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  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 . 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].
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 . 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 .
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 .
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,
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 .
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 .
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 . 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 . 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.
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.