Clell V. Bagley, D.V.M.
Extension Veterinarian
Utah State University

 Certain metabolites of some fungi (molds) are poisonous (toxic). Because the prefix myco- refers to fungi, these toxins are termed mycotoxins. Penicillin is derived from a fungus and could be termed a mycotoxin, in relation to bacteria which are sensitive to it.11.

There are thousands of species of fungi but a relative few of these grow on agricultural products and only a fraction of these are capable of producing mycotoxins.7

It is known that about 100 fungi which grow on standing crops or stored feeds, produce toxic substances and approximately 20 of these have been associated with naturally occurring diseases.11

A primary inoculum of the fungus onto the feed or food product is necessary for the fungus to develop, but abundant spores are usually present in soil, air and water. The practice of mixing low grade or contaminated grains with high quality commodities will certainly increase the inoculation potential.7

The toxigenic fungi are relatively ubiquitous and they can germinate, grow and elaborate their toxins into a variety of substrates when conditions of moisture, temperature and aeration are favorable. The optimal conditions for toxin production by different fungi may be quite variable. A great portion of the mycotoxin problem is associated with stored grains and other concentrate rations, especially high-moisture corn, silage, cottonseed, peanuts and, to some extent, soybeans. Because of the involvement of stored feeds, the mycotoxicoses are especially important in groups of animals such as feedlot cattle, dairy cattle, market swine and poultry.11,13

The best known of the mycotoxins is aflatoxin. Others known include rubratoxin, ochratoxin, T-2 toxin, and zearalenone (F-2 toxin). Derivatives of this latter toxin are currently being used to promote growth efficiency in livestock.11

Many of the mycotoxins are heat stable and are not inactivated by pelleting, canning and other processes. They are also non-antigenic, so immunity is not developed.11

History of Mycotoxins

 Fungal poisons have been known for many years, but they were not considered a major factor in animal disease, until the last decade. Ergotism which is caused by the mycotoxin ergot, has been  known to affect man and animals since medieval times. It causes either dry gangrene or nervous derangements depending on the characteristics of the ingested ergot.

Two other mycotoxicoses were recognized during World War II: alimental toxic aleukia (ATA) of man and stachybytryotoxicosis of horses. Then in 1960, in Great Britain, 100,000 turkey poults died in a disease outbreak. The cause of death was identified as aflatoxin contaminated peanut meal. In retrospect, it was recognized that disease outbreaks in other animals were due to aflatoxins and other mycotoxins.11

Aflatoxin Sources and Characteristics

 Aflatoxins are a group of chemically related mycotoxins and are classified as B1, B2, G1, and G2,. The most common and toxic of the aflatoxin group is B1. 2,8
Aflatoxin was first identified in stored grain, as a product of the fungus Aspergillus flavus, or A. flavus, hence the prefix afla-. It was later found that similar toxins were produced by toxigenic strains of Aspergillus parasiticus.11

Aflatoxin has been found in wheat, corn, barley, oats, coconut oil and meal, cottonseed, cassava, dry peameal and other raw food products.7

The molds which produce aflatoxin usually do not grow in silage, but aflatoxins already present can survive the acids produced during the ensiling process. The corn should be examined, and tested if necessary, prior to harvest.8

Several factors are able to modify mold development, including moisture, temperature aeration and substrate. Moisture is probably the most important of these factors but the type of mold and whether or not a toxin is produced will depend on the interplay of all these factors. Maximum yields of a aflatoxin develop during incubation at 24-25.C, but reduction of oxygen from 5% to 1% greatly inhibits the mold growth and aflatoxin production. Contaminated cotton bolls are usually on the lower third of the plant, especially in irrigated fields, where the higher humidity is more conducive to mold growth.7

Physical damage, such as breakage and stress cracks in grain, will increase the likelihood of fungal growth.7  To prevent production of the toxin on harvested products, care should be taken to prevent physical damage at harvest and to reduce the moisture level soon after harvesting.3  A "blacklight" can be used to detect the presence of mold growth on grain and is all right for use as an initial test. It must be realized, however, that this checks only for the presence of mold growth and even if this test is positive, it doesn't mean that an aflatoxin is present. Culture of mold or even isolation of specific molds from feeds means nothing. The specific toxin must be isolated and/or its toxicity demonstrated.4,11

Effects of Aflatoxicosis

The principle effects of aflatoxin seem to be those induced in the liver. If the dose is sufficient to produce an acute toxicity, it results in an increased clotting time and hemorrhage, especially in the intestinal lumen. There is also edema of the gall bladder. Acute poisoning causes hepatitis and necrosis of liver cells. resulting in the prolonged blood clotting time, with affected animals dying from severe hemorrhages.11
In subacute cases of poisoning, the liver lesions are those of regeneration and repair rather than necrosis. The bile duct cells proliferate and scar tissue forms. The rate of protein formation and the growth rate are depressed but the animal may or may not die.11,10

In chronic cases, the lesions are those of chronic liver dysfunction. These usually include icterus, fibrosis or cirrhosis of the liver, ascites and pulmonary edema. The changes may be so subtle that they are overlooked, but it can result in decreased appetite, poor feed conversion, reduced growth rate and decreased resistance to disease. It may also cause diarrhea, bloody diarrhea, abortion, or deformities of the fetus.8,8,10,11

Acquired immunity is impaired by aflatoxins but the mechanism by which this occurs is still unclear. The species of animal involved and the time of intake relative to immunization both result in variations in the affect of aflatoxins.12

Other mycotoxins may act synergistically with aflatoxin to accentuate many of its effects. The lesions are probably due to the combined effects of several compounds.9

Aflatoxins may also be carcinogenic and result in tumor formation in some animals after prolonged consumption of feed containing as little as 0.5 to 15 parts per billion (pbb). This is evident in laboratory animals and especially in trout.11  Two bears. who were mates in a zoo, died within a short time of each other and the cause of death in both cases was primary liver tumors. On investigation it was found that the day-old bread the bears had been fed as part of their diet, was actually several weeks old instead and was contaminated with fungi.3

Toxic Levels

Feed containing 1 part per million (ppm) can cause acute poisoning in some animals.11  Ruminants are relatively resistant, although there are some effects on cattle such as decreased growth rate or decreased milk production. Sheep are quite resistant and no natural cases have been reported, but hepatic cell carcinomas and nasal tumors were produced in an experimental study after several years feeding peanut meal containing a level of 1.75 ppm of anatoxin.10

Dogs and poultry are quite sensitive. Ducklings are used for bioassays because of their sensitivity and rapid response to toxins. The rabbit is very sensitive and trout are the most sensitive discovered to date, with even 4 ppb resulting in liver tumors.10

Young animals are usually more sensitive than are older ones. In calves, 150-200 ppb (or 0.5 mg/day) results in unthrifty animals. It would require 300-400 ppb to cause first calf heifers to lose weight and 2400 to 3100 ppb to reduce the appetite and markedly decrease production of adult dairy cows. However, 10,000-12,000 ppb (10-12 ppm) would cause the death of even an adult cow in about five days. Even lower levels may have an effect in the farm situation, compared to when a purified toxin is used in the laboratory. This is evidently due to the synergistic effects of other molds and their products in the farm situation.3,8

The sample used for analysis is extremely important because only one or two contaminated kernels in 635 bushels of corn is sufficient to give levels over 20 ppb.4  The levels present in samples have ranged from 5 to 8,000 ppb.7

The aflatoxins do not persist for long periods in the animal's body and will usually be excreted within 96 hours. Because of this, it may be possible to use moldy feed for short term or low level feeding to certain animals. Contaminated feeds should not be given to lactating cows, pregnant Cows or calves under 8 weeks of age. A ration containing 50 ppb could be given to calves 2-6 months of age, and heifers over 6 months of age could be given up to 300 ppb.10

The processing of various products may have an effect on the aflatoxin content of the final products. For example when oil is extracted from contaminated, whole cottonseed, the aflatoxin which is removed with the oil will be removed or destroyed during processing. But that which remains with the resulting cottonseed meal, approximately 70% of the original, will still be toxic. Treatment of feeds with ammonia has shown promise of decreasing the level of toxin contained in the treated feed, but other research must still be performed.1  Although acetic and propionic acid derivatives seem to be effective in controlling fungal growth, they do not affect any mycotoxins which may have been present before their addition.11

Regulations and Public Health Aspects

Human exposure to aflatoxin can result from direct consumption of aflatoxin contaminated foods. whether processed or unprocessed. And aflatoxin ingested by food producing animals may also be transferred within the animal's body into meat, milk, or eggs and these would be potential sources for human exposure. Aflatoxin contaminated foods are deemed adulterated.13  The concern for human hazard varies for different food products. For example, only dry-milled corn products, such as corn meal or grits, are of concern. Refined corn oil, fresh sweet corn and other corn food products prepared by wet milling, such as corn starch, present no hazard because the toxins are either removed or destroyed during processing. Residues of aflatoxtin B1, have been found in the musculature and certain organs of poultry and swine after they were given aflatoxins in their feed. Aflatoxin B1 in the dairy ration is transformed to a metabolite, labeled aflatoxin M1, in the cow's milk. The metabolite is as potent a carcinogen as is the parent toxin. When aflatoxin B1 is present in the cow's feed at a concentration of 100 mg/kg (ppb) it can result in aflatoxin Ml in the milk at a concentration of 1 m g/L.13  

Aflatoxin is conjugated by the liver and incorporated into the tissues, to a certain extent, although about 90% of it is rapidly excreted from the animal body. The conjugated form is not detectable with the classical methods of analysis, but if these tissues were ingested by another animal, the conjugate could be broken down and the toxin released in the new animal.3

 Because the aflatoxins have been shown to be carcinogenic, federal law requires that the FDA establish a zero tolerance level for them in food. However, in practical application this is limited by the methods of measurement, so guidelines are provided to define the baseline for sampling and assay procedures.7

 These guidelines apply to primary agricultural products because complete freedom of contamination cannot be achieved with present methods and technology. In 1965, the original were set at 30 ppb (mg/kg) in raw or finished products. In 1969 this was lowered to 20 ppb, and in the future it may be lowered further as technology improves. If there is evidence that the contamination could have been avoided, the zero tolerance level can be used by the regulatory officials for enforcement.13

 The surveillance for aflatoxin is implemented through inspection and sampling procedures of storage facilities and food and feed manufacturers. Commodities such as peanuts, Brazil nuts, pistachio nuts, cottonseed, corn, almonds, pecans, walnuts and copra are recognized as having a demonstrated potential for contamination with aflatoxin. Others considered to have a potential for contamination include filberts, chestnuts, rice, dried beans and milk products.13

 Corn or other grains which contain in excess of 20 ppbm, cannot be shipped across state lines or blended with toxin-free grains. Products used for human food cannot contain more than 20 ppb. For animal feeding, feeds higher than 20 ppb are often acceptable, but there is a question of livestock safety. If it contains less than 100 ppb, it can usually be fed intermittently to mature, non dairy animals without affecting the meat or causing other serious problems.4  Feeds containing over 20 ppb cannot be legally fed to lactating dairy cattle, if the milk is shipped interstate.6

 An action level of 0.5 ppb has been established by the FDA for aflatoxin M1 in fluid milk. In a 1977 study in the Southeast, over half (153/302) of the milk samples collected contained 0.1-0.4 ppb of aflatoxin, and 6% (19/302) contained 0.5-3.9 ppb. Because of this, the study concludes that corn which is over 20 ppb is being fed to many dairy cattle in the Southeast.6