Mycotoxins

Author/s : Dr Hinner Köster – Technical Director: OTK Feeds
Mycotoxins are secondary metabolites (by-products) produced by various generations of fungi when they grow on agricultural products before or after harvest or during transportation or storage. In general, they are differentiated between field and storage fungi. It must also be kept in mind that the presence of specific fungi does not necessarily mean that the toxin is present and therefore in all cases the presence of the mycotoxin must be tested chemically. It is, however, important to determine the fungi to specie level so that one can know what mycotoxins to test for since testing is expensive and because there are so many toxins known to man. Fungal growth and mycotoxin production is influenced by both intrinsic and extrinsic factors. The intrinsic factors include water activity, redox potential and pH [1] while extrinsic factors that affect mycotoxin production and type include temperature, relative humidity, the availability of oxygen, type of crop, susceptibility of the crop, insect or mechanical damage, storage conditions, and any subsequent handling [1, 2]. The toxicity of individual mycotoxins with relation to animal species depends on the breed, sex and age of the animal. It is estimated that approximately 25% of all food commodities produced on earth are contaminated with mycotoxins. They are found in feed ingredients such as maize, sorghum, grain, barley, wheat, cottonseed oilcake, groundnuts and other legumes.

Research on mycotoxins has been going on since 1961 when the first group of mycotoxins was isolated and described [2]. These were made up of aflatoxins and were followed up in 1965 by the discovery of ochratoxins. Scientists estimate that 300 to 400 different mycotoxins are presently identified with more being isolated as new techniques and processes evolve. The major mycotoxins that are important in South African commodities include aflatoxin, ochratoxin, fumonisin, deoxynivalenol (DON, vomitoxin), zearalone and T-2 toxin. The last four are also grouped as trichothecene mycotoxins, which are a chemically related family of compounds that are produced by the Fusarium species of fungi. Different mycotoxins have different chemical structures and biological activities and are classified accordingly. They may be carcinogenic (e.g. aflatoxin B1, ochratoxin A, fumonisin B1), oestrogenic (zearalenone), neurotoxic (fumonisin B1), nephrotoxic (ochratoxin), dermatotoxic (trichothecenes) or immunosuppressive (aflatoxin B1, ochratoxin A, and T-2 toxin) [1, 2].

Aflatoxin
Aflatoxins are the best known mycotoxins and are highly toxic chemicals that are mainly produced by the mold fungi Aspergillus flavus and A. parasiticus. Usually food and feed originating from areas with higher temperature and humidity are contaminated with these mycotoxins. The temperature optimum for A. flavus is 28-30ÝC and the minimum moisture content is 8-10 % in peanuts [2]. Research conducted into the growth of aflatoxin has indicated that the fungus also needs some form of stress in the plants for the fungus to invade. The stress may be in the form of drought that weakens the plant system, extended periods of high temperature, damage from insects or birds, high crop density, or competition from weeds. All of these conditions weaken the host or provide a means of entry to the spores to establish a foothold in/on the host. Animals differ in their reaction to these toxins. Therefore there are guidelines according to livestock specie as to the maximum toxin level that can be safely fed. For example, maximum levels for maize in the USA are 20ppb for immature animals and dairy feed, 100 ppb for breeding beef cattle, breeding pigs and poultry, and 200 ppb for finishing pigs and finishing beef cattle [2, 3, 4]. The European Union is much stricter and established maximum levels of 4 ppb for aflatoxins in agricultural commodities and 2 ppb for aflatoxin B1, the most potent compound [2].

Ochratoxin
These mycotoxins are primarily produced by the typical storage fungi Penicillium and Aspergillus. They occur most often in stored grain that is not dried properly. Ruminants are much less susceptible than monogastric species to this highly carcinogenic toxin, since microbial degradation of the toxin takes place in the forestomach of the ruminant animal. The official maximum limit of ochratoxin A has been established in several European countries as 5ppb in cereals and cereal products [2].

Fumonisin
Fumonisin is a recently identified mycotoxin discovered in South Africa. This toxin is produced by Fusarium moniliforme (causes Fusarium ear rot in maize) and F. proliferatum and is especially hazardous to horses. Because little is known about the effects of this toxin, only recommended levels exist. These are no higher than 5ppm in the non-roughage portion of horse diets, 10 ppm for pigs and 50 ppm for beef cattle and poultry [2, 3, 4].

Deoxynivalenol (DON, Vomitoxin)
Fusarium graminearum is the parent fungi for DON or vomitoxin. Wheat and barley are the most commonly effected grains but the same fungus infects maize. It is a natural toxin that forms when conditions are cool and wet where the grain is grown. The current guidelines for grain and grain by-products within the FDA in the USA is 10ppm for poultry and ruminating beef and feedlot cattle older than 4 months (if grain and grain by-products do not exceed 50% of the diet) and 5ppm for pigs (if grain and grain by-products do not exceed 20% of the diet) [2, 3, 4].

Zearalenone
Zearalenone is a mycotoxin produced by the typical ground fungi of the genus Fusarium. It is mostly found in maize and very similar to deoxynivalenol. It has estrogenic-like effects that were determined to cause reproductive problems in pigs. No suggested guidelines for this mycotoxin exist, although 0.5 ppm serves as a general guide for the interim [2].

T-2 Toxin
Fusarium tricinctum and some strains of F. roseum produce this mycotoxin of which very little is known. T-2 has been found in maize in the field, silage and prepared feeds made with maize. Some work in North Dakota, USA shows that this toxin has the same critical level as zearalenone of 0.5 ppm and may currently be considered as guideline [2].

Detoxification
Since fungal contamination is closely linked to environmental conditions and handling of harvested crop, it is extremely important to store commodities under the correct conditions. However, environmental conditions are impossible to control, and therefore, many companies and researchers have tried to find a means of detoxifying mycotoxin contaminated feeds and raw materials to ensure a continuous safe feed supply. Most of the techniques and treatments currently used for detoxification of mycotoxin contaminated feedstuffs are ineffective and expensive or have side effects on the end products that cancel the benefits of detoxification. The processing of feeds or raw materials with toxins generally also does little to remove the toxin. A further approach used is to reduce absorption of mycotoxins from the gastrointestinal tract by adding a sorptive substance such as charcoal, bentonite, clay or zeolitic material to the diet. Although scientific studies show positive effects, the use of these additives in practice remains controversial and inconsistent.

In practice, the best way to prevent fungal growth is by agricultural and technical means. In some cases, if the mycotoxin levels are known, it is also possible to dilute out the effects of certain contaminated raw materials by blending to produce a final blended feed below the critical level of the specific mycotoxin. Sometimes, raw materials containing certain mycotoxins can be used in feeds for species that are less sensitive than others.

Who is responsible?
Clients often hold feed companies responsible for mycotoxin contamination in their feeds. The difficulty and cost of analysing for mycotoxins, as well as costs and ineffectiveness of detoxyfying feeds or adding absorptive substances on a continuous basis makes it very difficult for feed companies to take sole responsibility for mycotoxin contamination. Traditionally, expensive and time-consuming chemical procedures such as HPLC, GC or TLC are used to analyse for mycotoxins in feed and raw materials [3]. HPLC has replaced TLC recently in being the officially approved method of testing for mycotoxins by the Association of Official Analytical Chemists. Newer processes have been developed for quicker and less expensive tests that will give results in a shorter period of time with less hazardous or toxic chemicals and procedures. These tests include methods that use direct competitive Enzyme Linked Immunosorbent Assay technology as well as fluorescence technology and have an accuracy that is acceptable for the grain industry [3]. HPLC is still used as a reference method to gauge the accuracy of these quick tests and in laboratory use where greater accuracy is needed.

Due to the complexity of the problem, the best way of reducing mycotoxin contamination still remains if it is already addressed much earlier in the feed chain. The process starts with the producer and storer of grain and other feed ingredients and ends with the user and storer of the final mixed feed. It is therefore important that every role player in this feed chain realises and accepts his or her responsibility and understands the factors that affect fungal growth if economic losses attributed to mycotoxin infection is effectively going to be reduced.


References
1. Food safety and quality as affected by animal feedstuff. Twenty second FAO Regional Conference for Europe. Porto, Portugal, 24-28 July 2000.
2. Höhler, D. 2000. A brief survey on important mycotoxins and possible detoxification methods. Feed Tech, Vol.4, number 5/6, pp 44-46.
3. Purdue University Agricultural Communications. Grain Fungal Diseases & Mycotoxin Reference. http//www.isco.purdue.edu/mycotoxins.htm.
4. Purdue University Agricultural Communications. Mycotoxins in feed grains. http//www.isco.purdue.edu/mycotoxins.htm.
 
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