absorption in the small intestines.

What are Mycotoxins?

Mycology is the study of fungi which includes moulds and yeast. The number of fungi species is huge and range from the useful like the mushrooms that we eat, the yeast we use to make bread and beer and the penicillium from which we extract penicillin, to the highly toxic like certain toadstools and those that produce mycotoxins like Fusarium.

Mycotoxins are secondary metabolites produced by fungi. The fungal toxins are chemically diverse — representing a variety of chemical families — and range in molecular weight from about 200 to 500. Mycotoxosis is the name given to the group of illnesses that they cause in animals and humans. Mycotoxins have names like zearalenone, fumonsin, vomitoxin and aflatoxin. They are produced mainly by the various members of the Aspergillus, Fusarium and Penicillium genera. It is estimated that 25% of the world production of cereals is contaminated with various mycotoxins and in certain parts of the world up to 90% can be affected. They can be found in nearly all agricultural commodities including maize, wheat, oats, rice and barley and under certain conditions can be found in hay, straw, grass and maize silage.

Mycotoxins are invisible, tasteless and odourless so it’s impossible to do any on-farm testing. Mould in hay does not necessarily indicate mycotoxin contamination – but it could, and the conditions that favour mould growth also favour mycotoxin production. i.e baling and storing at too high a moisture content.

Mycotoxins exert their effects through several means. Broadly they cause reduced food intake, reduce nutrient absorption, impair metabolism, alter endocrine and exocrine systems, suppress immune function and alter gut microbial growth and health. The effects are often subtle. In the trials that have been carried out so far horses have shown quite good resistance to the challenge from certain single toxins. However, in real life this is unlikely to occur and the most likely challenge is from a cocktail of mycotoxins. There may well be a synergistic effect between these which then presents a far bigger challenge to the horse.

How do mycotoxins affect horses?

Although there is not much scientific literature on the subject mycotoxins have been implicated with a variety of conditions including depressed appetite, colic, abnormal liver function, hypersensitivity, neurological disorders and brain lesions. In other species ingestion and inhalation of mycotoxins has been shown to also affect the immune system, kidney and lung function and reproductive ability. It is not too big a leap of faith to expect the same sort of effects in horses. So long term exposure to low levels of mycotoxins may well result in immune suppression, reduced growth rate, fertility problems, frequent respiratory problems, reduced performance and possibly laminitis.

Because it is likely to be non acute there is a danger that early mycotoxicity may not be recognised. The problems caused may well be blamed on something else when in fact the mycotoxins are an important co-factor. However as we see with staggers, as exposure increases the effects on performance and health can become more pronounced. It is interesting that researchers studying the incidence of colic and mycotoxin contaminated feed found that some degree of mycotoxin contamination occurred in every case of colic.

As with other conditions the effects of mycotoxins can be amplified by stress. Horses in full training, breeding mares, stallions and rapidly growing foals are more susceptible to mycotoxins than horses used occasionally for less rigorous riding. Although many of the effects of mycotoxins are perhaps insidious a well known and dramatic seasonal problem is grass staggers, which can be caused by infected ryegrass or tall fescue. Staggers affects many herbivores including horses. It occurs mainly in warmer areas but as temperatures increase it is becoming more common in cooler regions. It is most common in the summer and autumn.

Ryegrass affected animals become anxious when they are approached and seem to become stressed more readily than usual. In severe cases the animals move with stiff legs, as if they have sore feet – taking small careful steps. Another way of checking is to clap your hands in the direction of a horse with suspected staggers. Unaffected horses behave as expected and usually run away. Those with the condition appear stiff, unable to move easily and act in an uncharacteristic manner. The biggest problem in mild cases is not the staggers itself but the risk of falling into fences, streams or other obstacles. Tall fescue affected animals show slightly different symptoms. They include hyperthermia (elevated body temperature), lower feed intake and consequent weight loss, lower pregnancy rates and decreased milk production. These conditions although more apparent during hot weather, can occur at any time of the year. Horses with tall fescue toxicity are especially prone to developing serious reproductive problems such as abortions, difficult births, and foal deaths.

Both of these conditions are caused by the presence of neurotoxins in the grass the horse has eaten. The neurotoxins are produced by endophytes, fungi that live inside the grass stems. Acremonium coenophialum is the fungus which mainly infects tall fescue producing the toxin ergovaline. Acremonium lolii mainly infects perennial ryegrass, and the major toxin it produces is lolitrem B. The toxins have a specific damaging effect on an area of the cerebellum; the part of the brain that coordinates movement and balance. There are several toxins involved, some of which are quite similar to LSD. This has led to suggestion that they may cause the animals to hallucinate. It is also thought that prolonged exposure may predispose an animal to brain tumour development. It is interesting to note that after the first occurrence of these diseases in an animal, each  ubsequent attack is likely be increasingly severe.



The first line of defence is obviously to exclude mycotoxins from the diet and environment. Never easy, and often impossible. In staggers the replacement of grass species with endophyte free strains is an important part of control but because the fungus is endemic reinfection can occur. The inclusion of alternative species such as chicory and plantain in the pasture is very useful.

In general however it can sometimes be impossible to prevent the ingestion or inhalation of mycotoxins so strategies have been developed to mitigate or treat the effects, which is the second line of defence. The use of toxin binders is a theoretically simple way to prevent the toxin from entering the bloodstream even though it may be in the horse’s gut. So the addition of an appropriate binder to contaminated diets or potentially contaminated diets has been considered the most promising approach to reduce the effects of ingested mycotoxins. The theory is that the binder decontaminates mycotoxins in the feed by binding them strongly enough to prevent toxic interactions with the animal and to prevent absorption through the gut. Potential absorbent materials include activated charcoal, aluminosilicates (clay, bentonite, montmorillonite, zeolite, phyllosilicates, etc.), complex indigestible carbohydrates (cellulose, certain polysaccharides in the cell walls of yeast and bacteria, glucomannans, peptidoglycans, and others), and synthetic polymers such as cholestryamine and polyvinylpyrrolidone and derivatives.

Research with mycotoxin binders has been conducted for over 20 years, and although there are several products in the market further work is needed to establish which products are best. It seems that different toxins respond best to different binders, which makes the problem a whole lot more complex. So although there is an excellent potential for binders to help manage the mycotoxin problem at the moment they are not the whole answer.

There are only limited measures that can be taken to deal with inhaled mycotoxins because under the right conditions levels can become quite high in dust from stable bedding, dry pastures or training grounds. So it is important to keep arenas and bedding clean and as dust free as possible. The practise of soaking hay prior to feeding is also well worth considering for those susceptible individuals.

The herbal approach to prophylaxis would include the use of clays or other inert binders which would be used in conjunction with plant sourced toxin binders to give the widest possible binding spectrum. The herbs to consider are those with mucilaginous characteristics including Psyllium (Plantago ovata), Slippery Elm (Ulmus rubra) Fenugreek (Trigonella foenum-graecum), Marshmallow Root (Althaea officinalis), Comfrey (Symphytum sp), Bladderwrack (Fucus vesiculosus) and Konjac (Amorphophallus konjac). The mucilage or gum these plants contain are generally high molecular weight polysaccharides which act as toxin binders. They have broad spectrum binding ability and can be used to help with a range of toxins. Of the herbs mentioned Psyllium and Fenugreek are the most readily available. Slippery Elm is good but harvesting destroys the tree and supply is limited. Psyllium and Fenugreek are both seeds of annual plants which can be readily cultivated, both can be added to feed, are available as dry powder and have other beneficial actions.


Sooner or later most organisms will adapt to their environment and develop ways of dealing with the toxins that confront them. For example it was estimated in 2002 that at least 155 plant species had become resistant to various herbicides, in the same way that internal parasites eventually become resistant to the chemicals we use to try and kill them. Mammals are equally adept at dealing with toxins and it is only when the levels become excessive that individuals begin to show signs of suffering, but even if the signs are not evident the toxins within the body still need to be eliminated.

To deal with these foreign substances be they naturally occurring or man made the liver has developed a three stage detoxification process (oxidation, reduction and hydrolysis). This is to make these substances less harmful and prepare them for elimination by the main excretory routes; the gut, kidney or skin. In unusual circumstances some of these foreign substances can be transformed into new chemically active metabolites and free radicals. The liver is at the heart of these processes and is quite vulnerable to the effects of chemicals that can not be metabolised or excreted.

The herbal approach to detoxification is based upon the premise that the body is a self healing and homeostatic organism. This is true to a large degree but herbal medicine seeks to help this by using basic, simple and safe herbs to assist the eliminative process. So detoxifiers are herbs that protect the liver and aid in the detoxification and removal of actually or potentially toxin substances. To do this they may bind to the toxic substance or aid the body in its processing of it, offer protection to the liver or aid in the imination of the toxin or its metabolite.


There are pretty well no conventional treatments for mycotoxosis, but there are a range of herbs that can be of great benefit. The major problems which the toxins cause relate to the liver but any herbal treatment should encourage detoxification throughout the body. The actions to look for include hepatics to support and protect the liver and diuretics to aid with waste removal through the kidneys. Alteratives also have an important role in toxin binding, supporting the body in stress and normalising processes.

Tannins are a chemical class found in many plants that are known to provide detoxifying actions through binding, as well as antioxidant protection. Of particular note are the condensed tannins whose binding and antioxidant actions improve toxin clearance and reduce the likelihood of cellular damage. This binding action is to toxins in general but more specific work is needed to establish their specific activity in mycotoxicity.

Herbs to consider.

Hepatics; Burdock (Arctium lappa),Dandelion Root (Taraxacum off.), Globe Artichoke,(Cynara scolymus), Milk Thistle ( Silybum marianum), Schisandra ( Schisandra chinensis) and Yellow Dock ( Rumex crispus).


Diuretics; Dandelion Leaf (Taraxacum off. ) and Nettle (Urtica dioica)



Alteratives; Alfalfa/Lucerne (Medicago sativa ), Bladderwrack(Fucus vesiculosus), Sarsparilla, (Smilax ornata) and Red Clover (Trifolium pratense)



Tannins; Black or Green Tea (

Camellia sinensis ), Grape Seed (Vitis vinifera), and Witch Hazel (Hamamelis virginiana).



Antioxidants; Milk Thistle, Thyme (Thymus vulgaris) and Rosemary (Rosmarinus off.)


Increasing bile flow from the liver is another important part of the elimination of toxins. Herbs researched for this action are Calendula (

Calendula off.), Milk Thistle,Dandelion Root, Yellow Dock, and Gentian(Gentiana lutea). Herbs that have been shown to help regenerate damaged liver tissue may also be of benefit, including Milk Thistle, Schisandra and Globe Artichoke.

You may well find other herbs to assist but for the herbs to be of use they need to have some basic qualities. Do they increase toxin clearance? Do they have free radical scavenging or other binding activity? Do they provide antioxidant protection to prevent damage to cells in contact with the toxin? Do they have direct liver protective, supportive or stimulatory effects?

Finally when looking to treat a case it is important to have regard for the physical state of the individual horse; is it old, young, sick in other ways, pregnant or lactating?, and to also look at any additional effects that the toxicity is causing. For example in ryegrass staggers there can be neurological damage.

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