Beneficial and harmful products that fungi produce draw researchers’ attention worldwide. Antibiotics inhibit microbial activity when administered at sublethal doses, providing important tools to manage infections, primarily bacterial, in humans and other animals. Fungi can produce chemicals called mycotoxins that are toxic to animals, including humans. The symptomology syndrome expressed by these substances is called mycotoxicoses.
The toxins produced by fungi can be present in food or feed products long after all visible signs of the fungus have disappeared. Some mycotoxins can withstand the normal cooking and processing conditions. Thus mycotoxins, produced by fungi colonizing parts of plant, such as peanuts, grains, or beans, will potentially be present in food products consumed by humans around the world. Also, there are indications that the mycotoxins present in animal feed may remain as residues (mycotoxins or their breakdown products) in meats, eggs, or milk (including cheese and yogurt) produced by animals fed these mycotoxin-containing feeds. Humans and other animals can also suffer mycotoxin effects via inhalation, skin exposure, and intravenous drug use.
Fungi associated with mycotoxin production are found worldwide and may produce toxins on almost any food source that will support their growth. Aspergillus flavus and A. parasiticus have drawn more attention than other genera due the acutely toxic nature and powerful carcinogenic effect their naturally-occurring mycotoxins have on humans and other animals. Aflatoxin is the name of the toxin this genus of fungi produces; 14 toxins or derivatives have been identified within this group (see below). Certain strains of A. flavus do not produce aflatoxin and these strains occur naturally in the American Southwest albeit at low frequencies. The presence of strains that lack mycotoxin production can be found for other fungi that produce mycotoxins such as Fusarium spp. The strain A. flavus AF36 can displace aflatoxin-producing strains, when applied to cotton or corn fields, and harvested grain has been shown to have lower levels of mycotoxins. Research is underway with experimental biopesticides or biocontrol agents and offers hope of reducing the presence of mycotoxins.
The liver is the organ aflatoxins most often affect. Fat or glycogen storage often is increased considerably. The young of the species appear to be most susceptible. Incidence of liver problems (including cancer) has been observed in selected geographic areas where aflatoxin intake was high. The current guideline is 20 parts per billion (20 ppb) of total aflatoxins for primary agricultural commodities and for some directly-derived products. Other mycotoxins are associated with gastro-intestinal problems, immune system depression, infertility, and sick-building syndrome. The effects of mycotoxins appear to be more pronounced with food levels are insufficient.
Poor feed efficiency, breeding problems, general unthriftiness, and death have been observed in livestock that consumed feed that was supporting or had supported fungal growth. Since 1960, medical and agricultural research in mycotoxins has been greatly stimulated by reports from Britain of a large number of turkeys and other birds poisoned from feeding on contaminated peanut meal. Analysis of the peanut meal showed that it contained an aflatoxin produced by the common mold, Aspergillus flavus.
Numerous crops are susceptible to contamination by toxin-producing fungi. Cotton seed, soybeans, corn, rice, sorghum, a number of other small grains, and forage crops are among the farm commodities in which mycotoxins have been found. Mycotoxins also are produced by some other parasitic fungi. For example, the grain diseases ergot and scab are caused by fungus parasites that develop in growing cereals and produce toxin–ergot by certain Claviceps species and scab by some Giberella and Fusarium species.
Tolerance varies widely among various species of warm-blooded animals. Ducklings are extremely susceptible and are used widely as test animals to confirm the presence of aflatoxin. Sheep, on the other hand, are relatively resistant. A British scientist has ranked a number of the common domestic animals in declining order of susceptibility as follows: ducklings, turkeys, pheasants, pigs, cattle, and sheep. The young of most species are more susceptible to aflatoxins than old animals.
High moisture is the single most important condition contributing to mold. Mold grows most rapidly at temperatures of 60°F to 100°F in combination with high relative humidity. The modern trend in storing farm commodities is to harvest at higher moisture content than in the past. Harvesting and feeding the commodity at the high moisture level increases feed efficiency. However, the chance of fungal infestations increases under these conditions also. Insect or rodent damage increases problems with mycotoxins; a potent example is the case of corn earworm and ear rots. Research workers have been searching for answers to this problem. Recently it has been found that treating the commodity with organic acid (proponic acid and/or acetic acid) has greatly reduced the amount of fungus growth in high-moisture products. Also, the deployment of GMO corn containing a Bt gene resulted in reduced earworm injury and subsequent reduction in mycortoxin levels.
Basic Approaches to Mold Prevention
Mold prevention is thought to begin with properly planting and growing the crop. Use fungus-free, viable seed, fertilize properly, and control plant pests and diseases. Harvest promptly at maturity. Moisture content is higher in immature crops and their susceptibility to mold damage greater. But permitting the crop to stand in the field after ripening can be an invitation to insect or mold invasion.
Test for moisture content after harvest, and dry promptly to a level for safe storage. Provide adequate aeration. Recommended safe moisture levels refer to all seeds and kernels in the lot—not to average moisture content. Remove mold-damaged material before storage or processing.
Clean oilseed and grain crops thoroughly to remove as much foreign matter and damaged seed as possible.
Keep moisture from stored product.
- Use weathertight bins.
- Provide proper aeration.
- Keep crop clean and storage area dry and cool.
- Examine stored crop frequently.
Use antifungal substances, which must meet food additive requirements to be acceptable.
Some Mycotoxin Problems and Their Causes
Ergotism Ingesting grains infested with Claviceps purpurea.
Fescue Foot Ingesting grass forage infested with Fusarium sp., Cladosporium sp., and Epicoccum sp.
Rhizoctonia Toxicosis Ingesting legumes (forage) infested with Rhizoctonia leguminicola.
Stachybotryotoxicosis Ingesting feeds infested with Stachybotryos atra.
Facial Eczema Ingesting perennial ryegrass infested with Pithomyces chartarum (a similar problem is associated with Periconia sp.).
Aflatoxicosis Ingesting grains infested with Aspergillus flavus, A. parasiticus, and other Aspergillus spp.
Vulvovaginitis in Swine Ingesting grains infested with Fusarium graminearum, F. tricinctum, F. verticillioides.
Alimentary Toxic Aleukia Ingesting millet infested with Fusarium sporotrichoides, F. poae, F. lateritium, Cladosporium sp., and Alternaria sp.
Grain Toxicosis Certain species of the genera Aspergillus, Penicillium, Fusarium, Cladosporium, and Alternaria have been associated with moldy grain toxicosis syndrome.
Hemorrhagic Syndrome in Poultry Ingesting moldy feed mixes. Isolates of Aspergillus chevalieri, A. clavatus, A. flavus, A. fumi-gatus, A. glaucus, Paecilomyces variota, P. citrinum, P. purpuro-genum, P. rubrum, and Alternaria sp. have been associated with the problem.
USDA-ARS. 1965. Preventing Mold-caused Toxins in Farm Commodities. Special Report.
Master manual on molds and mycotoxins. 1972. Farm Technology and Agri-fieldman. 28:5, 19-69a.
Smalley, E.B. The mycotoxin problem. Department of Plant Pathology, University of Wisconsin, Madison. Mimeo.