Mycotoxins of world-wide Importance

Those moulds and mycotoxins which are currently considered to be of world-wide importance (Miller, 1994) are shown in Table 1 and Figure 4.

An 'important' mycotoxin will have demonstrated its capacity to have a significant impact upon human health and animal productivity in a variety of countries.

Table 1 - Moulds and mycotoxins of world-wide importance

The Aflatoxins

The optimal water activity for growth of A. flavus is high (about 0.99). The maximum is at least 0.998 whereas the minimum water activity for growth has not been defined precisely. Pitt and Miscamble (1995) report a minimum of approximately 0.82. In general, production of toxins appears to be favoured by high water activity. A. flavus is reported to grow within the temperature range 10 - 43^oC. The optimal growth rate occurs at a little above 30^oC, reaching as much as 25 mm per day. The aflatoxins are produced by A. flavus over the temperature range 15 - 37^oC, at least. It is not possible to specify an optimum temperature for the production of the toxins, although production between 20 - 30^oC is reported to be significantly greater than at higher and lower temperatures.

The effect of water activity and temperature on the behaviour of A. parasiticus is similar to that described above for A. flavus. Pitt and Miscamble (1995) have reported a minimum for growth of about 0.83; and a minimum for aflatoxin production of about 0.87. There are only limited data on the effect of temperature on the growth of A. parasiticus and the production of the aflatoxins. It was reported that optimal growth and toxin production occur at approximately 30 and 28^oC, respectively.

The term 'aflatoxins' was coined in the early 1960s when the death of thousands of turkeys ('Turkey X' disease), ducklings and other domestic animals was attributed to the presence of A. flavus toxins in groundnut meal imported from South America (Austwick, 1978).

Figure 4 Some Important Mycotoxins

(Although the aflatoxins are the major toxins associated with this mycotoxicosis, another mycotoxin - cyclopiazonic acid (Figure 5) - has been implicated (Bradburn et al., 1994) in the aetiology of Turkey X disease.) The chronic effects of low dietary levels (parts per billion) of aflatoxin on livestock are also well documented (Coker, 1997) and include decreased productivity and increased susceptibility to disease.

The aflatoxin-producing moulds occur widely, in temperate, sub-tropical and tropical climates, throughout the world; and the aflatoxins may be produced, both before and after harvest, on many foods and feeds especially oilseeds, edible nuts and cereals (Coker, 97).

Although the aflatoxins are predominantly associated with commodities of sub-tropical and tropical origin, their occurrence has also been reported (Pettersson et al, 1989) in temperate climates in acid-treated grains.

Aflatoxin B₁ is a human carcinogen (IARC, 1993a) and is one of the most potent hepatocarcinogens known. Human fatalities have also occurred (Krishnamachari et al., 1975) from acute aflatoxin poisoning in India (in 1974), for example, when unseasonal rains and a scarcity of food prompted the consumption of heavily contaminated maize. If the immunosuppressive action of the aflatoxins in livestock is similarly manifested in humans, it is possible that the aflatoxins (and other mycotoxins) could play a significant role in the aetiology of human disease in some developing countries, where a high exposure to these toxins has been reported.

Lubulwa and Davis (1994) have studied economic losses attributable to the occurrence of aflatoxin only, in maize and groundnuts, in Southeast Asian countries (Thailand, Indonesia and the Philippines). They concluded that contaminated maize accounted for about 66 per cent of the total loss, whereas losses attributable to spoilage and deleterious effects on human and animal health were 24, 60 and 16 per cent of the total, respectively. However, the study considered losses associated with morbidity and premature death caused by cancer only. Consequently, it is likely that when the additional effects on human health caused by the immunotoxic effect of aflatoxin (and other mycotoxins) are included, the loss associated with aflatoxins will be significantly increased.

Figure 5 The Structure of Cyclopiazonic Acid

The Trichothecenes

Surprisingly little is known about the effects of water activity and temperature on the behaviour of the Fusarium moulds, including the production of mycotoxins.

In the case of F. graminearum, the temperature limits for growth have not been reported, although the optimal temperature has been estimated at 24 - 26 ^oC. The minimum water activity for growth is 0.9; the maximum limit is recorded as in excess of 0.99. No information is available on the effect of water activity and temperature on the production of deoxynivalenol, nivalenol and zearalenone.

The minimum water activity for the growth of F. sporotrichioides is 0.88, whereas the maximum limit is reported as >0.99. The minimum, optimal and maximum temperatures for growth are -2.0, 22.5 - 27.5 and 35^oC, respectively. As with the other Fusarium moulds, there is no information on the conditions required for the production of T-2 toxin.

T-2 toxin and deoxynivalenol (Figure 4) belong to a large group of structurally-related sesquiterpenes known as the ´trichothecenes'.

T-2 toxin is produced on cereals in many parts of the world and is particularly associated with prolonged wet weather at harvest. It is the probable cause of ´alimentary toxic aleukia' (ATA), a disease (IARC, 1993b) which affected thousands of people in Siberia during the Second World War, leading to the elimination of entire villages. The symptoms of ATA included fever, vomiting, acute inflammation of the alimentary tract and a variety of blood abnormalities. T-2 toxin is responsible for outbreaks of haemorrhagic disease in animals and is associated with the formation of oral lesions and neurotoxic effects in poultry. The most significant effect of T-2 toxin (and other trichothecenes) is the immunosuppressive activity which has been clearly demonstrated in experimental animals; and which is probably linked to the inhibitory effect of this toxin on the biosynthesis of macromolecules. There is limited evidence that T-2 toxin may be carcinogenic in experimental animals.

Deoxynivalenol (DON) is probably the most widely occurring Fusarium mycotoxin, contaminating a variety of cereals, especially maize and wheat, in both the developed and developing world. The outbreaks of emetic (and feed refusal) syndromes amongst livestock, caused by the presence of DON in feeds, has resulted in the trivial name, vomitoxin, being attributed to this mycotoxin.

The ingestion of DON has caused outbreaks (IARC, 1993c; Bhat et al., 1989; Luo, 1988) of acute human mycotoxicoses in India, China and rural Japan. The Chinese outbreak, in 1984-85, was caused by mouldy maize and wheat; symptoms occurred within five to thirty minutes and included nausea, vomiting, abdominal pain, diarrhoea, dizziness and headache.

To date, nivalenol-producing isolates of F. graminearum have been observed, on rice and other cereals, only in Japan and have been associated with the occurrence of red mould disease ('Akakabi-byo'). Symptoms include anorexia, nausea, vomiting, headache, abdominal pain, diarrhoea and convulsions (Marasas et al., 1984).

Zearalenone

Zearalenone is a widely distributed oestrogenic mycotoxin occurring mainly in maize, in low concentrations, in North America, Japan and Europe. However, high concentrations can occur in developing countries, especially when maize is grown under more temperate conditions in, for example, highland regions.

Zearalenone is co-produced with deoxynivalenol by F. graminearum and has been implicated, with DON, in outbreaks of acute human mycotoxicoses.

Exposure to zearalenone-contaminated maize has caused (Udagawa, 1988) hyperoestrogenism in livestock, especially pigs, characterised by vulvar and mammary swelling and infertility. There is limited evidence in experimental animals for the carcinogenicity of zearalenone.

The Fumonisins

The fumonisins are a group of recently characterised mycotoxins produced by F. moniliforme, a mould which occurs worldwide and is frequently found in maize (IARC, 1993d). Fumonisin B₁ has been reported in maize (and maize products) from a variety of agroclimatic regions including the USA, Canada, Uruguay, Brazil, South Africa, Austria, Italy and France. The toxins especially occur when maize is grown under warm, dry conditions.

The minimum water activity for the growth of F. moniliforme is 0.87; the maximum limit is recorded as >0.99. The minimum, optimal and maximum temperatures for growth are 2.5 - 5.0, 22.5 - 27.5 and 32 -37^oC, respectively. There is no information on the conditions required for the production of fumonisin B₁.

Exposure to fumonisin B₁ (FB1) in maize causes leukoencephalomalacia (LEM) in horses and pulmonary oedema in pigs. LEM has been reported in many countries including the USA, Argentina, Brazil, Egypt, South Africa and China. FB1 is also toxic to the central nervous system, liver, pancreas, kidney and lung in a number of animal species.

The presence of the fumonisins in maize has been linked with the occurrence of human oesophageal cancer in the Transkei, southern Africa and China. The relationship between exposure to F. moniliforme, in home-grown maize, and the incidence of oesophageal cancer has been studied in the Transkei during the ten-year period 1976-86 (Rheeder et al, 1992). The percentage of kernels infected by F. moniliforme was significantly higher in the high-risk cancer area during the entire period; and FB1 and FB2 occurred at significantly higher levels in mouldy maize obtained from high-risk areas in 1986.

Previously, an evaluation by the International Agency for Research on Cancer had concluded that there is sufficient evidence in experimental animals for the carcinogenicity of cultures of F. moniliforme that contain significant amounts of the fumonisins; whereas there is limited evidence, in experimental animals, for the carcinogenicity of fumonisin B₁ (IARC, 1993d). However, the results of a recently completed study of the toxicology and carcinogenesis of fumonisin B₁ has been reported (NTP, 1999) by the National Toxicology Program of the US Department of Health and Human Services. Although the report is still in draft form, it concludes that there is clear evidence of carcinogenic activity of fumonisin B₁ in male F344/N rats based on the increased incidences of renal tubule neoplasms; and that there is also clear evidence of carcinogenic activity of fumonisin B₁ in female B6C3F₁ mice based on the increased incidences of hepatocellular neoplasms. There is no evidence of carcinogenic activity of fumonisin B₁ in female rats or male mice.

Ochratoxin A

A. ochraceus grows more slowly than both A. flavus and A. parasiticus, but can grow at a water activity as low as 0.79. Growth has also been reported within the temperature range 8 - 37 ^oC, with an optimum variously reported as 25 - 31^oC. Ochratoxin A is produced within the temperature range 15 - 37 ^oC, with an optimal production at 25 - 28 ^oC.

P. verrucosum grows within the temperature range 0 - 31^oC and at a minimum water activity of 0.80. Ochratoxin A is produced over the whole temperature range. Significant quantities of toxin can be produced at a temperature as low as 4^oC, and at a water activity as low as 0.86.

Exposure (IARC, 1993e) to ochratoxin A (OA) appears to occur mainly in wheat and barley growing areas in temperate zones of the northern hemisphere. The levels of OA reported in these commodities ranges from trace amounts to 6000 µg/kg, in Canadian wheat. In the UK, reported levels have varied from <25 to 5,000 and from <25 to 2,700 µg/kg in barley and wheat respectively. It also occurs in maize, rice, peas, beans, cowpeas, vine fruits and their products, coffee, spices, nuts and figs.

The ability of OA to transfer from animal feeds to animal products has been demonstrated by the occurrence of this toxin in retail pork products, and the blood of swine, in Europe.

Although cereal grains are considered to be the main human dietary source of OA, it has been suggested (IARC, 1993e) that pork products may also be a significant source of this toxin. Ochratoxin A has been found in blood (and milk) from individuals in a variety of European countries, including France, Italy, Germany, Denmark, Sweden, Poland, Yugoslavia and Bulgaria. One of the highest reported levels is 100 ng/ml OA in blood from Yugoslavia (Fuchs et al, 1991); whereas 6.6 ng/ml OA in milk has been recorded in Italy (Micco et al, 1991).

Existing or proposed regulations for OA are available in at least eleven countries, the permitted levels ranging from 1 to 50 µg/kg in foods and from 100 to 1000 µg/kg in feeds. In Denmark, the acceptability of pork products from a specific carcass is determined by analysing the OA content of the kidney. The pork meat and certain organs can be consumed as food if the OA content of the kidney is no more than 25 and 10 µg/kg respectively (van Egmond, 1997).

A provisional tolerable weekly intake of 100 ng/kg bw, of OA, approximating to 14 ng/kg body weight per day, has been recommended by a WHO/FAO Joint Expert Committee on Food Additives, JECFA (JECFA, 1996a).

Ochratoxin A has been linked with the human disease Balkan endemic nephropathy, a fatal, chronic renal disease occurring in limited areas of Bulgaria, the former Yugoslavia and Romania. OA causes renal toxicity, nephropathy and immunosuppression in several animal species and it is carcinogenic in experimental animals.

There is sufficient evidence in experimental animals for the carcinogenicity of OA (IARC, 1993e).

Patulin

Patulin (Figure 4) is an antibiotic produced by a number of moulds. It occurs in rotten apples contaminated by Penicillium expansum and, consequently, may occur in apple juice and other apple-based products.

Experimental studies have demonstrated that patulin is a neurotoxin and that it produces marked pathological changes in the viscera. Although patulin has been reported as inducing local sarcomas, no mutagenic activity has been discernible in most short-term tests.

JECFA (JECFA, 1996b) has established a provisional maximum tolerable daily intake of 400 ng/kg bw for patulin.