Mycotoxins are toxic secondary metabolic products of molds present on almost all agricultural commodities worldwide. Unlike primary metabolites (sugars, amino acids and other substances), secondary metabolites are not essential in the normal metabolic function of the fungus.15 Other known secondary metabolites are phytotoxins and antibiotics.
Currently there are around 400 mycotoxins reported. These compounds occur under natural conditions in feed as well as in food. Some of the most common mycotoxins include: aflatoxins, trichothecenes, fumonisins, zearalenone, ochratoxin and ergot alkaloids.10 Mycotoxins are produced by different strains of fungi and each strain can produce more than one mycotoxin. The major classes of these mycotoxin-producing fungi are listed in Table 1.13, 15, 25
Each plant can be affected by more than one fungus and each fungus can produce more than one mycotoxin. Consequently, there is a high probability that many mycotoxins are present in one feed ingredient, thus increasing the chances of interaction between mycotoxins and the occurrence of synergistic effects, which are of great concern in livestock health and productivity. Synergistic effects occur when the combined effects of two mycotoxins (even at low levels) are greater than the individual effects of each toxin alone. Simple additive effects can also occur with the combined effects of two mycotoxins being equal to the sum of the effects of each toxin on its own.10, 14, 19
Mycotoxins are invisible, tasteless, chemically stable and resistant to temperature and storage. They are resistant the normal feed manufacturing processes.
Mycotoxin producing fungi can be divided into two groups.13, 25
- Field fungi (such as Fusarium sp.) typically produce mycotoxins in the field (“pre-harvest”)
- Storage fungi (such as Aspergillus and Penicillium sp.) typically occur after harvest (“post-harvest”)
However, in special cases like under unusually hot or dry conditions Aspergillus and Penicillium species can also affect crops during the growing season. On the other hand, field fungi can continue growing and produce mycotoxins during transport and storage.7
Mycotoxins cause economic losses at all levels of food and feed production, including crop and animal production, processing and distribution.6, 21, 26 According to the FAO (Food and Agriculture Organization) around 50% of the world’s crop harvests may be contaminated with mycotoxins.11
|Major genera of mycotoxin producing fungi||Species||Mycotoxins|
(B1, B2, G1, G2)
(syn. F. moniliforme)
|Fumonisin (B1, B2, B3)|
Type A Trichothecenes:
T-2 toxin, HT-2 toxin,
Type B Trichothecenes:
|P. roqueforti||Roquefortine C|
|Cyclopiazonic acid (CPA)|
Tall fescue toxins:
Mycotoxins can occur despite negative analytical results due to two main reasons. Firstly, they are often located in so-called “hot-spots” (Figure 1) and therefore they might stay undetected depending on sampling procedures.1, 18, 22
Secondly, masked mycotoxins could occur in feed. Masked mycotoxins are product of a specific biochemical reaction where mycotoxins can be bound to certain molecules including e.g. glycosides, glucuronides, fatty acid esters and proteins. Due to the biochemical modification, these masked mycotoxins are not detectable with conventional analytical methods, i.e. when using a conventional testing method samples may show up as having no (or low) contamination whereas, in reality, there is a mycotoxin risk present.3, 5, 16 The mycotoxin-molecule bond may be cleaved in the gastrointestinal tract of the animal and the mycotoxin is released (figure 2).4, 8
Mycotoxins differ in their structure, which explains the great variation of symptoms.9, 13, 23 Toxicity of mycotoxins can be acute and chronic. In acute cases, the effects of the toxin will appear after a short exposure time (seconds, minutes or hours). Usually, acute toxicity is the result of exposure to high doses and is characterized by the presence of easily recognizable severe symptoms.9, 17 Chronic toxicity is characterized by weaker symptoms that might only occur after an initial period of exposure. Chronic toxicity can occur with long-term exposure to low doses of mycotoxins. A chronic effect of some mycotoxins is the induction of cancer, especially of the liver (e.g. aflatoxin). The major acute and chronic effects of mycotoxins are listed in table 2.17
|Acute effect||Chronic effects|
|Deterioration of liver and kidney function||Liver cancer|
|Jaundice (yellow skin)||Chronic hepatitis|
|Emesis||Slow developing jaundice|
|Anorexia||Hepatomegalite (abnormal size of liver)|
|Ascites (fluid in the peritoneal cavity)||Liver cirrhosis|
|Gastrointestinal hemorrhage (bleeding)||Immune suppression|
Toxic effects of mycotoxins can be reversible and irreversible. Reversible effects include minor damages that can heal such as skin irritation. Irreversible effects involve permanent damage to health. One example, is vasoconstriction, (blood vessel constriction) caused by ergot alkaloids, leading to necrosis of extremities (e.g. toes, ears or tail).13, 17
The main toxic effects of mycotoxins are carcinogenicity, genotoxicity, nephrotoxicity, hepatotoxicity, estrogenicity, reproductive and digestive disorders, immunosuppression and dermal effects.6, 10, 13, 17 There are several factors which influence symptoms:13
- Type of mycotoxins consumed, intake level and duration of exposure
- Animal species, sex, breed, age, general health, immune status
- Farm management: hygiene, temperature, production density
- Possible synergism between mycotoxins simultaneously present in feeds
Primary toxic effects of mycotoxins are summarized in table 3.17
|Mycotoxin||Primary mechanism of action|
|Aflatoxin||Binds to guanine (DNA-adduct) after metabolic activation in the liver|
|Trichothecenes||Inhibition of protein synthesis|
|Zearalenone||Binds to mammalian estrogen receptor|
|Ochratoxins||Blocks protein synthesis|
|Ergot alkaloids||Binding to adrenergic, dopaminergic and serotonin receptors|
|Fumonisins||Inhibit ceramide synthase (sphingolipid biosynthesis)|
Mycotoxins are capable of direct-target toxicity towards certain organs such as the liver, nervous system, kidney, skin, cardiovascular, reproductive and immune systems. Non-direct-target effects include carcinogenicity, teratogenicity and mutagenesis.6, 10, 13, 17
Mycotoxins are absorbed through the gastro-intestinal tract (GIT), the lungs, the skin and other organs like the eyes.17 In the GIT, mycotoxins can be absorbed in the mouth and esophagus (minimal absorption), in the stomach and in the small intestine. In the stomach, absorption is generally correlated with the fraction of non-dissociated contaminants like weak acids.17 The small intestine, is where the maximal absorption takes place. A fraction of mycotoxins can also be absorbed in the colon.9, 10, 17
In the lungs, mycotoxins that are generally carried by dust are absorbed in the alveoli (i.e. ochratoxin A).17
Once in the body, mycotoxins can be distributed through different pathways. Some mycotoxins can bind to plasma proteins and be transported into the blood plasma (e.g. ochratoxin A).17, 20, 23 Mycotoxins can also be lipophilic and accumulate in the fat tissues. Lipophilic compounds can easily penetrate the blood-brain barrier and the placental barrier.6, 13, 17 The half-life elimination time, or the time required to reduce the initial plasma concentration of the toxin, can be very long in the case of chronic exposure (ochratoxin A half-life elimination time in humans is longer than 560 hours.9, 17
Mycotoxins can be partially excreted through several pathways. Usually polar and hydrophilic substances are excreted via the kidneys with the urine. Accumulation of mycotoxins in kidneys has been observed and can produce toxic effects (i.e. ochratoxin A).17
Compounds that are characterized by high molecular weight are usually excreted via the bile. Other pathways of excretion include transfer to milk (aflatoxin B1) and elimination through sweat and saliva. Unabsorbed mycotoxins may be excreted in feces but could still have had an effect on the gut wall during passage.17
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- Gareis, M., Bauer, J., Thiem, J., Plank, G., Grabley, S., & Gedek, B. (1990). Cleavage of zearalenone-glycoside, a "masked" mycotoxin, during digestion in swine. Journal of Veterinary Medicine, Series B, 37(3), 236-240.
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