Chemically Speaking - September 2006
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Mycotoxins and Bt Corn
Transgenic Bt corn contains a gene from the soil bacterium Bacillus thuringiensis that encodes for a crystal (Cry) protein that is toxic to common lepidopteran corn pests. Because of reduced pest damage, one indirect benefit of Bt corn is lower levels of mycotoxin contamination. Mycotoxins are secondary metabolites of fungi that can be toxic, carcinogenic, or both to humans and animals. Insect damage is one factor that predisposes corn to mycotoxin contamination, because insect herbivory creates kernel wounds that encourage fungal colonization, and insects themselves serve as vectors of fungal spores.
Two of the most important mycotoxins in corn are fumonisins and aflatoxins. Fumonisins are produced by the fungi Fusarium 
verticillioides and Fusarium proliferatum. Consumption of fumonisin has been associated with elevated human esophageal cancer incidence in various parts of Africa, Central America, and Asia and among the black population in Charleston, South Carolina. Because fumonisin B1 reduces uptake of folate in different cell lines, fumonisin consumption has been implicated in neural tube defects in human babies. Fumonisins can be highly toxic to animals, causing diseases such as equine leukoencephalomalacia (ELEM) in horses and porcine pulmonary edema (PPE) in swine. Aflatoxins are produced by the fungi Aspergillus flavus and Aspergillus parasiticus, and are the most potent chemical liver carcinogens known. Acute aflatoxicosis, characterized by hemorrhage, acute liver damage, and death, can result from extremely high doses of aflatoxin. In poultry, aflatoxin consumption results in liver damage, decreased egg production, inferior egg-shell quality, inferior carcass quality, and increased susceptibility to disease. In cattle, the primary symptoms are reduced weight gain, liver and kidney damage, and reduced milk production.
Where insect pests are present, Bt corn has been shown to have lower levels of certain mycotoxins than non-Bt isolines. In the Corn Belt region of the United States, when insect damage from the European corn borer or southwestern corn borer is high, fumonisin concentrations are significantly lower in Bt corn compared with their near-isogenic, non-transgenic counterparts. In France, Italy, Turkey, and Argentina, Bt corn has been shown in field trials to have significantly lower fumonisin levels than non-Bt isolines. Compared with fumonisin, insect pest damage is less strongly correlated with aflatoxin concentrations in corn. The insects that are controlled by Bt corn are not as important in predisposing plants to infection by A. flavus as they are for F. verticillioides and F. graminearum; and A. flavus can infect corn not just through kernel wounds caused by insects, but through the silks. Hence, field tests of aflatoxin reduction in Bt corn show a mixed record.
A model to assess the economic benefits in the United States due to Bt corn’s reduction of mycotoxins identified three classes of economic impacts from mycotoxins: market effects, animal health, and human health. High quality corn (i.e., low levels of mycotoxin) can be sold as human-food-grade corn at the highest market price. Corn contaminated with levels of mycotoxins between the highest-permitted levels of food and feed can be sold for animal feed at a lower price, and corn with high levels of mycotoxins is either sold for non-food-non-feed uses at an even lower price or rejected outright. The proportions of the total crop that are rejected at each of these levels depend on the national or international standards for mycotoxins in food and feed. In the United States, where roughly a quarter of total field corn acreage is planted with Bt corn, the annual benefits that Bt corn provides in terms of lower fumonisin and aflatoxin contamination are estimated at about $23 million. It is likely that animal and human health benefits of Bt corn would be more prominent than market gains in areas such as Latin America and sub-Saharan Africa, and the northern regions of China where corn is a staple in animal and human diets and mostly exchanged locally. In the future, as mycotoxin standards may become stricter and thereby harder to meet, mycotoxin-control technologies such as Bt corn may grow in popularity among corn-exporting nations worldwide. (ISB News Reports, 9/7/06).
