An introduction to mycotoxins
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by J.l. Pitt
Mycotoxins are toxic metabolises produced by fungi, especially by saprophytic moulds growing on foodstuffs or animal feeds. They must always have been a hazard to man and domestic animals, but until the past 30 years their effects have been largely overlooked. Although poisonous mushrooms are carefully avoided, moulds growing on foods have generally been considered to cause unaesthetic spoilage, without being dangerous to health. Between 1960 and 1970 it was established that some fungal metabolises, now called mycotoxins, were responsible for animal disease and death. In the decade following 1970 it became clear that mycotoxins have been the cause of human illness and death as well, and are still causing it.
It is now well established that mycotoxicoses (the diseases caused by mycotoxins) have been responsible for major epidemics in man and animals at least during recent historic times. The most important have been ergotism, which killed thousands of people in Europe in the last thousand years, alimentary toxic aleukia (ATA) which was responsible for the death of many thousands of people in the USSR in the 1940s; stachybotryotoxicosis, which killed tens of thousands of horses and cattle in the USSR in the 1930s; and aflatoxicosis, which killed 100,000 young turkeys in England in 1960 and has caused death and disease in many other animals, and perhaps man as well. Each of these diseases is now known to have been caused by growth of specific moulds which produced one or more potent toxins, usually in one specific kind of commodity or feed.
It is important to distinguish between the effects of bacterial toxins and mycotoxins. The classic bacterial toxins are proteins, which produce characteristic symptoms in only a few hours, as the human body recognises them, and produces antibody mediated reactions to them. Fungal toxins on the other hand, are almost all low molecular weight chemical compounds which are not detected by antigens, and hence produce no obvious symptoms. Mycotoxins are insidious poisons.
Mycotoxins can be acutely or chronically toxic, or both, depending on the kind of toxin and the dose. In animals, acute diseases include liver and kidney damage, attack on the central nervous system, skin disorders and hormonal effects. Nerve toxins may cause trembling or even death. Skin disorders may be necrotic lesions or photosensitivity, while hormonal effects include abortions in cattle, swollen genitals in pigs and a variety of poorly defined disorders including vomiting in pigs, feed refusal and failure to thrive. Toxins which act on the liver and kidney are especially difficult to detect and levels much lower than those producing acute effects are often carcinogenic. When eaten in minute quantities in the daily diet, they can cause cancers in experimental animals long after the time of eating. It is probable that humans can be affected the same way.
Table 1 lists a number of mycotoxins, some of the moulds which are known to produce them, and known or possible acute diseases with which they may be involved. In some cases, the connection between mycotoxin and disease is fairly well documented. In other, cause and effect are less certain. The most important disease which may have been due to mycotoxins, are reviewed briefly below.
Ergotism occurred throughout the past thousand years in central Europe, and has certainly killed many thousands of people. The fact that it was caused by a fungus has been known for a long time, since at least 1750. The fungus, Claviceps purpurea, grows in the ovaries of grains, especially rye, and the resulting sclerotia, called argots, are difficult to separate from normal grain at milling, and become dispersed in flour made from the grain.
Ergotism causes constrictions in blood vessels leading to the hands and feet. In extreme cases death of cells (necrosis), bacterial infections (gangrene) and effects on the mind (hallucinations) may occur, and in some cases death results. The toxins in argots are now known to be alkaloids, some of which find use in pharmaceuticals. The last outbreak of ergotism in Europe occurred in 1954. So far as I am aware, ergotism has not occurred in Asia, but it has occurred in Ethiopia quite recently (King, 1979).
Alimentary toxic aleukia
ATA caused the deaths of many thousands of people in the USSR, especially in the Orenburg District around the Caspian Sea, from 1942 to 1948. In some localities, mortalities were as high as 60% of those afflicted, and 10% of the entire population. Records show that ATA was also prevalent in earlier years.
ATA is an exceptionally nasty disease, causing fever, bleeding from the skin, nose, throat and gums, necrosis, and suppression of the immune system. These features are similar to radiation poisoning, and quite different from those caused by most other mycotoxins, or bacterial toxins.
During World War II, labour was very scarce in Russia, resulting in delays in harvest, and also food was very scarce, causing consumption of poor quality grain. Early studies on ATA were inconclusive. During the 1970s it became clear that ATA was a mycotoxicosis, and that the toxin responsible for ATA was the trichothecene toxin known as T-2. It was produced by the growth of Fusarium species, F. sporotrichioides and F. poae, in grain allowed to remain in the fields unharvested during winter.
Trichothecenes are now regarded as probably the most important mycotoxins, believed to be responsible for a variety of diseases of both man and domestic animals. Most have occurred in Europe, the USSR, Japan and the United States (Ueno, 1980). Toxicoses are usually acute, but Marasas et al. (1979) have suggested that trichothecenes may be involved in the high incidence of oesophageal cancer in the Republic of Transkei (South Africa).
Trichothecenes are also at the centre of the "Yellow Rain" controversy which occurred earlier this decade. According to some sources, trichothecenes were used as a chemical warfare agent in South East Asia, causing the deaths of thousands of villagers in Laos and along the ThaiKampuchean border (Watson et al., 1984). The facts in the Yellow rain controvery have become obscured by politics, but two things are clear: people have died from chemical poisoning in those areas, and trichothecenes are sufficiently toxic to have been the cause.
There are no records of ATA in Asia, and it is not clear if trichothecene production is likely in this region. We are currently carrying out studies aimed at providing information about this point.
Acute cardiac beriberi
Another human mycotoxicosis of significance, acute cardiac beriberi was a common disease in Japan, especially in the second half of last century. This disease is characterised by difficulties with breathing, nausea and vomiting, and after 2 to 3 days, severe pain and distress. Progressive paralysis may lead to respiratory failure and death.
Beriberi is the general name for vitamin deficiencies resulting from the consumption of polished rice. Careful work by Uraguchi (1971) showed that acute cardiac beriberi may not be a vitamin deficiency, but a toxicosis. In 1910 the incidence of acute cardiac beriberi suddenly decreased in Japan: Uraguchi points out that this coincided with implementation of a government inspection scheme which dramatically reduced the sale of mouldy rice. The incidence of true beriberi, resulting from the consumption of polished rice, was unaffected. It is notable that victims of this acute cardiac beriberi were often young healthy adults.
Acute cardiac beriberi is caused by citreoviridin, a mycotoxin produced by the comparatively rare species, Penicillium citreonigrum. Although it no longer occurs in Japan, there is no proof that acute cardiac beriberi does not still exist in some other part of Asia.
Onyalai is an acute disease characterised by haemorrhaging lesions in the mouth. It has been endemic in Africa, especially in Southern Sahara regions for at least 80 years (Rabie et al., 1975). It is much more common in rural than urban populations, Since many of the people affected by onaylai subsist on millet, Rabie et al. (1975) suggested the possible role of a mycotoxin in this disease. Toxigenic isolates of Phoma sorghina were found to be common in millet consumed by affected populations, and Rabie et al. (1975) were able to reproduce many of the symptoms of onyalai in rats fed maize and wheat on which P. sorghina had been grown.
Some of the toxins discussed in this section (Table 2) may produce acute effects, but they are more significant because of their ability to cause long term disease. The best known and most studied of chronic mycotoxicoses are produced by aflatoxins.
Aflatoxins were discovered in 1960 following the deaths of 100,000 young turkeys in England, and high incidences of liver disease in ducklings in Kenya and hatchery reared trout in the United States, English scientists soon established the cause of all these problems to be toxins produced by the common moulds Aspergillus flavus and A. parasiticus. Assay techniques were devised and preliminary toxicological studies carried out by 1963 (Sargent et al., 1963).
Aflatoxins are named by letters and subscripts. Aflatoxin B1, the most toxic compound, is usually associated with aflatoxin B2: these compounds are usually formed by both A. flavus and A. parasiticus. Aflatoxins G1 and G2 are formed only by A. parasiticus (Klich and Pitt, 1988). Aflatoxins M1 and M2 are formed in milk when aflatoxin B1 and G1 are ingested in feed.
Aflatoxins have both acute and chronic toxicity in animals, and produce four quite different effects: acute liver damage, liver cirrhosis, induction of tumours and teratogenic and other genetic effects.
Acute toxicity of aflatoxins to humans has been encountered only rarely (Shank, 1978). In 1967, 26 Taiwanese in two farming communities became ill with apparent food poisoning. Nineteen of those affected were children of whom three died. Rice from affected households was blackish green and mouldy, and appeared to be of poorer quality than rice from households which were unaffected. Samples of the mouldy rice contained about 200 g/kg of aflatoxin B1, which was probably responsible for the outbreak. Post mortem examinations were not carried out.
In 1974, an outbreak of hepatitis that affected 400 Indian people, of whom 100 died, was almost certainly due to aflatoxins. The outbteak was traced to corn heavily contaminated with Aspergillus flavus and containing up to 15 mg/kg aflatoxins. Consumption by some of the affected adults was estimated to be 2-6 mg in a single day.
It has been suspected for some time that aflatoxin may be a factor in Reye's syndrome, a common cause of death in South East Asian children. Shank et al. (1971) found significant levels of aflatoxins (1-4 g/kg) in livers of 23 Thai children who had died of Reye's syndrome. Children who have died from Reye's syndrome in Czechoslovakia and in New Zealand have also been found to have had aflatoxins in their livers at autopsy.
Kwashiorkor, a disease of children in Northern Africa and elsewhere in undernourished populations, which is usually attributed to nutritional deficiencies, may also be related to aflatoxin intake (Hendrickse et al. 1982). Aflatoxin-induced liver damage may make these children less able to cope with the high protein diets usually recommended as the cure for kwashiorkor (Newell, 1983).
Aflatoxins and primary liver cancer
Scarcely two years after the discovery of aflatoxins came the first warnings that they may cause human liver cancer. This disease has a high incidence in central Africa and South East Asia. When epidemiological evidence suggested a possible correlation with mycotoxins in the food supply, field studies were initiated on an international basis. Epidemiological data were coupled with analyses of those foods that form the staple diets of stable indigenous populations. Stability in both diet and population is essential in studies of this kind because of the long induction period (10-20 years) for human liver cancer.
Studies of this kind were carried out in Kenya, Swaziland, Uganda, Thailand, Mozambique and rural areas in the Southeastern United States. By 1976, sufficient data existed to allow plotting and statistical analysis (van Rensburg, 1977). These indicated a positive correlation between the logarithm of aflatoxin ingestion and the occurrence of human primary liver cancer, at least in Africa and South East Asia.
Epidemiological studies in the USA have produced results differing from those of van Rensburg (1977). Stoloff and Friedman (1976) estimated that children in rural communities in the southern states of the USA may ingest as much as 40 µg aflatoxin per kilogram body weight per day, mostly from maize. From van Rensburg's figures, such a level should produce four to ten deaths from primary liver cancer per 100,000 population per year.
The actual level, however, is about one, less than in some other regions of the USA where aflatoxin is unlikely to be ingested in significant amounts.
Some doubt has recently been cast on the role of aflatoxin in human liver cancer. Earlier or simultaneous exposure to hepatitis B virus appears to be a prerequisite. Equally or more important may be the course of events after the cancerous lesion is initiated by aflatoxin. Campbell (1983) has shown that in rats a high protein diet after exposure to aflatoxin caused a higher rate of liver cancer than a low protein diet. Thus it would seem that nutrition can play a vital role in the induction of liver cancer by aflatoxins.
In the early 1970s, observers in Denmark noted a high incidence of nephritis (kidney inflammation) in pigs at slaughter. A search for possible causes eventually showed the presence of ochratoxin A, a mycotoxin originally reported from Aspergillus ochraceus. Analysis of pig feeds showed that 50% of samples contained ochratoxin A at levels up to 27 mg/kg. The mould responsible was reported to be Penicillium viridicatum, but has more recently been shown to be P. verrucosum (Pitt, 1987). This species occurs commonly in Danish barley (Frisvad and Viuf, 1985).
The discovery of ochratoxin led to analyses of pork and bacon. It was found that a significant proportion of ingested ochratoxin lodged unchanged in depot fat. The risk to humans is difficult to assess, but as pig meats are an important part of the Danish diet and rural populations usually eat their own uninspected pigs, a risk certainly exists. Death rates from kidney failure are high in some Danish rural areas and it is resonable to suppose the cause is ochratoxin.
Penicillium verrocusum has not been reported to occur in Asia. However, Aspergillus ochraceus and related species which also produce ochratoxin do. The significance of ochratoxin A in tropical climates has not yet been assessed however.
The potential role of aflatoxins and other mycotoxins in cancer should be sufficient incentive for further investigations, especially in Asia and other tropical areas where the occurrence and significance of mycotoxins has not yet been fully assessed.
While it may not be possible to produce a food supply completely free of mycotoxins, improvements in storage and handling of grains, nuts and other commodities can minimise mould growth, and so reduce the risk of mycotoxin contamination in food supplies.
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