Aflatoxins

Main types of aflatoxins

Aflatoxins are named for their respective innate fluorescent properties². Aflatoxin B₁ and B₂ exhibit a blue fluorescence while Aflatoxin G₁ and G₂ exhibit a green fluorescence. Aflatoxin B₁ is considered the most toxic aflatoxin. This mycotoxin is metabolized in the liver, where it forms secondary toxic metabolites like the hydroxylated form aflatoxin M₁.^{2, 3, 4, 5}

• Aflatoxin B₁ and B₂ are mainly produced by A. flavus and A. parasiticus
• Aflatoxin G₁ and G₂ are primarily produced by A. parasiticus
• Aflatoxin M₁ is the metabolized form of aflatoxin B₁ in humans and animals

Production and occurrence

Production of aflatoxins is greater during hot and humid conditions (optimally with day temperatures around 32°C and night temperatures around 24°C).^{2, 5, 6} Aflatoxins often develop during storage, though contamination of food and feed material can also occur in the field e.g. on crop parts damaged by pests or drought. Inadequately dried material or high humidity contribute to the production of aflatoxins in storage.^{2, 5, 7}

The fungi that produce aflatoxins spread by spores (microconidia) through rain splash, wind and insects. Spores germinate and the fungi can penetrate plant material often facilitated by pest or other damage.² Grains that are stored with high moisture content (e.g. >14%), at high temperatures (>20°C) and with damage can potentially be contaminated (Table 1).² Because individually damaged grains can have a high aflatoxin content , there can be a very heterogeneous distribution of aflatoxins and sampling needs to be conducted carefully to avoid missing a contamination problem.

Table 1. Conditions favoring Aspergillus flavus growth

Aflatoxins are very stable compounds and resist several food processing techniques like roasting, extrusion, baking and cooking, hence they represent a severe risk for humans and animals.^{5, 6, 7, 8}

A ten-year survey of more than 74,000 samples reveals that aflatoxin B1 is most prevalent in samples gained from South Asia (82.2%), Sub-Saharan Africa (76%), Southeast Asia (57.4%) and Southern Europe (28.9%). The most affected crops analyzed in the study were maize and rice with a prevalence of 24% and 31% and a median concentration of 4 ppb and 5 ppb, respectively.⁹ Also foodstuff including dried fruits (figs), coffee, spices and peanuts are regularly contaminated with Aflatoxins.

Toxicity and effects

Aflatoxin B1 is considered the most toxic aflatoxin. The main target organ for aflatoxins is the liver where it is metabolized and forms secondary toxic metabolites like the hydroxylated form, aflatoxin M₁.^{2, 3, 4, 5} This mycotoxin is excreted in the urine and in the milk. Studies have shown that aflatoxin M1 can be detected in milk just 12-24 hours after exposure.^{2, 5} Additionally, a consistent aflatoxin M1 presence in milk occurs after 3-6 days of continuous consumption of aflatoxin B1 contaminated feed.^{2, 5, 8}

Aflatoxins are genotoxic compounds and target several organs like the liver, kidneys, reproductive and immune systems.^{2, 5, 6, 7} They are potent carcinogens for humans and all animal species investigated, and young animals have been shown to be more susceptible than mature ones.¹⁰ These mycotoxins were included in the International Agency for Research on Cancer’s (IARC) group 1 (substances that are highly carcinogenic for humans) in 2002.^{5, 8} Aflatoxins are the only mycotoxins that are strictly regulated in markets like the EU and USA.^{3, 4, 8} 

Aflatoxins in corn (maize)

In 1920, Aspergillus flavus was reported to cause an ear mold of corn, but it took up to the 1960s until the fungus could be recognized as toxigenic when it was shown to be associated with Turkey X disease.² 

The fungus can be identified by the yellow-green or grey-green spore masses visible at sites of kernel damage. Heavily damaged kernels that are cracked open may fluoresce bright greenish-yellow (BGYF) and provide a “presumptive” indication of the presence of aflatoxin. Individual kernels of corn may contain as many as 400,000 ppb (μg/kg) of aflatoxin. Therefore, sampling is very important in analysis for levels of contamination in bulk grain lots,² as aflatoxin contamination can result in dramatic economic losses.¹¹

Aflatoxin M₁ in milk

Aflatoxin M₁ is a hydroxylated metabolite of aflatoxin B₁, which was first found present in milk and can be detected in urine as well. It is not produced directly by Aspergillus flavus or any other aflatoxin-producing fungus, therefore, it is not considered as a contaminant of feed grains but of milk and milk products. Aflatoxin M₁ is very stable at high temperatures.

Regulations

Counteraction

Aflatoxins are primarily considered field mycotoxins. Different strategies can be used to reduce the levels and occurrence of aflatoxins in agricultural products.

These strategies are divided into two methods:

1. Prevention: management systems to reduce crop contamination in the field and during storage
2. Detoxification: degradation or removal of aflatoxins from crops, adsorption in the gastrointestinal tract

Detoxification

The prevention of aflatoxin formation in the field and during storage is the first step to reduce the risk of aflatoxin contamination. Detoxification strategies aim to deactivate, destroy or remove the toxin and fungal spores, while retaining the nutrient value and acceptability of the feed by the animal. Physical and chemical processes can be effective against aflatoxins but can be time consuming and costly. Biological methods include the use of adsorbant materials to prevent aflatoxicosis. These compounds bind aflatoxins in the gastrointestinal tract, thus reducing the amount of aflatoxins entering the bloodstream.

Only Bentonite with an aflatoxin B1-binding capacity above 90% is authorized by the European commission as a substance for reduction of the contamination of feed aflatoxin B₁ for ruminants, poultry and pigs (European Commission Regulation (EC) No 1060/2013).¹⁴