Origin and Discovery of Aflatoxins: The Turkey X Disease Case

A wide range of agricultural products are susceptible to contamination by certain fungi capable of producing harmful secondary metabolites known as mycotoxins. Among them, aflatoxins are particularly important due to their toxic impact on humans, poultry, and livestock. The issue of aflatoxin contamination first came to light in 1960 during a major outbreak of a condition known as 'Turkey X Disease' in the United Kingdom, which led to the death of more than 100,000 young turkeys. Investigations revealed that the cause was a toxin present in peanut meal contaminated with Aspergillus flavus, and this toxin was subsequently identified and named aflatoxin.Read more

Natural occurrence:

Aflatoxins contaminate a wide range of food products including cereals such as maize, sorghum, pearl millet, rice, and wheat; oilseeds like groundnuts, soybeans, sunflower seeds, and cottonseed; various spices including chillies, black pepper, coriander, turmeric, and ginger; as well as tree nuts such as almonds, pistachios, walnuts, and coconuts. Additionally, aflatoxins can be found in milk from animals that have ingested contaminated feed.

Physical and chemical properties:

Aflatoxins are highly toxic, carcinogenic, mutagenic, and immunosuppressive compounds produced as secondary metabolites primarily by the fungi Aspergillus flavus and Aspergillus parasiticus. Among the 18 known aflatoxin types, the most common and significant are aflatoxins B1, B2, G1, and G2. Aflatoxin B1 (AFB1) is usually the most abundant both in fungal cultures and contaminated food products. Pure AFB1 appears as a pale-white to yellow crystalline, odorless solid. Aflatoxins are soluble in solvents such as methanol, chloroform, acetone, and acetonitrile. Typically, A. flavus produces AFB1 and AFB2, whereas A. parasiticus produces AFG1, AFG2, along with AFB1 and AFB2. Minor aflatoxins including M1, M2, B2A, and G2A have also been isolated from cultures of these fungi. Moreover, closely related compounds like aflatoxin GM1, parasiticol, and aflatoxicol are produced by A. flavus. Aflatoxins M1 and M2 are notable as they are metabolites of AFB1 and AFB2, respectively, and are found in the milk of animals that have consumed aflatoxin-contaminated feed.

Chemical reactions of aflatoxins

The interactions of aflatoxins with various physical conditions and chemical reagents have been extensively studied due to their potential use in detoxifying contaminated materials.

Heat:

In a dry state, aflatoxins are highly stable up to their melting point. However, when moisture is present and temperatures are elevated, aflatoxins gradually degrade over time. This degradation can occur in oilseed meals, roasted peanuts, or aqueous solutions at neutral pH. Although the exact degradation products are not fully characterized, it is believed that heat and moisture cause the lactone ring of aflatoxins to open, possibly followed by decarboxylation.

Alkalis:

Under alkaline conditions, hydrolysis of the lactone ring takes place, but this process is reversible. Upon acidification, the lactone ring can reform. At higher temperatures near 100°C, further reactions occur, including ring opening and decarboxylation, potentially leading to the removal of methoxy groups from the aromatic ring. Similar reactions are observed when aflatoxins are exposed to ammonia and various amines.

Acids:

Exposure to mineral acids converts certain aflatoxins by adding water across a double bond in the furan ring, resulting in derivative forms. In the presence of reagents like acetic anhydride combined with hydrochloric acid, these reactions proceed further to form acetoxy derivatives. Other acid reagents, such as formic acid-thionyl chloride, acetic acid-thionyl chloride, and trifluoroacetic acid, also produce similar modified compounds.

Oxidizing agents:

A variety of oxidizing chemicals including sodium hypochlorite, potassium permanganate, chlorine, hydrogen peroxide, ozone, and sodium perborate—react with aflatoxins, altering their molecular structure. These changes often result in the loss of fluorescence characteristic of aflatoxins. However, the precise mechanisms and the structures of the resulting products largely remain unidentified.

Reduction:

Hydrogenation converts certain aflatoxins into less toxic forms by saturating double bonds, yielding specific reduced derivatives. Further reductions with agents like sodium borohydride open the lactone ring and reduce acid and keto groups, leading to the formation of novel reduced aflatoxin compounds.

Biology of aflatoxin-producing fungi

The fungi responsible for aflatoxin production are closely related species that act as saprophytes, growing on decaying plant material in soil and crop residues. They are globally distributed but are more prevalent in tropical regions with variable rainfall, temperature, and humidity. These fungi are characterized by specialized structures that produce spores. Their colonies display distinctive colors and microscopic features which differentiate species, including variations in spore arrangement, size, surface texture, and presence or absence of certain hardened structures.

   Terminal portion of a conidiophore of A. flavus showing the basal portion of the vesicle and distribution of radiation phialides (arrows). X 1000.

Phialides and chains of conidia of A. parasiticus illustrating basipetal development of conidia. Those at the base of the chains (arrows) are least mature. X 3000.