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Aflatoxin producing members of Aspergillus are common and widespread in nature. They can colonize and contaminate grain before harvest or during storage. Host crops are particularly susceptible to infection by Aspergillus following prolonged exposure to a high humidity environment or damage from stressful conditions such as drought, a condition which lowers the barrier to entry.
The native habitat of Aspergillus is in soil, decaying vegetation, hay, and grains undergoing microbiological deterioration and it invades all types of organic substrates whenever conditions are favorable for its growth. Favorable conditions include high moisture content (at least 7%) and high temperature.
Crops which are frequently affected include cereals (maize, sorghum, pearl millet, rice, wheat), oilseeds (peanut, soybean, sunflower, cotton), spices (chile peppers, black pepper, coriander, turmeric, ginger), and tree nuts (almond, pistachio, walnut, coconut, brazil nut).
The toxin can also be found in the milk of animals which are fed contaminated feed.
High-level aflatoxin exposure produces an acute necrosis, cirrhosis, and carcinoma of the liver exhibited by hemorrhage, acute liver damage, edema, alteration in digestion, and absorption and/or metabolism of nutrients.
No animal species is immune to the acute toxic effects of aflatoxins including humans; however, humans have an extraordinarily high tolerance for aflatoxin exposure and rarely succumb to acute aflatoxicosis.
Chronic, subclinical exposure does not lead to as dramatic of symptoms as acute aflatoxicosis. Children, however, are particularly affected by aflatoxin exposure which leads to stunted growth and delayed development. Chronic exposure also leads to a high risk of developing liver cancer, as the metabolite aflatoxin M1 can intercalate into DNA and alkylate the bases through its epoxide moiety.
Detection of aflatoxin in humans
There are two techniques that have been used most often to detect levels of aflatoxin in humans.
The first method is measuring the AFM1-guanine adduct in the urine of subjects. Presence of this breakdown product indicates exposure to aflatoxin in the past 24 hours. However, this technique has a significant flaw in that it only produces a positive result in approximately one-third of positive test subjects. Additionally, due to the half-life of this metabolite, the level of AFM1-guanine measured can vary significantly from day to day, based on diet, and thus is not useful for assessing long term exposure.
Another technique that has been used is a measurement of the AFB1-albumin adduct level in the blood serum. This approach is significantly more accurate, as positive results are generated in 90% of positive test subjects. This test is also useful for measuring long-term exposure, as it remains positive for two to three months.
Aflatoxin in pets
Aflatoxin in dry dog food manufactured by Diamond Pet Foods was responsible for at least 23 dog deaths due to liver failure between Dec 2005 and early 2006. In an April 12 2006 letter FedEx'd from the Department of Health and Human Resources to a manufacturing plant, the FDA warned Gary Schell, president of Schell and Kampeter Inc. of Missouri that independent testing of three samples of incoming corn to their processing plant showed between 90 and 1851 ppb, while paperwork on three (of four samples) showed aflatoxins levels <20 ppb, and other sample was not recorded. The results of this letter are unknown.
Major types of aflatoxins and their metabolites
At least 13 different types of aflatoxin are produced in nature. Aflatoxin B1 is considered the most toxic and is produced by both Aspergillus flavus and Aspergillus parasiticus. Aflatoxin G1 and G2 are produced exclusively by A. parasiticus. While the presence of Aspergillus in food products does not always indicate harmful levels of aflatoxin are also present, it does imply a significant risk in consumption of that product.
Aflatoxins M1, M2 were originally discovered in the milk of cows which fed on moldy grain. These compounds are products of a conversion process in the animal's liver. However, aflatoxin M1 is present in the fermentation broth of Aspergillus parasiticus.
Interaction of aflatoxin with the Hepatitis B virus
Studies have shown that concurrent infection with the Hepatitis B virus (HBV) during aflatoxin exposure increases the risk of hepatocellular carcinoma (HCC). As HBV interferes with the ability of hepatocytes to metabolize aflatoxins, an aflatoxin M1-DNA conjugate exists for a longer period of time in the liver, increasing the probability of damage to tumor supressor genes such as p53. This effect is synergistic with the resulting damage far greater than just the sum of aflatoxin or HBV individually. (Williams, 2004)
Decreasing HBV infection levels through vaccination is an effective and simple approach that can be taken to reduce these harmful synergistic effects, thus decreasing the impact of chronic aflatoxin exposure. This strategy may prove to be highly effective – many regions of the world which have high aflatoxin rates, such as western Africa and China, also have high HBV infection rates.
As of January 2007, there are but two primary manufacturers (as distinguished from re-packers and re-sellers) of pure aflatoxins known:
Customers use these compounds for instance as internal standard when monitoring foodstuffs for aflatoxin contaminants.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Aflatoxin". A list of authors is available in Wikipedia.|