Aflatoxins (AfTs) are the most important mycotoxins worldwide, for toxicity, occurrence and economic impact. Different forms of AfTs exist and the main types are aflatoxin B1 (AfB1) which is the major toxin produced, B2 (AfB2), G1 (AfG1), and G2 (AfG2); farm animals metabolize these forms and transform them in the less harmful aflatoxins M1 (AfM1)andM2 (AfM2),which are found in milk, meat and other dairy products. AfTs are produced primarily by the fungi Aspergillus flavus (AfB1 and AfB2) and A. parasiticus (AfB1,AfB2,AfG1 and AfG2). Other Aspergillus species, viz. A. nomius, A. sergii, A. bombycis, A. minisclerotigenes, A. parvisclerotigenus, A. pseudocaelatus, A. pseudotamari, and A. ochraceoroseus also are aflatoxigenic (Varga et al., 2011), but their significance for mycotoxin occurrence is negligible. Generally regarded as saprophytes, Aspergillus spp. grow on a large number of substrates. Under favorable conditions, some species exhibit limited parasitic abilities and can colonize crops in the field. Aspergillus flavus and A. parasiticus are xerophyte species that can grow well and proliferate in environments with low water activity (aw ≤ 0.85) and under drought condition.
AfT contamination occurs in major crops such as maize, peanuts, cottonseed, pistachios, figs, tree nuts, cereal grains and spices, and derived foods and feeds (Council for Agricultural Science and Technology CAST, 2003). In maize, A. flavus develops on the surface of the ears, with the infected parts that become covered with masses of gray-green spores. AfTs may be produced by the fungus in the field until the kernel reaches about 15% of moisture. Cottonseed, may become contaminated with AfTs if the seed bearing capsules are damaged in conjunction with high humidity and warm temperatures before or after harvest. Tree nuts such as pistachios and almonds may also be infected following injuries, such as the splitting of hulls. In peanut A. flavus and A. parasiticus cause a disease known as yellow mold, and high levels of AfT contamination may result following relatively high temperatures and moderate humidity. Infection of crops by aflatoxigenic species and production of AfTs in the field are affected by various factors including environmental conditions (hot and humid weather), drought plant stress in conjunction with high temperatures, mechanical injuries caused by plant grazing insects, and plant genotype (Diener et al., 1987; Council for Agricultural Science and Technology CAST, 2003). Williams et al. (2004) defined the sub tropical and warm temperate regions located at latitudes between 401N and 401S as the “hot zones” for AfT risk, and estimated that 4.5 billion people living in those areas are chronically exposed to AfTs. It is expected that in the future temperate regions will become more subjected to AfT outbreaks, because of global Earth warming. AfTs often occur together with cyclopiazonic acid, another A. flavus mycotoxin, in groundnuts and with cyclopiazonic acid and the Fusarium toxins fumonisins in maize.
Usually, field contamination increase during postharvest activities, e.g., crop drying, or in storage if conditions, particularly moisture, are favorable to molding. However, in some cases, molding and AfTs contamination can occur even in the absence of previous field infection, if the commodity is stored improperly. AfTs are temperature-stable compounds and can resist to heat treatments used for food and feed processing, such as ultrahigh-temperature (UHT) treatment, pasteurization, roasting, and baking, and also to cold storage.
The AfTs are potent liver toxins and most animal species exposed to AfTs show signs of liver disease, ranging from acute to chronic. AfB1 is a potent hepatocarcinogen and is classified as a Group 1 compound (carcinogenenic to humans) by the Inter national Agency for Research on Cancer (International Agency for Research on Cancer IARC, 2012a). AfB1 is also an immune suppressor and cause malnutrition and growth impairment in both humans and animals (Rushing and Selim, 2019). The kidney can also be affected in pigs and goats.
AfTs have been linked to cases of acute poisoning and chronic aflatoxicosis in humans and animals (Pitt et al., 2012). In humans, AfTs have caused outbreaks of acute hepatitis. The association of ingestion of AfT-contaminated grains with inflammation of the liver is well supported by epidemiological evidences, most notably by the observations of outbreaks of acute hepatitis associated with consumption of maize heavily contaminated with aflatoxin in Kenya in 1981 and 2004, which resulted in several hundred deaths. Previously, sporadic cases of aflatoxicosis were reported in Western India (1974) and Malaysia (1988). The intake of total AfTs estimated to result in a risk of fatality was ˃ 1 mg/day, i.e., ˃ 20 mg/kg body weight (bw)/ day in adults, while aflatoxicosis without fatality may occur with 5–10-fold lower doses (Wild and Gong, 2010). Among the livestock, the most sensitive animals for acute toxicity are the avian species, primarily ducklings, followed by turkeys, chicks and quail. In other farm animals, decreasing level of sensitivity is shown by rabbits, young pigs, calves, mature cattle and sheep (Pitt et al., 2012). Dogs are also quite sensitive to acute toxicity. The susceptibility of a species to toxicity of AfTs depends on its ability to metabolize the toxin efficiently and quickly to less toxic or nontoxic metabolites. Undernourished or stressed animals are more sensitive to AfTs.
