From Aflatoxin to Zearalenone: Key Mycotoxins You Should Know - Aflatoxins

Mycotoxins are substances produced by fungi that infect grain crops like maize and small grains and cause ear and kernel rots. Exposure to mycotoxins can lead to chronic or acute toxicity in humans and animals. In addition, mycotoxins can lead to market losses, discounts, rejection of grain lots at elevators, and a reduction in livestock efficiency and productivity.

The most economically important mycotoxins include aflatoxins (AF), deoxynivalenol (DON, also known as vomitoxin), fumonisins (FUM), zearalenone (ZEA), ochratoxin A (OTA), T2, HT-2, ergot alkaloids, and patulin (PAT). The fungal species that produce mycotoxins have worldwide distribution; therefore, mycotoxin contamination occurs everywhere grain crops are grown. Accordingly, mycotoxins have been detected in feed, silage, food, and beverages derived from cereal grains, and products from animals exposed to contaminated feed.

This is part one of an ongoing series covering the mycotoxins that are important in our agronomic systems.

Aflatoxins

Aflatoxins are hepatotoxic, carcinogenic, mutagenic, and immunosuppressive compounds. In humans, aflatoxin ingestion can lead to liver cancer, growth impairment, immunosuppression, and death. Animal response to aflatoxin exposure will vary according to the level of exposure, the duration, the animal species, and developmental stage. Reduced feed uptake, efficiency, and immunosuppression are common responses to aflatoxin-contaminated feed.

Based on their chemical structure, the major types of aflatoxins include aflatoxin B1 (AFB1), aflatoxin B2 (AFB2), aflatoxin G1 (AFG1), aflatoxin G2 (AFG2), and aflatoxin M1 (AFM1). Fungi do not produce aflatoxin M1; instead, it is a form of aflatoxin (similar to AFB1) secreted in the milk of humans and livestock exposed to aflatoxins in their diet.

Which fungi produce aflatoxins?

The fungal species Aspergillus flavus and Aspergillus parasiticus are the most common AF-producers (aflatoxigenic), although many other Aspergillus species can produce them.

How does the contamination of maize occur?

Infection of corn kernels by aflatoxigenic fungi can occur in the field, during crop development, or after the crop has matured.

Aspergillus species survive in the soil and corn residue. High temperature and humidity favor the production of fungal spores (Robertson, 2005), which are carried to the ears by wind and insects and will enter the kernel through silks and wounds. Once Aspergillus reaches the corn ear, it may cause Aspergillus Ear Rot, characterized by a powdery olive-green (Figure 1) to brown fungal growth commonly found at the tip of the ear and associated with insect damage. However, fungal growth may cover the entire ear (Figure 2).

Figure 1. Olive-green fungal growth of Aspergillus on corn. Photo credit: Travis Faske, University of Arkansas - Division of Agriculture, Bugwood.org

Figure 2. Olive-brown spores of Aspergillus on corn. Photo credit: Gary Munkvold, Bugwood.org

It is important to note that Aspergillus strains vary in their ability to produce aflatoxins. Therefore, fungal growth does not necessarily mean that aflatoxins are present. On the other hand, aflatoxin can be present in kernels that look healthy.  

Risk factors for aflatoxin contamination

The risk of aflatoxin contamination in corn increases when high temperatures, drought, and insect infestations occur after silking and during grain fill. High temperatures and drought may favor fungal colonization of senescing plant parts, leading to seed contamination (Cotty and Jaime-García, 2007). High temperatures may also select for growth and dispersal of Aspergillus species with a greater ability to produce aflatoxins (Ching'anda et al., 2021).

Occurrence of aflatoxins

Aflatoxin contamination of corn is common in the Southern US, and it rarely occurs in the northern states. However, projection models based on weather variables and growth stages indicate that the risk of aflatoxin contamination will increase in all corn-producing areas in the US by 2031 to 2040 (Yu et al., 2022), with a shift in rainfall patterns and temperatures being the major drivers. One example of the effect of weather on AF contamination occurred in 2012 when a severe drought affected the most important corn-producing regions in the United States (Rippey, 2015) and Europe and led to an outbreak of aflatoxin contamination in corn in some European countries and millions of dollars in losses (Kos et al., 2018).

Aflatoxin regulations

Aflatoxins are the most toxic mycotoxins, therefore, the most regulated mycotoxins in the world. The Food and Drug Administration (FDA) has established action levels for total aflatoxins (sum of AFB1+AFB2+AFG1+AFG2) in food, animal feed, and AFM1 in milk. Feed ingredients and cereals intended for human consumption containing less than 20 ppb (parts per billion, or ug/kg) are considered safe. Certain commodities with levels of aflatoxins higher than 20 ppb can be used as animal feed, however, in this case, FDA advisory levels must be followed since the appropriate use of contaminated commodities will depend on the animal species, developmental stage, and weight. The advisory level for AFM1 in milk is 0.5 ppb.

Take-home messages:

• Drought and high temperatures increase the risk of aflatoxin contamination
• Scout your fields before harvest to determine if Aspergillus Ear Rot is present
• If more than 10% of the ears how Aspergillus ear rot, test your fields for aflatoxins
• Notify your insurance agent
• Follow proper harvesting, drying, and storage practices

References

Robertson, A. (2005). Risk of aflatoxin contamination increases with hot and dry growing conditions. ICM News. IC-494(23):185-186

Cotty P.J., and Jaime-Garcia, R. (2007). Influences of climate on aflatoxin producing fungi and aflatoxin contamination, International Journal of Food Microbiology, 119: 109-115.

Ching'anda, C., Atehnkeng, J., Bandyopadhyay R., Callicott K. A., Orbach M. J., Mehl H.L., Cotty P. J. (2021). Temperature Influences on Interactions Among Aflatoxigenic Species of Aspergillus Section Flavi During Maize Colonization. Front. Fungal Biol. 2:720276.doi: 10.3389/ffunb.2021.720276

Yu, J; Hennessy, D.A., Tack, J., and F. Wu. 2022. Climate change will increase aflatoxin presence in US Corn. Environ. Res. Lett. 17 (2022) 054017

Rippey, B.R. (2015) The U.S. drought of 2012. Weather and Climate Extremes 10: 57-64, DOI: 10.1016/j.wace.2015.10.004.

Kos, J., Hajnal E.J., Šarić, B., Jovanov, P., Mandić, A., Đuragić, O., and  Kokić, b. (2018) Aflatoxins in maize harvested in the Republic of Serbia over the period 2012–2016, Food Additives & Contaminants: Part B, 11:4, 246-255, DOI: 10.1080/19393210.2018.1499675