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Public Health Threat Posed By Aflatoxins

Toxic chemicals derived from fungi (known as mycotoxins) contaminate as much as 25% of the world's food supply. One of the most important of these toxins from the point of view of human health are the aflatoxins, which are produced by the fungi, Aspergillus flavus and Aspergillus parasiticus. Contamination of foods by these fungi occurs predominantly, although not exclusively, in developing countries with warm and humid climates, where they contaminate a variety of oilseeds and cereal crops, which are dietary staples in parts of the developing world. The foodstuffs most often and most heavily contaminated are maize and groundnuts (peanuts). The crops are contaminated both during growth and as a result of imperfect storage. Infestation of crops by Aspergillus occurs particularly when the crops are damaged by drought, high temperatures, or insect-induced or other forms of injury. Infestation during storage occurs because of poor storage conditions, such as the foodstuffs being damaged by too much moisture or heat, or by insects or rodents. The level of contamination in developing tropical and subtropical countries may exceed the regulatory limits (20 parts per billion or lower) by one or two orders of magnitude.

The most important public health threat posed by the aflatoxins is that they cause cancer in humans and many animal species. In fact, aflatoxins are among the most potent naturally occurring chemical carcinogens known to man. The most potent of the aflatoxins, aflatoxin B1,, was classified as carcinogenic to humans by the International Agency for Research on Cancer in 1993. The form of cancer produced by exposure to aflatoxins is cancer of the liver (hepatocellular cancer). This is one of the most common cancers world wide and is an extremely severe form of cancer because it is seldom amenable to surgical removal and it does not respond to other forms of treatment. Other toxic effects of the aflatoxins are acute liver injury and suppression of immunity resulting in an increased susceptibility to infections.

There are four aflatoxins, aflatoxin B1, B2, G1, and G2. Of these, the most potent cancer-causing agent is aflatoxin B1.

Epidemiological evidence is available that shows a strong direct correlation between high dietary exposure to aflatoxin B1 and a high incidence of hepatocellular cancer in humans. One way in which aflatoxin B1 causes hepatocellular cancer has been described. It does so by causing a mutation in one of the most important (if not the most important) tumour suppressor genes (anti-cancer genes). This mutation results in the complete inactivation of the gene and therefore facilitates cancer development.

There is a synergistic (additive) effect between the cancer-causing properties of aflatoxin B1 and another common and important cancer-causing agent in southern Africa, viz., the hepatitis B virus. Chronic infection with this virus occurs in approximately 10% of Black South Africans, including children. Geographical regions with high dietary exposure to aflatoxins are also frequently regions with high hepatitis B virus carrier rates. Regions in which both of these risk factors are present have hepatocellular cancer rates as high as 113 per 100,000 of the population per annum compared with less than 3 per 100,000 of the population per annum in regions where neither is common. Hepatitis B virus infection is predominantly acquired in early childhood and it poses a very high risk of later causing hepatocellular cancer (the risk may be as high as 100 times greater than the risk in someone not chronically infected with the virus).

Because of the frequency of hepatitis B virus infection and its devastating consequences, all children born in South Africa since April 1995 have been immunized against hepatitis B virus. This will prevent those children from becoming infected with the virus. However, those children born before this time (or who are not vaccinated for one or other reason) remain at risk of becoming chronic carriers of the virus and developing liver cancer. Therefore, virtually the entire school-going population is still at risk. Because of the synergistic risk of liver cancer between hepatitis B virus infection and aflatoxin exposure, those children who are carriers of the virus and get exposed to aflatoxin as a result of eating peanut butter contaminated by the toxin will be at an even greater risk of developing hepatocellular cancer.

The Regulations Governing tolerances for Fungus-Produced Toxins in Foodstuffs (GN. R.313 of 16 February 1990) promulgated in terms of the Foodstuffs, Cosmetics and Disinfectants Act, 1972 (Act 54 of 1972) limit levels of aflatoxin in foodstuffs. According to the regulations, foodstuffs are deemed contaminated if they contain more than 10 micrograms per kilogram (g/kg) of aflatoxin, of which not more than 5 g/kg may be aflatoxin B1.

Issued by the South African Department of Health. October 2004. (Update 2017 imminent)

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