Mycotoxins have probably existed for as long as producers have grown crops throughout the ages. It has only become clear in the last 40 years that fungi living in foods and feeds can produce mycotoxins.
Food can be contaminated with mycotoxins before or after harvesting, during the drying of grain and in storage. Mycotoxins are secondary metabolites produced by filamentous fungi and appear to have no role in the normal growth of these fungi.
These secondary metabolites are varied molecules as their structures range from single heterocyclic rings to irregularly arranged six to eight membered rings, and they are toxic to a wide range of organisms such as mammals, plants, birds and fish.
The term ‘mycotoxin’ was coined after the death of 100 000 turkey poults near London in 1962 due to aflatoxin contamination of the peanut meal that they consumed. The period between 1960 and 1970 was termed the mycotoxin gold rush due to many scientists who joined a well-funded search for mycotoxins.
All mycotoxins are of fungal origin but not all toxic metabolites produced by fungi are called mycotoxins. Some of the metabolites produced by fungi are beneficial to humans, such as the antibiotic penicillin, which is toxic towards bacteria.
Many secondary metabolites produced by either fungi or bacteria play a role in causing or exacerbating plant diseases. Toxins produced by fungi that have a deleterious effect on plants are called phytotoxins. Mycotoxins can be both mycotoxic (toxic to animals) and phytotoxic (toxic to plants).
Occurrence of mycotoxins
Cereal grains, oil seeds, tree nuts and dehydrated fruit are susceptible to fungal contamination and thus mycotoxin formation. Due to the fungal infection and subsequent mycotoxin production by these fungi, mycotoxins are often consumed directly by humans and animals because of their presence in the food chain.
It was previously perceived that mycotoxin contamination of grains was a post-harvest problem, but that perception has changed and it is now considered that mycotoxin contamination occurs under field conditions and during harvesting as well. Food and animal feed can be contaminated by a succession of fungi, which can produce a range of mycotoxins, and thus often more than one mycotoxin is present.
Mycotoxins have caused major epidemics in humans and animals over the ages. Such epidemics include:
- St Anthony’s fire, which caused gangrenous and convulsive effects in humans.
- Alimentary toxic aleukia (ATA), which was responsible for the death of at least 100 000 Russians between 1942 and 1948.
- Stachybotryotoxicosis, which killed thousands of horses in the Union of Soviet Socialist Republics (USSR) in the 1930’s.
Even in the modern society of today, mycotoxins are still often found in food such as animal feed, peanut butter, milk and flour, to mention a few. Major outbreaks are frequently observed, such as in Kenya during 2004, where 125 people died and 200 people needed medical treatment after consuming aflatoxin-contaminated maize, or dog food contaminated with aflatoxin in America in 2005, 2006 and 2011.
The toxic effects of mycotoxins
The toxicity of mycotoxins can be classified into four groups: Acute, chronic, mutagenic (agent that changes the genetic material of an organism) and teratogenic (agent that acts on the foetus to cause congenital abnormality) – see Table 1.
Mycotoxin poisoning causing acute toxicity usually results in the deterioration of liver or kidney function, whereas chronic mycotoxin poisoning causes no immediate effect, but eventually can cause cancer.
It is widely known that grain contaminated with mycotoxins causes a toxic effect in humans consuming it. It can also interfere with DNA replication and protein synthesis.
Most mycotoxins are highly stable during food processing and are thus a great threat to humans. Diseases caused by the ingestion of mycotoxins by humans and animals are termed mycotoxicosis.
Types of mycotoxins and the effect on grain crops
There are about seven main types of mycotoxins that appear in human food and animal feeds, and they are primarily produced by five genera of fungi.
The mycotoxin-producing fungi include genera such as Fusarium, Aspergillus, Penicillium, Alternaria and Claviceps (Table 2). Aspergillus, Fusarium and Penicillium are the major mycotoxin producers that are linked to food production.
Fusarium spp., which produces mycotoxins before or immediately after harvest, is one of the most destructive pathogens on cereal crops. The main mycotoxins produced by these fungi include aflatoxins, fumonisins, trichothecenes and ochratoxin (Table 2).
Aflatoxin is one of the most consumed mycotoxins and is predominantly produced by Aspergillus flavus and Aspergillus parasiticus. It is often found in commodities such as maize, sorghum, peanuts (groundnuts), soya beans and sunflower seed, to name a few.
Aflatoxins is the umbrella term for the entire group of aflatoxins, with aflatoxins B1 being the most toxic, and is a potent carcinogen often causing liver cancer in many animal species (Table 1 and 2).
The fumonisin group of mycotoxins is produced by over 50 species of Fusarium and has a history of infecting grain such as maize and wheat. The fumonisin B group (FB1, FB2 and FB3, Figure 1) is produced by Fusarium verticillioides, which is the causal agent for Fusarium ear rot in maize – one of the most common ear diseases in maize. It causes leukoencephalomalacia in equines and pulmonary oedema and hydrothorax in pigs (Table 1 and 2).
The trichothecenes are a large group of mycotoxins produced by a variety of fungi such as Fusarium, Trichoderma and Stachybotrys, to name a few. Deoxynivalenol (DON) is probably the most well-known mycotoxin of the group. It is produced on a host of crops, which includes maize, wheat (Fusarium head blight), barley and vegetables, and is formed under field conditions as well as post-harvest. Swine and other monogastric animals are the most sensitive to the toxin.
It causes a range of symptoms such as feed refusal, diarrhoea, bleeding and death in pigs, immunologic effects and haematological changes, to name a few (Table 1 and 2).
Zearalenone (F-2 toxin) is an estrogenic mycotoxin, with swine being the most sensitive to the toxin of all farm animals. It is mainly produced by Fusarium spp., with F. graminearum being the main culprit (Figure 2). It is often found in contaminated grain such as maize, wheat and barley. It affects the hormonal system of sows and thus causes fertility problems. Cattle and chickens are also affected by the mycotoxin (Table 1 and 2).
Ochratoxin is a group of mycotoxins primarily produced by the Aspergillus species and to an extent a couple of Penicillium species as well. Ochratoxin A is a potent nephrotoxin mainly affecting the kidneys on a range of hosts.
It is mainly found in a range of agricultural commodities, foodstuffs, fruits and nuts. It has also been detected in blood and other animal tissues and in milk, including human milk (Table 1 and 2).
Favourable conditions
Mycotoxin production is highly dependent on environmental conditions such as moisture and temperature, but it’s also dependent on other related factors such as insect and bird damage. Mycotoxin contamination usually occurs more in the field than in storage with maize.
Post-harvest mycotoxin formation is usually associated with improper drying and storage conditions. This is one of the biggest reasons why crops should be harvested at an adequate moisture level.
The ideal conditions for in-field mycotoxin formation are not easy to define. Aflatoxin and fumonisin production are favoured by high temperatures and drought conditions, whereas fungi, which produces trichothecenes, prefers milder and wetter conditions. Insect damage associated with fungal infection as seen in maize ear diseases is also favoured by warmer and dryer conditions.
Minimising risk
- It is of great importance to know that the fungi don’t just grow on the surface of food but can penetrate deep within food as well.
- Fungi usually grow on food and grain that are not dried properly or grain that was damaged in the field, as with ear disease in maize.
- Fungi usually don’t grow on dried grain and thus the most efficient way to prevent mycotoxicosis is to properly dry grain before it is stored for the long term and to maintain the dry conditions as well.
- Insect control is also of great importance, since insect damage on grain crops also favours disease formation. Insects can be controlled by either planting Bt hybrids or by using a chemical spray programme.
- Crop rotation and tillage will also be beneficial in controlling fungi that produce mycotoxins, but there are exceptions as the F. verticillioides affects a wide range of hosts. Other methods of prevention include a proper fertilisation programme and weed control.
References
- Bennett JW and Klich M, 2003. Mycotoxins. Clinical Microbiology Reviews 16:3, 497-516
- Coppock RW and Dziwenka M, 2014. Mycotoxins. Biomarkers in Toxicology, 549-562
- Klopper R and Tweer S, 2009. Gibberella ear rot fact sheet. https://pcraft.reload.co.za/assets/disease_fact_sheet/Gibberella_Ear_Rot.pdf [28 September 2023]
- Munkvold GP and White DG, 2016. Compendium of corn diseases. APS Press St. Paul, MN
- Pitt JI, 2000. Toxigenic fungi and mycotoxins. British Medical Bulletin 56:1, 184-192
- Steyn PS, 1995. Mycotoxins, general view, chemistry and structure. Toxicology Letters 82-83, 843-851
