Prof. Raymond D Coker
Founder & Director
Raymond Coker Consulting Limited
INTRODUCTION TO MYCOTOXINS
“Wailing and writhing men collapsed in the street: others fell over and foamed in epileptic fits whilst some vomited and showed signs of insanity. Many of them shouted “Fire! I’m burning”. It was an invisible fire that separated the flesh from the bones and consumed it. Men, women and children died in unbearable agonising pain.”…
These are the words used by a tenth century chronicler to describe a disease which affected many parts of Europe in 943 AD. The disease became known as ‘St Anthony’s fire’ because of the burning sensation experienced in the limbs of victims (ultimately leading to the onset of gangrene and the loss of limbs), and because many of the victims visited the shrine of St Anthony in France in the hope of being cured.
We now know that St Anthony’s Fire (ergotism) was caused by the consumption of rye contaminated with the ‘ergot alkaloids’, produced by the mould Claviceps purpurea (Bove, 1970; Beardall and Miller, 1994), and that it reached epidemic proportions in many parts of Europe in the tenth century. Toxic secondary metabolites, such as the ergot alkaloids which are produced by certain moulds, are described as ‘mycotoxins’, and the diseases they cause are called ‘mycotoxicoses’.
Mycotoxins occur in a wide variety of foods and feeds and have been implicated (Coker, 1997) in a range of human and animal diseases. Exposure to mycotoxins can produce both acute and chronic toxicities ranging from death to deleterious effects upon the central nervous, cardiovascular and pulmonary systems, and upon the alimentary tract. Mycotoxins may also be carcinogenic, mutagenic, teratogenic and immunosuppressive. The ability of some mycotoxins to compromise the immune response and, consequently, to reduce resistance to infectious disease is now widely considered to be the most important effect of mycotoxins, particularly in developing countries.
The condition of a crop is determined by a complex milieu involving a multitude of interactions between the crop, the macro- and micro-environment and a variety of biological, chemical, physical and socio-economic factors. A change within any one process will invariably bring about changes in one or more of the other processes. Action taken before harvest to control pest damage and/or increase production (e.g. selection of varieties, timing of harvest) can have a significant impact on the post-harvest quality of the crop. Hybrid white maize, for example, has much higher yields than traditional varieties but has poor on-farm storage characteristics.
The factors which primarily contribute to bioterioration (including mould growth) within an ecosystem, are moisture, temperature and pests. Moulds can grow over a wide range of temperatures and, in general, the rate of mould growth will decrease with decreasing temperature and available water. In grains, moulds utilise intergranular water vapour, the concentration of which is determined by the state of the equilibrium between free water within the grain (the grain moisture content) and water in the vapour phase immediately surrounding the granular particle. The intergranular water concentration is described either in terms of the equilibrium relative humidity (ERH, %) or water activity (aw). The latter describes the ratio of the vapour pressure of water in the grain to that of pure water at the same temperature and pressure, whilst the ERH is equivalent to the water activity expressed as a percentage. For a given moisture content, different grains afford a variety of water activities and, consequently, support differing rates and type of mould growth. Typical water activities which are necessary for mould growth range from 0.70 to 0.99, where the water activity, and the propensity for mould growth increase with temperature. Maize, for example, can be relatively safely stored for one year at a moisture level of 15 per cent and a temperature of 15oC. However, the same maize stored at 30oC will be substantially damaged by moulds within three months.
Insects and mites (arthropods) can also make a significant contribution towards the biodeterioration of grain because of the physical damage and nutrient losses caused by their activity, and also because of their complex interaction with moulds and mycotoxins. The metabolic activity of insects and mites causes an increase in both the moisture content and temperature of the infested grain. Arthropods also act as carriers of mould spores and their faecal material can be utilised as a food source by moulds. Furthermore, moulds can provide food for insects and mites but, in some case, may also act as pathogens.
Another important factor that can affect mould growth is the proportion of broken kernels in a consignment of grain. Broken kernels, caused by general handling and/or insect damage, are predisposed to mould invasion of the exposed endosperm.
Mould growth is also regulated by the proportions of oxygen, nitrogen and carbon dioxide in the inter-granular atmosphere. Many moulds will grow at very low oxygen concentrations; a halving of linear growth, for example, will only be achieved if the oxygen content is reduced to less than 0.14 per cent. Interactions between the gases and the prevailing water activity also influence mould growth.
The interactions described above, within granular ecosystems, will support the growth of a succession of micro-organisms, including toxigenic moulds, as the nutrient availability and microenvironment changes with time. In the field, grains are predominantly contaminated by those moulds requiring high water activities (at least 0.88) for growth, whereas stored grains will support moulds which grow at lower moisture levels.
It is well recognised that the main factors which influence the production of mycotoxins are water activity and temperature. However, given the complexity of the ecosystems supporting the production of mycotoxins, the conditions under which toxigenic moulds produce mycotoxins are equally complex.
Those mycotoxins which are currently considered to be of most importance within foods and feeds, together with their chemical structures and the crops with which they are especially associated, are shown below (Table 1 & Figure 1).
The major mycotoxin-producing mould species are; Aspergillus flavus & A. parasiticus (aflatoxins B1, B2, G1, G2); A. ochraceus & Penicillium verrucosum (ochratoxin A); Fusarium graminearum (deoxynivalenol & zearalenone); F. sporotrichioides (T-2 toxin) and F. verticillioides & F. proliferatum (fumonisin B1). Mycotoxins occur in a wide variety of crops, including cereals, edible nuts and oilseeds. Maize, especially, may be contaminated by a variety of mycotoxins (Table 1).
It is also evident that mycotoxins are represented by a wide range of chemical structures.
Mycotoxins are toxic towards a variety of key organs in animals and Man, impacting upon their health and productivity (Table 1). Those mycotoxins which compromise the immune system are especially important.
Aflatoxins B1, B2, G1 & G2
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 Aspergillus flavus toxins in groundnut meal imported from South America (Austwick, 1978).
The aflatoxin-producing moulds (e.g. A. flavus & A. parasiticus) occur widely, in temperate, sub-tropical and tropical climates, throughout the world, although the aflatoxins are predominantly associated with commodities of sub-tropical and tropical origin. Aflatoxins may be produced, both before and after harvest, on many foods and feeds especially oilseeds, edible nuts and cereals.
Aflatoxin B1 is a human carcinogen, and is one of the most potent hepatocarcinogens known.
The chronic effects of low dietary levels (parts per billion) of aflatoxins on livestock are also well documented (Coker, 1997) and include decreased productivity and increased susceptibility to disease.
Aflatoxin B1 in dairy feed can be metabolised and transferred to cow’s milk in the form of aflatoxin M1. The percentage carry-over rate typically lies within the range of 1-5 per cent depending upon, for example, the level of aflatoxin B1 within the feed and the productivity of the cow. However, the carry-over rate can vary significantly from cow to cow and, on an individual cow basis, from day to day.
T-2 toxin & Deoxynivalenol
T-2 toxin and deoxynivalenol 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 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 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.
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 are a group of mycotoxins produced primarily by Fusarium verticillioides and Fusarium proliferatum.
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.
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.
Ochratoxin A (OA) appears to occur mainly in wheat and barley growing areas in temperate zones of the northern hemisphere.
It 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.
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.
Other Moulds and Mycotoxins
There are a number of mycotoxicoses which are not widely occurring, but which are important to the exposed populations in the affected regions.
The mould and mycotoxins include those which have been associated with a variety of livestock diseases including ergotism, paspalum staggers, ryegrass staggers, facial eczema, fescue foot, lupinosis, slobber syndrome, stachybotryotoxicosis and diplodiosis (Table 2).
The Co-occurrence of Mycotoxins
The complex ecology of mould growth and mycotoxin production can produce mixtures of mycotoxins in foods and feeds, especially in cereals. The co-occurrence of mycotoxins can affect both the level of mycotoxin production and the toxicity of the contaminated material. The production of the aflatoxins in stored grains, for example, may be enhanced by the presence of trichothecenes, whereas the toxicology of naturally occurring combinations of trichothecene mycotoxins is reportedly (Schiefer et al, 1986) determined by synergistic interactions, in experimental animals. For example, in a study with swine, the effect of deoxynivalenol on weight gain and feed conversion was synergized by T-2 toxin.