Dr Edson Ncube: Agricultural Research Council – Grain Crops, Potchefstroom

Mycotoxins are by-products of fungal growth that can survive various forms of heat treatment and they are harmful to both humans and animals. Fungal species such as Aspergillus, Fusarium, and Penicillium infect grain crops such as maize, groundnut and sorghum to the extent that the yield and quality of the harvest is unsuitable for human and livestock consumption.

OCCURRENCE OF MYCOTOXINS

Fungal pathogens produce mycotoxins during pre-harvest and post-harvest stages of grain production (Table 1) and are detrimental to trade in grains. Mycotoxin occurrence is impacted by weather/environmental conditions, and the change in climatic conditions has a bearing on mycotoxin contamination of grain.

For instance, in areas with hot, humid, tropical climates, Aspergillus fungus will be predominant, producing aflatoxins in maize and groundnut in the field. In subtropical areas, the fungus Fusarium will predominate, thereby producing fumonisin mycotoxins in maize. Penicillium as well as Aspergillus are more commonly associated with grain during storage. The major mycotoxins affecting maize, sorghum, millet and groundnuts in southern Africa include fumonisins, aflatoxins, trichothecenes, ergot alkaloids and zearalenone.

Other risk factors for mycotoxin occurrence include Busseola fusca stem borer (Fig. 1) infestations, tillage and storage practices. For instance minimum and no tillage increase the risk of Aspergillus and Fusarium infection, as fungal spores from the previous crop remain on or near the soil surface where they reproduce and easily infect crops through wounds inflicted by insects as well as through the roots.

ECONOMIC AND HEALTH IMPACT OF MYCOTOXINS

Exposure to human food or animal feed contaminated with mycotoxins through ingestion, inhalation or absorption through the skin are known to result in reduced performance, supressed immunity, and sickness or death in humans and animals. Fumonisins have been associated with oesophageal cancer in humans in the Eastern Cape province of South Africa and the Cixian, Linxian and Shangqiu counties of China.

A high incidence of neural tube defects (NTD) in infants whose mothers were consuming fumonisin-contaminated maize during pregnancy in Mexico has also been reported. However, it is known that maternal folic acid protects the foetus against NTD, and fortification of all enriched cereal products with folic acid reduces the occurrence of NTD.

Aflatoxins suppress the immune system of susceptible populations in humans and it also causes liver cancer in humans. Acute aflatoxin poisoning has resulted in hundreds of human deaths in Kenya during the 2004/05 cropping season through the consumption of aflatoxin-contaminated maize. It has also been shown that in Benin and Togo, children exposed to aflatoxin had a 2 cm lower height gain than those exposed to the lowest level.

Each class of mycotoxins has its own specific economic and health impact (Table 1). Commercial grain producers are exposed to mycotoxin contamination resulting in market restrictions since aflatoxin and fumonisin contamination levels are legislated in many countries including South Africa.

This is however difficult to enforce locally since techniques for mycotoxin analyses are expensive and skilled personnel are not located throughout the country. In the subsistence farming sector, however, the food produced is consumed by the producers. This is where the mycotoxin problem becomes a major threat to human health. These farmers do not have access to technology used to screen grain for mycotoxins.

In many cases where yields are restricted by drought and other biotic stresses, the farmers have no choice but to consume contaminated grain. Moreover, in some cultures, visibly mouldy grain is used to brew traditional beer, resulting in high exposure to mycotoxins. In this context, the Agricultural Research Council conducted a study in Limpopo, Mpumalanga, KwaZulu-Natal and the Eastern Cape provinces of South Africa to determine mycotoxin levels in subsistence farming systems.

High levels of fumonisins exceeding the maximum permitted levels of 2 parts per million or µg/g set by the World Health Organisation and The Department of Health in South Africa have been found in maize destined for human consumption in certain areas in KwaZulu-Natal, Limpopo and the Eastern Cape provinces (Fig. 2). This has necessitated interventions in mycotoxin awareness. Therefore it is important that field crops such as maize and sorghum are monitored for mycotoxin contamination through surveys in various jurisdictions in order to promote trade in safe commodities as well as the adoption of uniform regulations for the management of mycotoxin contamination in food and feed.

Table 1. Mycotoxins produced by different fungal genera in different crops and the impact of these mycotoxins in animals.

REDUCING EXPOSURE TO MYCOTOXINS

• Grain sorting (separating visibly mouldy grain from non-mouldy grain) – do not consume mouldy grain.
• A recent study by Dr Ncube (ARC – Grain Crops) showed that planting Bt (genetically modified) maize reduces fumonisins and ear rots (Fig. 3) in South Africa – control of stem borers.
• Practicing crop rotation and application of nitrogen fertilizers.
• Nixtamalisation (lime water treatment of maize grain prior to cooking as is done in Mexico) – removes the outer layer of the grain where mycotoxins are concentrated.
• Use of mycotoxin binders in animal feed. Binders are indigestible substances designed to capture the mycotoxins before they reach the digestive tract of livestock. The binders include diatomaceous earth, yeast cell walls, and bentonite.
• Storage practices such as drying the grain to a moisture level below 16%, use of hermetic (air-tight) storage facilities such as hermetic bags, plastic or metal tanks, control of weevils and the larger grain borer are also important for the management of mycotoxins during storage. The control of weevils, the larger grain borer and other pests can also be achieved through the use of diatomaceous earths.

CONCLUSION

With maize and to a lesser extent sorghum being staple foods in Southern Africa, the high intake of maize per person per day highlights the relevance of mycotoxin contamination, given that the recommended daily intake of food contaminated with mycotoxins were calculated based on the average intake of maize in developed countries where human consumption of maize is far less than in developing countries.

The quality of grain consumed thus determines the quality of life. For instance, reduced immunity as a result of consumption of mycotoxin-contaminated grain could increase the burden of human diseases in rural areas.

Therefore, it is important to determine the extent of mycotoxin contamination of grain produced by small-scale farmers since mycotoxin contamination is sporadic and seasonal.

• External funding is required in order to address the mycotoxin challenge in rural areas.
• More research over several seasons is required to determine the extent of mycotoxin contamination of food in rural areas.

For more information, contact Dr Edson Ncube at ncubee@arc.agric.za or +27 18 299 6374

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