Mold/aflatoxins
Many different fungi may grow as molds on stored grains. Fusarium and Aspergillus fungi
are among the most common grain molds. Not all fungi produce toxins, but
Aspergillus, which produces aflatoxin, is among the most common grain mold fungi.
Aflatoxins are poisonous, carcinogenic byproducts produced during the growth of several
species of the mold fungus Aspergillus. These byproducts are produced as the fungi grow
in feed grains, processed feed, and food products.
In South Dakota, aflatoxins are primarily a problem in corn but can also occur in other grain
crops. Aflatoxins are highly toxic to livestock, poultry, and people. Consumption of low
concentrations by animals sensitive to aflatoxins can lead to death in 72 hours. In general,
at nonfatal levels, the health and productivity of animals fed contaminated feed are seriously
impaired. As a result, the Food and Drug Administration (FDA) has set an action level for
aflatoxins in corn at 20 parts per billion (ppb). Corn containing aflatoxin levels of 20 ppb
or more cannot be sold in interstate commerce, and, in general, should not be fed to young
poultry, swine, and livestock, or to lactating animals, and must not be milled for human
consumption.
Understanding Aspergillus
and Aflatoxin Contamination
Development in the Field
The development of aflatoxins depends on the infestation and growth of the Aspergillus mold
in grain. High temperatures and high humidity favor the infection of corn kernels through the
silks by the Aspergillus fungi. In the southeastern United States and the eastern Corn Belt this
environmental condition occurs more frequently and is the main reason Aspergillus infections
and aflatoxin contamination occur more frequently.
Below-normal soil moisture (drought stress) has also been found to increase the number
of Aspergillus spores in the air. Therefore, when drought stress occurs during pollination,
the increased inoculum load (spores in the air) greatly increases the chances of infection.
Furthermore, drought stress, nitrogen stress, and other stresses that affect plant growth
during pollination can increase the level of aflatoxins produced by Aspergillus fungi.
Often, Aspergillus will grow on the unfilled portions of the ear.
In the past, insect injury to the maturing ear of corn was considered a requirement for
infection to occur. This is now known to be false. However, insect damage to ears provides
wounds that allow Aspergillus to more readily infect the kernels. Insects also transport
Aspergillus spores to the silks and the kernels. Therefore, insect damage, especially during
pollination in drought-stressed corn, can increase the occurrence of Aspergillus and the levels
of aflatoxins.
FS 907
by E. Kim Cassel, Extension Program Leader for Agriculture and Natural Resources
Bill Campbell, Extension Farm Machinery and Safety Specialist, Ag & Biosystems Engineering Department
Martin Draper, Extension Plant Pathologist, Plant Science Department
Bill Epperson, Extension Veterinarian, Veterinary Science Department
Cooperative Extension Service / College of Agriculture & Biological Sciences / USDA
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Aflatoxins
The yellow mold is Aspergillus flavus, a
leading source of aflatoxin. Many molds
may grow in corn and different mycotoxins
are associated with each mold.
SOUTH DAKOTA STATE UNIVERSITY
Serological tests are now considered to be more reliable and
their accuracy has been validated by comparison to more costly
and time-consuming analytical procedures. Serological test kits
using such methods as ELISA do not require specialized
labs, equipment, or training and when conducted according
to manufacturer’s instructions can give accurate results for the
presence (qualification) and amount (quantification) of aflatoxin
in grain samples.
Sampling for Aflatoxins
Regardless of the test procedure used, the single most important
factor for reliable and accurate testing of grain for aflatoxins is
obtaining a representative sample. The ideal sample size should
be at least 10 pounds of corn. The sample should consist of
several smaller subsamples (10 or more 1-pound samples) that
have been taken from different spots and then mixed together.
Handle each bin or truck separately, and take a 10-pound
sample from eachsource.
Place samples in a cloth or paper container that allows air
exchange. Air-tight containers or plastic bags allow condensation,
which raises the moisture content, resulting in the possibility
of continued growth and toxin production of the fungus.
Send or take samples to a testing lab as quickly as possible.
Monday through Wednesday is the preferred time of week to ship
samples for testing. Samples mailed later in the week are more
likely to be delayed in transit. This gives the fungus a chance to
continue to grow and produce toxins. Samples should be properly
identified and include: the source of the sample (truck or bin), the
sender of the sample, full address, and telephone number.
Figure 4: An ELISA test can
detect very small quantities
of specific contaminants in
grain. The development of
color (blue in this test)
indicate the presence of the
toxin. Intensity of the color
can indicate the amount of
toxin present.
Several other factors play a role in the development of
Aspergillus mold and aflatoxin production. Because drought
stress plays such an important role, practices that reduce
drought stress in plants should reduce the levels of infection
and aflatoxin production.
Irrigation has been shown to be very effective in reducing
Aspergillus infection and aflatoxin development, even if done
only during pollination. Tillage practices have not been as
effective and have only been demonstrated to reduce aflatoxin
by subsoiling in areas with hardpans. Occasionally during
droughty periods, hybrids of differing maturities or those
planted early will pollinate during periods when drought stress
is less often observed in South Dakota. Escaping drought with
planting dates and hybrid maturity may differ from one year
to the next.
Time of harvest has also been shown to be important in
influencing the occurrence and levels of aflatoxin because
Aspergillus does not compete well with other molds when corn
is above the 20 percent moisture content. Harvesting corn when
moisture content is above 20 percent followed by rapid drying
to at least a moisture content of 14 percent within 24 to 48
hours of harvest keeps further Aspergillus growth and toxin
production at a minimum.
Aflatoxin Development in Storage
Mature corn that remains in the field or corn that is stored without
adequate drying can be subject to Aspergillus growth and
aflatoxin production. Temperatures between 80F and 100F and
relative humidity of 85 percent (corresponding to 18 percent
grain moisture) are optimum for growth of Aspergillus. Growth
of the fungus is poor below 55F, but if the grain is moist
enough, toxins can still be produced. However, simply reducing
the moisture content to as low as 12 percent does not kill the
fungus and does not reduce the levels of toxins that have
already been produced. If moisture levels rise again above
12 percent anytime during storage, and temperatures are high
enough, then mold growth and toxin production will resume.
It is important to note that conditions favoring the growth of
Aspergillus also favor the growth of other fungi that can have
harmful effects on humans if they are inhaled or ingested while
working in grain handling facilities. Always wear a dust respirator
when working in grain or feed storage and handling areas.
Detecting Aflatoxin Contamination
Once aflatoxin is produced, it is stable. Heat, cold and light
do not affect it. It is also colorless, odorless and tasteless, and
because of the low concentrations involved and the uneven
distribution in grain bins, aflatoxins are difficult to detect.
In the past, elevator operators and buyers used the blacklight
test, but this test simply detects compounds that
fluoresce (aflatoxins and others) and should only be used
to select samples that require further testing. Similarly,
minicolumn tests are no longer recommended, as they were
prone to give false positive results if used improperly.
Preventing Aflatoxin Contamination
Resistance to aflatoxin accumulation in corn kernels has been
recently identified. Hybrids resistant to aflatoxin and other
fungal toxins should become available in the near future.
To minimize aflatoxin contamination of corn products:
• Reduce plant stress.
Use recommended production practices to minimize plant
stress and maximize yields. These include insect, weed, and
disease control practices, and the use of recommended plant
populations and fertility practices. When possible, irrigation
during pollination can decrease predisposition of the crop to
aflatoxin. Plant corn as early as possible, and plant several
different hybrids of different pollination periods to reduce
the chance of environmental stress at pollination in at least
part of the crop. Care should be taken to store uncontaminated
hybrids away from those that are contaminated.
• Harvest corn early and dry it immediately.
Harvesting corn when it is above the 20 percent moisture
content and drying it within 24 to 48 hours to a moisture
content no greater than 14 percent greatly reduces the
infection, growth, and toxin production by Aspergillus.
• Avoid damaged kernels.
Damaged kernels are more likely to become infected with
molds both in the field and in storage. Corn hybrid selection
and insect control can play a role in reducing kernel damage.
Corn hybrids with good husk coverage of the ear have been
shown to have less infection and aflatoxin development.
Also, Bt corn hybrids derived from transformational events
that express the Bt trait in the ears and the silks are less likely
to become infected with toxin-producing fungi as a result
of reduced kernel damage. Adjustment of the combine
to reduce mechanical damage of the kernels at harvest is
a very important means of reducing contamination in storage.
• Store corn at 12 percent moisture content.
Maximum moisture content for corn storage should be
14 percent. Moisture content at or below 12 percent is ideal
for storage of corn, because growth and toxin production
by Aspergillus cannot occur.
• Keep storage and feeding facilities clean.
Aspergillus fungi can survive on residues left in storage areas.
When environmental conditions become favorable, infection
and toxin production can reoccur year after yearin storage
systems that are not properly cleaned and disinfected.
Using Contaminated Corn
Recommended levels are 0 ppb aflatoxins in feed. However,
aflatoxin-contaminated feed can be tolerated by some livestock,
particularly older animals. Obviously, the higher the level of
contamination, the greater the risk in feeding contaminated corn
to animals. Furthermore, continued proper storage is essential
so that aflatoxin levels do not continue to increase in the corn
or feed before use.
Detoxification of feed continues to be an elusive goal. However,
certain feed additives have been successfully used to inhibit
mold growth and to reduce the incidence of aflatoxicosis in
animals. Organic acids such as propionic, sorbic, and benzoic
acids as well as their salts such as calcium propionate and
potassium sorbate, and copper sulfate can be used to inhibit
mold growth in feed. Mineral clays such as zeolite and bentonite
as well as hydrated sodium calcium aluminosilicate (HSCAS)
can bind to aflatoxin, protecting animals from absorbing the
toxin that may be in the feed. These products, according
to FDA rules, cannot as yet be labeled as mycotoxin binders,
and are sold as anti-caking and free-flow feed additives.
There are no clear-cut safe levels for different animal species
regarding their resistance or tolerance to aflatoxins.
The following section on aflatoxicosis and ruminants and
the general guidelines for dealing with aflatoxin-contaminated
feed may assist you in deciding whether to assume the risk,
Aflatoxicosis and Livestock
Aflatoxicosis is a disease caused by the consumption of
aflatoxins. Aflatoxins are secondary mold metabolites produced
by some strains of Aspergillus flavus and other related species
of Aspergillus fungi. The four most common aflatoxins are B1,
B2, G1, and G2. Contaminated grains and grain byproducts are
the most common sources of aflatoxin. Corn silage may also be
a source of aflatoxins, because the ensiling process does not
destroy toxins already present in silage.
Aflatoxins are metabolized in ruminants by the liver and are
excreted in the bile. Aflatoxin B1 is the most potent mycotoxin
(toxic substance produced by a mold) to affect cattle.
B1 increases the apparent protein requirement of cattle and is
a potent carcinogen (cancer causing agent). When significant
quantities of B1 are consumed, the metabolite M1 appears
in milk within 12 hours. Research suggests M1 is not as
carcinogenic or mutagenic as B1, but it does appear to be as
toxic as its parent compound.
Symptoms
Beef and dairy cattle are more susceptible to aflatoxicosis than
sheep or horses, although other mycotoxicoses occur in these
species, such as facial eczema in sheep and leukoencephalornalacia
in horses. Young animals of all species are more
susceptible than mature animals to the effects of aflatoxin.
Pregnant and growing animals are less susceptible than young
animals, but more susceptible than mature animals.
Feed refusal, reduced growth rate and decreased feed efficiency
are the predominant signs of chronic aflatoxin poisoning.
In addition, listlessness, weight loss, rough hair coat and mild
diarrhea may occur. Anemia along with bruises and subcutaneous
hemorrhage are also symptoms of aflatoxicosis. The disease
may also impair reproductive efficiency, including abnormal
estrous cycles (too short and too long) and abortions. Other
symptoms include impaired immune system response, increased
susceptibility to disease, and rectal prolapse.
Pathology
Clinical laboratory findings vary with the animal species, level
of aflatoxin in the ration, and the duration of feeding. There are
no consistent diagnostic changes in hematocrit, hemoglobin,
and differential cell counts in animals fed aflatoxin.
Leukocytosis may occur in animals with secondary bacterial
infections. Serum bilirubin levels may be elevated and typically
serum protein levels are decreased.
Lesions observed at necropsy related to either acute or chronic
liver disease are dependent upon the level of aflatoxin and the
duration of feeding. A majority of acute liver damage observed
has been the result of experimentally high doses, while chronic
liver damage is a more common field observation. The liver is
usually pale tan, yellow or orange. Hepatic fibrosis and edema
of the gallbladder may also be observed.
Diagnosis
The diagnosis of aflatoxicosis is often difficult because of the
variation in clinical signs, gross pathological conditions and the
presence of infectious diseases due to the suppression of the
immune system. On the farm, more than one mold or toxin
may be present in the contaminated feed, which often makes
definitive diagnosis of aflatoxicosis difficult.
The prognosis of aflatoxicosis depends upon the severity of
liver damage. Once overt symptoms are noticed the prognosis
is poor. Treatment should be directed at the severely affected
animals in the herd and further poisoning prevented.
Treatment
Aflatoxicosis is typically a herd rather than an individual cow
problem. If aflatoxicosis is suspected, the ration should be
analyzed immediately. If aflatoxins are present, the source
should be eliminated immediately. Levels of protein in the
ration and vitamins A,D,E,K and B should be increased as
the toxin binds vitamins and affects protein synthesis. Good
management practices to alleviate stress are essential to reduce
the risk of secondary infections. Secondary infections must
receive immediate attention and treatment.
Prevention
Aflatoxicosis can only be prevented by feeding rations free
of aflatoxin. Preventing aflatoxin contamination is outlined
on the preceding page, but since preventing contamination is
not always possible, here are a few keys facts to remember
when dealing with contaminated feeds in animal rations:
• The recommended feeding level is 0 parts per billion (ppb).
• The level of aflatoxin an animal can tolerate will depend upon
the age and sex of the animal, its health status, and overall
management level of the farm.
• To avoid contamination of milk, lactating dairy cattle should
not receive more than 20 ppb in the total ration.
• Calves should not receive milk from cows fed in excess
of 20 ppb, because they can ingest aflatoxin from the milk.
• Beef cattle can tolerate slightly higher levels of aflatoxin, but
yearlings and mature cows should not receive more than 400
ppb in the total ration. Weanlings should not receive more
than 100 ppb in their total daily ration.
• Poultry and swine are more sensitive to aflatoxin contamination.
Under no circumstances should these livestock species be
fed more than 20 ppb aflatoxin in their daily rations.
The above are only guidelines. This does not suggest that
feeding at these levels or below will reduce or eliminate the
potential for aflatoxicosis. There are no clear cut safe feeding
levels. Safe levels vary with each individual animal. Remember
that ingestion of aflatoxins at levels even lower than those
listed in the guidelines may cause some undesirable side effects
and depends on such factors as age, sex, and general health of
the animals. To feed at a level other than 0 ppb is a risk assumed
by the person making the decision to do so. In all cases,
monitor animal health closely and discontinue the use of
contaminated feed immediately if undesirable effects are noticed.
Conclusion
Aflatoxins are highly toxic to livestock, poultry, and people.
Even when fed at nonfatal levels, aflatoxin can seriously impair
animal health and productivity. For lactating dairy cattle, do not
exceed 20 ppb aflatoxin in rations to avoid exceeding the Food
and Drug Administration level of 0.5 ppb in milk. Aflatoxin is
just one of many mycotoxins that can adversely affect animal
health and productivity.
Text adapted with permission from University of Maryland
Cooperative Extension Service Fact Sheet No. 444 & 445.
