Mycotoxins are poisonous compounds that are naturally produced by certain types of moulds (fungi) which can contaminate horse feed and pasture plants.

Although contamination of commercial horse feeds is not very common in Australia, it can occur, particularly under warm and rainy conditions.

At the Fuelling Wellness Global Animal Nutrition Summit which took place during August in Guleph, Ontario, Dr. Younes Dr Charfi from the University of Montreal presented a webinar lecture on Mycotoxins in Horse Feed.

Dr Charfi, an Associate Professor at Montreal with both DVM and PhD degrees, defined mycotoxins as organic compounds associated with toxicosis in animals. Being monogastric, horses and pigs are affected to a greater degree than ruminant animals.

Dr Charfi said the incidence of contamination is fairly low in Canada but more prevalent in feed imported from the United States, especially from southern areas. However, he warned that warming climates may cause increasing contamination worldwide and horse owners should remain vigilant. It is estimated that 25% of the world’s agricultural commodities contain mycotoxins.

Mycotoxin contamination can occur in cereal grains, commercial feed, hay, pasture and straw used for bedding. It is important to note that grinding and mixing preparation of commercial feed does not destroy the mycotoxins.

Most mycotoxins are produced by fungi, but the presence of fungi alone does not mean there is contamination. This is because fungi tend to produce harmful mycotoxins when they are under stress, as a result, for example, of extreme weather conditions (especially excess moisture), improper storage, overgrazing, and insect and disease damage.

This is why a particularly rainy season may result in greater amounts of toxins in the next growing season. Harvesting grain and hay in overly moist conditions will also result in molds producing harmful substances.

The conditions which favor the growth of mycotoxins are humidity in the feed between 10 and 20%, relative atmospheric humidity greater than70%, temperature above 20 degrees Celsius and the availability of oxygen. The toxins themselves persist, even if the fungi which produced them are destroyed.

One fungus can produce multiple mycotoxins and multiple fungi may produce the same toxin.

There has not been a lot of research on tolerance levels in horses, but more research has been conducted with pigs. In the few studies involving horses, they appear to have a higher tolerance than pigs.

Common types of mycotoxins and their related affects

The main contaminants discussed in Dr Charfi’s presentation were: deoxynivenol (DON), T-2/ HT-2, zearalenone, fumonisin, fusaric acid, orchratoxin A, and ergot alkyloids.

DON and related compounds can occur in cereal grains. It binds to the ribosomes in cells, thus inhibiting protein synthesis and at least during the initial exposure, causing a reduction of grain intake but not forage consumption. It also causes an increase in liver enzyme activities. While it’s been found to weaken immune response in pigs, so far, no effect on immune response has been shown in horses. Many other toxins also show a decrease in concentrate feed consumption, but it is unclear if this is caused by the horse smelling the contaminant or an actual biochemical reaction to consumption of the effected feed.

Zearalenone is similar in chemical structure to estrogen and thus binds with estrogen receptors and generates an estrogen-like response including enlargement of the uterus, vulvular prolapse, feed refusal, prolapsed vagina, internal hemorrhage and flaccidity of genitals in males.

Fumonisin contamination (also known as moldy corn disease) results in neurological symptoms such as drowsiness, hyper excitability, tremors, jaw paralysis and enlarged left brain hemisphere.

There have been limited reports of T-2 symptoms in horses, but in pigs it causes decreased feed intake and performance, skin lesions, tail necrosis, and intestinal hemorrhage.

Ochratoxin produced by aspegillus and penicillium can cross the placental barrier in pregnant mares and may affect the fetus, but there has been little research on this.

Fusariums cause a synergistic effect, increasing the reaction to other mycotoxins present.

Ergots and related fungi found on paspalum, perennial ryegrass, tall fescue and rye results in vasoconstrictors which cause lameness, reproduction disorders, abortion, foal death and lack of milk production. This can best be prevented by switching to ergot resistant or endophyte free varieties of hay, pasture and grain. It is also important to remove pregnant mares from such pastures or hay a few months before foaling.

Staggers is another disease seen in perennial ryegrass infections. The horse may suddenly tremble or lose coordination.

Rhizoctonia or black patch on red clover and other legumes causes excess salivation, lacrimation, colic and diarrhea.

Sweet clover poisoning occurs in molded sweet clover hay and prevents blood clotting. It is treated by removing contaminated hay and dosing with Vitamin K to aid clotting mechanism.

While Dr Charfi did not mention specific respiratory effects in horses, he did say that mycotoxins can also have a negative effect on the respiratory system.

Various feed additives are sold which claim to alleviate some of the results of mycotoxin contamination. In experiments in pigs, a blend of antioxidants and amino acids gave the best results when combined with DON contaminated feed. The aim of additives is to bind the toxins or degrade them.

Testing feed stuffs for mycotoxins is difficult because the contamination is often concentrated in ‘hot spots’ and not evenly mixed in the feed. This is why feed sampling for testing should be conducted multiple times, in multiple locations.

Mycotoxins are so prevalent that complete elimination is difficult. But careful management of harvesting, feed storage and pastures help to eliminate the risk.

A well managed, biodiverse pasture and preventing overgrazing helps avoid pasture and hay related problems. And careful observation, both visual and olfactory, of any feedstuff can alert the handler to problematic hay or grain.