New Hope for Improved Food
Safety in Sub-Saharan Africa
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|Scientists at the International Institute of
Tropical Agriculture (IITA) have developed a safe and effective
method for biological control of aflatoxins. These are toxic
chemicals of fungal origin, which contaminate maize and other major
food crops, posing a chronic threat to human health in sub-Saharan
With the new method, strains of the fungi that produce aflatoxin
are overwhelmed through the introduction of related but entirely
harmless strains. These were identified and tested through several
years of meticulous research supported by the German Agency for
Technical Cooperation (GTZ) and carried out in collaboration with
the Agriculture Research Service of the US Department of
Agriculture, University of Arizona in the USA, University of Bonn
in Germany and University of Ibadan in Nigeria.
Researchers found that inoculum containing the beneficial
strains can be produced most efficiently on sorghum grain,
resulting in a dry formulation, which can then be broadcast on
moist soil in farmers' maize fields. Laboratory and field tests
have demonstrated that the harmless strains spread quickly from the
soil to maize plants, where they reduce aflatoxin-producing strains
on maize grain by 91 to almost 100 percent. A single application is
sufficient to control the problem in the crop treated, though a few
additional applications may be required to achieve long-term
control in farmers´ fields. Large-scale testing of the new method
is currently under way in Nigeria.
An insidious threat
Aflatoxins are produced by various fungi, such as Apergillus
flavus and A. parasiticus, which grow as molds on
staple grains and root crops both before harvest and in storage.
Contamination of maize causes particular concern, because it is
sub-Saharan Africa's most important cereal.
The toxins are especially damaging to children. Continuous
exposure has been shown to stunt growth and even contribute to
infant mortality when coinciding with kwashiorkor, a form
of malnutrition caused by dietary deficiency of protein and other
nutrients. The insidious combination of impaired development and
undernourishment accounts for about half of the 4.5 million deaths
of children under the age of 5 occurring annually in sub-Saharan
Aflatoxins are also believed to affect the human immune system,
making people more vulnerable to infectious diseases, such as
malaria and HIV/AIDS. In addition, the toxins are linked to liver
disorders and can act in synergy with the Hepatitis B virus to
cause hepatocellular carcinoma. This is the most common cancer in
sub-Saharan Africa, accounting for as many as 10 percent of adult
male deaths in parts of West Africa.
Aflatoxins further damage the well-being of Africa's rural
families by limiting exports of maize and groundnut in particular.
Grain-importing countries maintain high food quality standards,
with especially strict controls on aflatoxin content. African food
and feed products showing levels of contamination above the
acceptable limits cannot penetrate major grain markets, resulting
in significant loss of agricultural income.
Except when people die of acute poisoning, as happened in Kenya
during 2004-2006, aflatoxins seldom receive adequate attention in
the region, even though they clearly have serious consequences and
are quite widespread. In Benin and Togo, for example, researchers
found that aflatoxin levels are about five times the safe limit of
20 parts per billion in up to 30 percent of household grain stores.
According to other results from a study carried out in those
countries and Nigeria, 99 percent of blood samples collected
randomly from children contained aflatoxins.
In pursuit of a biocontrol strategy
In an effort to reduce aflatoxin contamination, researchers at
IITA and elsewhere have deployed various methods, involving, for
example, modifications in grain drying, storage and food
preparation practices. To complement strategies that have already
proved effective, scientists have also actively pursued in recent
years the option of biological control, building on the
Institute's long and extraordinary record of success in using
this approach to combat major pests such as the mango and cassava
mealybugs, cassava green mite, desert locust and banana nematodes
The biocontrol strategy that appears to be effective against
aflatoxin employs a mechanism that researchers refer to as
"competitive exclusion." This is made possible by the
presence in nature A. flavus populations, not only of
"toxigenic" strains, which produce copious amounts of
aflatoxin, but also "atoxigenic" strains, which lack this
capacity. In order for the strategy to work, researchers must
identify and successfully introduce harmless strains that show a
large competitive advantage over the dangerous ones.
In the resulting biological struggle, explains IITA plant
pathologist Ranajit Bandyopadhyay, "the good strains of A.
flavus virtually eliminate their highly toxic relatives and
ensure that the 'bad guys' cannot re-emerge."
Such a strategy has proved effective for controlling aflatoxin
on cottonseed, groundnut and maize in the USA and on groundnut in
The challenge for scientists at IITA was to identify entirely
safe atoxigenic strains of A. flavus that are indigenous
to Africa and serve effectively as biocontrol agents. For this
purpose, they first collected more than 4,200 samples of the fungus
from different ecological zones of Nigeria. In these they
identified 2,127 distinct strains, of which 1,000 proved to be
atoxigenic. Only 26, though, were selected for further testing.
This involved an extremely laborious series of procedures, one
of them, for example, involving.more than 30,000 crosses between
different strains. The purpose was to group the selected strains
according to "vegetative compatibility" and make sure
they belong to groups consisting only of atoxigenic strains that
cannot cross with toxigenic strains in nature. This procedure
ensured that release of the biocontrol agents in the field would be
absolutely safe, leading to a drastic reduction, rather than an
inadvertent boost, in aflatoxin levels.
From the 14 unique atoxigenic groups identified, one strain each
from eight groups was chosen for further evaluation for ability to
compete with toxigenic strains. When maize grains were inoculated
in the laboratory and field with both a highly toxigenic A.
flavus strain as well as atoxigenic strains, one of the latter
reduced aflatoxin levels by 91 percent and two others by nearly100
In experimental field plots, various atoxigenic strains proved
capable of spreading quickly from the soil on which they were
released to maize plants. One especially promising strain was found
on nearly 99 percent of the maize grains analyzed.
The eight promising strains were further screened for their
ability to get established rapidly after release, continue
spreading and survive in plant debris. Strains showing these
abilities can achieve effective biocontrol of aflatoxin-producing
strains over multiple years, with only a few additional
applications after initial release.
Having identified several excellent candidate strains to serve
as agents of biocontrol, IITA researchers are further testing these
in large-scale trials at various locations in Nigeria. To permit
wide release of these biocontrol agents in the fields, researchers
have developed safe, efficient and effective methods for producing
and applying inoculum. IITA now seeks further support to
disseminate the biocontrol technology as part of a
"basket" of simple aflatoxin management practices that
can reduce aflatoxin levels in Africa's food and improve the
health of its people.
Photos provided by IITA
Releasing aspergillus in the
Aspergillus sporulating on
Cob inoculation being
Infected maize cob from farmer's
For more information, contact Ranajit Bandyopadhyay (email@example.com).