Model for sorption of mycotoxins onto the surface of clay.
Current research into mycotoxin reduction involves the mixing of
certain types of clays with the contaminated grain. Image credit:
Dr. Tim Phillips, TAMU.
Molds have served humans for centuries in the production of
foods (e.g., ripening cheeses and sausages). These living
organisms produce various fungal metabolites, some of which we
use as antibiotics (i.e., penicillin), and some with the
potential to produce severe adverse health effects.
The toxic substances which are produced by molds are termed
mycotoxins. A mycotoxin is produced under special conditions of
moisture and temperature. Not all types of mold (fungi) can
produce mycotoxins. In fact, some fungi are able to produce
mycotoxins only in special conditionssuch as at specific levels
of moisture, stress and the correct temperature. Even those
with the ability to produce mycotoxins may not produce them all
the time. The absence of mycotoxins doesn't ensure the absence
of fungal spores, so it's possible for fungi to "appear" when
the temperature and humidity are right.
Mycotoxins are defined as "secondary fungal metabolites that
are produced from primary metabolites and secreted into the
microenvironment around the mold, and when consumed or absorbed
by animals and humans can cause illness or behavioral changes"
(Cousins, et al., 2005). Mycotoxins represent a diverse group
of chemicals that can occur in and on a variety of plants from
which food is derived, but those representing the greatest
hazard occur in commodities, which tend to be consumed in large
amounts by a large percentage of the population (such as
peanuts and the cereal grains, barley, corn, rye and
wheat).
Poisoning by a mycotoxin is termed mycotoxicosis. Active
research on mycotoxins is a fairly recent development, since
reports of mycotoxicosis were relatively infrequent and
occurred in remote places of the globe where proper
agricultural practices were not exercised. As a result,
interest in mycotoxins remained only a curiosity for
researchers until publication of a series of reports in
1960-1963. These reports linked the death of turkeys in England
(so-called "turkey X disease") and ducklings in Uganda with the
consumption of peanut meal feeds containing mold products
produced by the contaminant Aspergillus flavusa condition
termed aflatoxicosis.
The additional discovery of aflatoxin metabolites (e.g.,
aflatoxin M1 found in the milk of cows consuming
aflatoxin-contaminated feed) led to more intensive studies of a
number of mycotoxins and to the identification of a variety of
these toxins associated with adverse human and animal health
effects. Aflatoxins were first identified by their blue or
green fluorescence with ultraviolet light, but now, subgroups
are identified as B1, B2, G1, and G2. It is also now known that
at least one additional mold produces aflatoxini.e.,
Aspergillus paraciticus.
Aflatoxins are most often described as potent liver toxins,
but their effects in animals vary with species (and breed),
dose, length of exposure and nutritional status. Among the
various mycotoxins, the aflatoxins have been the subject of the
most intensive research because of the extremely potent
hepatocarcinogenicity and the observed toxicity of aflatoxin B1
in ratsthe most commonly used experimental model. Suppression
of the immune system seems to be a common effect, making the
animal more susceptible to newly-acquired disease, a senescent
disease becoming fulminant or an infection resulting from
stress or trauma. The losses sustained from companion animal
toxicity and death are incalculable.
As noted earlier, aflatoxin is a product of the mold growing
on the commodity. The two primary sources of mold contamination
emerge during the growth of the plant, with certain conditions
favoring Aspergillus sp. growth such as drought and other
stresses which allow insect damage and mold attack, as well as
inadequate storage conditions.
Since the discovery of their potential threat to health,
progress has been made in decreasing the level of aflatoxin in
specific commodities. Control measures include use of resistant
crops (with genes inserted to produce natural pesticides
preventing the survival of insects that carry mold spores on
their bodies) and ensuring adequate storage conditions
(primarily by lowering the moisture content of the grain,
preventing mold growth). These precautionary steps are followed
by careful monitoring of susceptible commodities for aflatoxin
level and banning the lots that exceed the regulatory "action
level" of 20 parts per billion for aflatoxin B1 (because it is
the most toxic type and is regarded as the "sentinel" substance
for all other aflatoxins).
Once a batch is found to contain a higher than tolerable
amount of aflatoxin, the batch must be discarded in its
entirety. Under certain circumstances, a batch may be
rehabilitated with the use of radiation to destroy the
mycotoxin, or the batch may be blended with mycotoxin-free
grain to lower the overall amount. Current research into
mycotoxin reduction involves the mixing of certain types of
clays with the contaminated grain to absorb the mycotoxin, but
this technique is still in the experimental stages.
Aflatoxin represents just one type of mycotoxin produced by
Aspergillus. It is known that another mycotoxin, cyclopiazonic
acid (a muscle tissue toxin), is often produced concomitantly
with aflatoxin, but the effects of aflatoxin (especially at
high levels) are so dramatic that the effects of cyclopiazonic
acid are not seen.
Aspergillus flavis represents only one of approximately
70,000 known species of fungi. There are potentially hundreds
of thousands of unique mycotoxins, but only 300 have been
positively identified. Discovery of a new mycotoxin most often
follows a tragic poisoning after exposure to large amounts; but
because mycotoxins usually occur in small amounts, it is
possible they account for a number of insidious conditions
(including birth defects, impaired immunity and decreased
disease resistance, cancer promotion, mental illness and
generalized, non-specific conditions resulting in decreased
life span of man and animals).
At this point in the development of knowledge of mycotoxins,
researchers are unwilling to guess what types of mycotoxins
might be discovered and what the sum of their effects might
be.