Mycotoxins

What are mycotoxins? from URL http://www.knowmycotoxins.com/vequine.htm

The word mycotoxin stems from the Greek word "mykes" meaning mould and "toxicum" meaning poison. Mycotoxins are toxic secondary metabolites produced by fungi growing on crops in the field, during handling and in storage.  Mycotoxins are capable of causing disease, which may be potentially fatal. Fungi only produce mycotoxins under favourable conditions such as specific moisture levels, oxygen levels in the air and temperature, in other words the presence of fungi does not necessarily indicate the presence of mycotoxins. There are thousands of different species of fungi, but only a relatively small number (about 100 species) have the ability to produce mycotoxins.

Mycotoxins are often found in cereals such as barley, maize, wheat and oats and their by-products. They may also be found on pasture and therefore conserved forage in certain environmental conditions. Forage and traditionally cereals play a huge role in the diet of horses, particularly cereals in performance horses. Mycotoxins enter the horse via feed, forage or bedding. In fact mycotoxins are odourless and horses cannot taste them in feed, making ingestion more likely.  Ingestion of contaminated feed or forage and inhalation from mould-infested bedding, hay and haylage are all-important causes of exposure. Low-level intakes of mycotoxins over long periods are likely to produce chronic toxicological symptoms. This may affect athletic performance and breeding capability possibly without actual obvious disease symptoms. General symptoms such as loss of appetite, weight loss, unthriftiness, poor performance, increased susceptibility to infectious diseases, reduced growth rates and poor breeding performance, for example increased red bag delivery, may all be associated with chronic mycotoxin intake. Because mycotoxins negatively affect equine performance and health, mycotoxin control is crucial for equine welfare and feed safety reasons.

Historically, human cases of ergotism or St. Anthony's Fire have been described in Europe since the Middle Ages and are now known to be caused by mycotoxins produced in rye by the mould Claviceps purpurea. In 1960, an outbreak of Turkey X disease in England and the subsequent discovery of the aflatoxins stimulated great interest in the field of mycotoxin research. Since then many more mycotoxins, such as trichothecenes, zearalenone, ochratoxins and fumonisins have been discovered.

The term mycoses refers to actual growth of fungi on the animal host whereas the term mycotoxicoses refers to the disease caused by ingestion or inhalation of toxic fungal metabolites by the animal. Fungi may be present within the plants and these are known as endophytes. These fungi have a symbiotic relationship with the host plant, making it more resistant to drought or invasion by insects etc. In return the fungi can take nutrients from the host plant. Other fungi grow on the outside of the plant and these are known as saprophytes. They are often damaging to plant health and serve no useful purpose to the plant.

Fungi may be found on pasture or in cereal grains in varying amounts each year depending upon the environmental conditions. A cooler wetter season is more likely to result in greater fungal contamination of crops. High moisture levels will increase fungal growth and cooler temperatures result in the formation of mycotoxins by the fungi.

There are hundreds of known mycotoxins, but few have been extensively researched in horses and fewer still have good methods of available analysis. Mycotoxins vary greatly in their severity.  As horses are hindgut fermenters it is thought they may be more susceptible to the effects of mycotoxins than ruminants, which are able to degrade many mycotoxins in the rumen. Horses are hindgut or post gastric fermenters and as such will take mycotoxins into the body following ingestion of contaminated feed or forage. Eventually they will enter the small intestine where they may exert their effects on the intestinal wall or be absorbed in to the horses’ body via the blood.

Mycotoxins have been implicated in causing colic, organ disease (brain, liver, kidneys) reduced growth rate, respiratory problems, poor feed efficiency, reduced fertility and even death.
Mycotoxins exert their effects through four primary mechanisms:

• Intake reduction or feed refusal
• Alteration in nutrient content of feed in terms of nutrient absorption and metabolism
• Effects on the endocrine system
• Suppression of the immune system

These effects often lead to rather unspecific symptoms, which can also be caused by many other factors making if difficult to properly diagnose mycotoxin problems in horses. The symptoms of mycotoxicosis will depend upon the mycotoxin type and also the health status, age, sex and work level of the exposed horse. Mycotoxicoses may be categorised as chronic or acute. Acute toxicity may be observed as rapid onset of symptoms following intake of the toxin whereas chronic exposure to the mycotoxin leads to chronic toxicity symptoms, which are often difficult to pinpoint.

General symptoms (reduced performance, impaired immunity) are seen when dealing with moderate mycotoxin levels, while symptoms caused by higher mycotoxin levels are often more specific and obvious. Further complications in mycotoxicosis diagnoses can be caused by secondary symptoms resulting from opportunistic disease related to the suppression of the immune system following mycotoxin exposure. In addition, mycotoxicoses increase the horses’ vulnerability to microbial diseases.

In order to effectively identify mycotoxicosis, experience with mycotoxin-affected horses is important. This experience, combined with adequate feed and tissue analyses, provide the basis for the most accurate diagnosis of mycotoxicosis.

Approximately 300-400 compounds are currently recognised as mycotoxins and these are often difficult to classify due to their diverse chemical structures. Sometimes mycotoxins may be referred to in relation to the organ affected, for example nephrotxoins, hepatotoxins, immunotoxins and neurotoxins.

Mycotoxin concentrations are expressed in a number of ways but mainly given as ppb (parts per billion). Parts per million is shown as ppm.

Table 1: Occurrence of different key mycotoxins

Adapted from Bhatnagar et al., 2004

Problems commonly associated with mycotoxicosis in horses include:

• Reduced growth rates
• Decreased appetite
• Poor performance
• Colic
• Respiratory problems
• Hypersensitivity
• Reduced reproductive performance
• Increased susceptibility to disease
• Brain lesions
• Neurological disorders
• Organ damage, in particular  liver, kidneys

The degree of exposure to mycotoxins i.e. length of time and the concentration of mycotoxin in the feed will affect the symptoms shown. For example long-term exposure to low levels of mycotoxins may produce quite vague symptoms. In addition contaminated feed is likely to contain more than one fungal species and therefore multiple mycotoxins. It is also known that mycotoxins can act synergistically with other toxins.

Other factors will also affect the response of horses to mycotoxin exposure including age, workload, stress level, nutritional status and immune status of the horse. Horses in hard work or lactating are likely to have much higher intakes of cereal-based concentrate feeds and this should also be taken into account, when assessing possible mycotoxin involvement.

Optimising feed quality is vital for reducing mycotoxin levels in equine diets. Mouldy feeds should never be fed to horses even though they might not necessarily contain mycotoxins. In addition, significant concentrations of mycotoxins may be found in equine bedding such as straw and shavings and this should not be overlooked where mycotoxicosis is suspected.  Straw may already be contaminated with significant concentrations of mycotoxins at the time of harvest, although any type of bedding may be contaminated during unfavourable storage conditions.  It is important to note that mycotoxins may be present in bedding and feedstuffs even though they do appear to be fungus free.

Mycotoxins in copra meal.