Mycotoxins

Mycotoxins are chemicals that are produced by filamentous fungi that affect human or animal health. By convention, this excludes mushroom poisons. These fungi are called toxigenic fungi. All of these species are deuteromy-cetes (asexual) some of which have a known ascomycetous (sexual) stage. All of the mycotoxins discussed here are secondary metabolites of the fungi concerned, that is, compounds that are produced after one or more nutrients become limiting (1-3). The occurrence of mycotoxins is entirely governed by the existence of conditions that favor the growth of the fungi concerned. Under environmental conditions, different fungal species are favored as diseases of crop plants or as saprophytes on stored crops and sometimes other materials. When the conditions favor the growth of toxigenic species, it is an invariable and unfortunate rule that one or more of the compounds for which the fungus has the genetic potential are produced. Modern methods of agriculture appear to be selecting for increased prevalence of toxigenic strains (4).

With modern studies of toxigenic fungi, it has been appreciated that they can produce many compounds, often from different biosynthetic families. The ecological significance of the occurrence of these mixtures has been a fertile area of study in recent years. As it relates to human and animal toxicology, the potency of the contaminated material is due to the mixtures present (5,6). At the time of the discovery of penicillin and the first wave of fungal-derived antibiotics, the researchers believed that these compounds were active in nature. In the postwar period, secondary metabolites were characterized as everything from waste products to the consequences of "displacement activities" of the fungi. The current view is that these compounds are important as virulence factors and as mediators of interference competition; that is, they exclude competing microbes and animals from the food source (4,7,8).

Mycotoxins have affected human populations since the beginning of organized crop production. Ergotism is discussed in the Old Testament of the Bible. Some claim that the ancient Chinese used ergot for obstetrical purposes 5000 years ago. Many epidemics of ergotism were reported in western Europe from about a.d. 800. The screams of the victims, the stench of rotting flesh, extremities falling off, and death all feature in the descriptions of the disease. Large-scale mortalities persisted into the eighteenth century, when governments and the church promoted methods for the removal of sclerotia (9).

Outbreaks of ergotism have been reported in developing countries in modern times. However, because the cause of the problem—the consumption of ergot-infested grain—is something easily seen by the naked eye, ergotism is now uncommon. During the thirteenth century, rye was replaced by wheat in western Europe. The former species is resistant to Fusarium diseases and, as noted, the latter typically susceptible, leading to the accumulation of trichothecene mycotoxins. These compounds are prevalent in small grains in western Europe (10). In her book Poisons of the Past (11), the American historian Mary Matossian has analyzed the effect of weather and food consumption patterns during the plague epidemics in Europe. She found that plague epidemics took place when there were surpluses of small grains, the favourite food of rats. The occurrence of plague in the Middle Ages is very strongly associated with rainy and humid crop years. She found that the two years prior to the pandemic in Europe in 1378, the weather was extraordinarily rainy.

Although there are hundreds of fungal metabolites that are toxic in experimental systems, only five are of major agricultural importance: deoxynivalenol, aflatoxin, fumonisin, zearalenone, and ochratoxin (5). In grains, the toxins are concentrated into bran fractions during milling (12). All these toxins are stable in the processes typical of food and feed processing (12-15). Animal products can be a minor dietary source of ochratoxin and fumonisin; for the remaining three toxins, animal sources are not important under normal circumstances (16). Milk can contain aflatoxin Ml a derivative of aflatoxin B1 with much lower toxicity (17).

A recent report by the U.S. National Academy of Sciences notes that even with the high-quality food system in the United States, the carcinogenic mycotoxins in American diets may increase cancer rates. This is absolutely the case in many developing countries, where mycotoxins are a major population health problem (18,19). Information on regulations and guidelines for mycotoxins can be found in Van Egmond (20) and Kuiper-Goodman (21), and was discussed at the 1999 FAO Conference on Mycotoxins, in Tunis (http: / /www.fao.org).

A number of mycotoxins that occur from time to time in food in certain parts of the world will be considered. Human diseases associated with uncommon mycotoxins on rice and other crops are described in Pitt (22) and Beardall and Miller (23). There are also fungal toxins that occur in pastures that are not considered in this treatment. Inhalation exposure to mycotoxins is also not covered in this treatment but is a problem in farming and grain handling and in buildings with appreciable mold growth (24—27).

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