MYCOTOXINS

Mycotoxins are those secondary metabolites of fungi that have the capacity to impair animal health and productivity (D'Mello and Macdonald, 1998). The diverse effects precipitated by these compounds are conventionally considered under the generic term "mycotoxicosis", and include distinct syndromes as well as non-specific conditions. A list of the principal mycotoxins occurring in feeds and forages is given in Table 1, which also indicates the fungal species associated with the production of these contaminants. Mycotoxin contamination of forages and cereals frequently occurs in the field following infection of plants with particular pathogenic fungi or with symbiotic endophytes. Contamination may also occur during processing and storage of harvested products and feed whenever environmental conditions are appropriate for spoilage fungi. Moisture content and ambient temperature are key determinants of fungal colonization and mycotoxin production. It is conventional to subdivide toxigenic fungi into "field" (or plant-pathogenic) and "storage" (or saprophytic/spoilage) organisms. Claviceps, Neotyphodium, Fusarium and Alternaria are classical representatives of field fungi while Aspergillus and Penicillium exemplify storage organisms. Mycotoxigenic species may be further distinguished on the basis of geographical prevalence, reflecting specific environmental requirements for growth and secondary metabolism. Thus, Aspergillus flavus, A. parasiticus and A. ochraceus readily proliferate under warm, humid conditions, while Penicillium expansum and P. verrucosum are essentially temperate fungi. Consequently, the Aspergillus mycotoxins predominate in plant products emanating from the tropics and other warm regions, while the Penicillium mycotoxins occur widely in temperate foods, particularly cereal grains. Fusarium fungi are more ubiquitous, but even this genus contains toxigenic species that are almost exclusively associated with cereals from warm countries.

  An emerging feature is the co-production of two or more mycotoxins by the same species of fungus (Table 1). This observation has enabled a fresh interpretation of the causes of well-known cases recorded in the history of mycotoxicoses.

Aflatoxins

This group includes aflatoxin B₁, B₂, G₁ and G₂ (AFB₁, AFB₂, AFG₁ and AFG₂, respectively). In addition, aflatoxin M₁ (AFM₁) has been identified in the milk of dairy cows consuming AFB₁-contaminated feeds. The aflatoxigenic Aspergilli are generally regarded as storage fungi, proliferating under conditions of relatively high moisture/humidity and temperature. Aflatoxin contamination is, therefore, almost exclusively confined to tropical feeds such as oilseed by-products derived from groundnuts, cottonseed and palm kernel. Aflatoxin contamination of maize is also an important problem in warm humid regions where A. flavus may infect the crop prior to harvest and remain viable during storage.

  Surveillance of animal feeds for aflatoxins is an ongoing issue, owing to their diverse forms of toxicity and also because of legislation in developed countries (D'Mello and Macdonald, 1998). In the United Kingdom, analysis conducted during the 1987-1990 period indicated that all imported feedstuffs complied with legislation in force for AFB₁ levels. Elsewhere, however, aflatoxin levels in certain feeds still pose serious risks to animal health. Thus, in India total aflatoxin levels of 3 700 _g/kg were detected in a sample of groundnut cake. Of potentially greater significance is the contamination of maize samples in China and northern Viet Nam with combinations of AFB₁ and Fusarium mycotoxins. In China, 85 percent of maize samples were contaminated with both AFB₁ and fumonisin B₁ at levels ranging from 8 to 68 _g/kg and 160 to 25 970 _g/kg, respectively. Feed-grade maize in northern Viet Nam had AFB₁ levels ranging from 9 to 96 _g/kg, and fumonisin B₁ levels in the range of 271 to 3 447 _g/kg (Placinta, D'Mello and Macdonald, 1999). Between 1988 and 1989, analyses of farmgate milk in the United Kingdom showed low levels of AFM₁ contamination, but more than 50 percent of milk samples in the United Republic of Tanzania were found to contain the mycotoxin (D'Mello and Macdonald, 1998). The importance of aflatoxins in animal health emerged in 1960, following an incident in the United Kingdom in which 100 000 turkey poults died from acute necrosis of the liver and hyperplasia of the bile duct ("turkey X disease"), attributed to the consumption of groundnuts infected with Aspergillus flavus. This event marked a defining point in the history of mycotoxicoses, leading to the discovery of the aflatoxins. Subsequent studies showed that aflatoxins are acutely toxic to ducklings, but ruminants are more resistant. However, the major impetus arose from epidemiological evidence linking chronic aflatoxin exposure with the incidence of cancer in humans.

Ochratoxins

The Aspergillus genus includes a species (A. ochraceus) that produces ochratoxins, a property it shares with at least two Penicillium species. Ochratoxin A (OA) and ochratoxin B are two forms that occur naturally as contaminants, with OA being more ubiquitous, occurring predominantly in cereal grains and in the tissues of animals reared on contaminated feed. Another mycotoxin, citrinin, often co-occurs with ochratoxin. In recent Bulgarian wheat samples, OA and citrinin levels ranged from < 0.5 to 39 _g/kg and from < 5 to 420 _g/kg, respectively. In oats, higher levels of OA were detected (maximizing at 140 _g/kg) while citrinin was below detection limits (D'Mello, 2001).

  The ochratoxins and citrinin are nephrotoxic to a wide range of animal species. OA is frequently implicated in porcine nephropathy and in Balkan endemic nephropathy of humans. The role of citrinin in these syndromes has yet to be elucidated.

Fusarium mycotoxins

Extensive data now exist to indicate the global scale of contamination of cereal grains and animal feed with Fusarium mycotoxins (D'Mello and Macdonald, 1998). Of particular importance are the trichothecenes, zearalenone (ZEN) and the fumonisins. The trichothecenes are subdivided into four basic groups, with types A and B being the most important. Type A trichothecenes include T-2 toxin, HT-2 toxin, neosolaniol and diacetoxyscirpenol (DAS). Type B trichothecenes include deoxynivalenol (DON, also known as vomitoxin), nivalenol and fusarenon-X. The production of the two types of trichothecenes is characteristic for a particular Fusarium species. However, a common feature of the secondary metabolism of these fungi is their ability to synthesize ZEN which, consequently, occurs as a co-contaminant with certain trichothecenes. The fumonisins are synthesized by another distinct group of Fusarium species (Table 1). Three members of this group (fumonisins B₁, B₂ and B₃) often occur together in maize.

  Virtually all the toxigenic species of Fusarium listed in Table 1 are also major pathogens of cereal plants, causing diseases such as head blight in wheat and barley and ear rot in maize. Harvested grain from diseased crops is therefore likely to be contaminated with the appropriate mycotoxins, and this is supported by ample evidence. Surveillance of grain and animal feed for the occurrence of Fusarium mycotoxins has been the subject of many investigations over recent years (Tables 2 and 3). The global distribution of these mycotoxins is a salient feature, but striking regional differences should also be noted. Another aspect worthy of comment is consistent evidence of the co-occurrence of various Fusarium mycotoxins in the same sample. These issues have been considered at greater length by Placinta, D'Mello and Macdonald (1999) who, for example, referred to a German study in which 94 percent of wheat samples analysed were contaminated by between two and six Fusarium mycotoxins and 20 percent of the samples were co-contaminated with DON and ZEN (Table 2). The most frequent combination included DON, 3-ADON and ZEN. T-2 and HT-2 toxins were detected at levels ranging from 0.003 to 0.250 mg/kg and 0.003 to 0.020 mg/kg, respectively, but these mycotoxins only occurred in combination with DON, NIV and ZEN.

  In the Lublin region of southeastern Poland, type A trichothecene contamination of barley grain was linked with the natural incidence of fusarium head blight, in which the predominating organism was F. sporotrichioides (Placinta, D'Mello and Macdonald, 1999). Of 24 barley grain samples, 50 percent were positive for T-2 toxin, with a range of 0.02 to 2.4 mg/kg. In five of these samples, co-contamination with HT-2 toxin occurred, with a range of 0.01 to 0.37 mg/kg. Maize ears may also become naturally infected with Fusarium pathogens. The findings of one study in Poland indicated that infection with F. graminearum can result in contamination of cobs with DON (Table 2) and 15-ADON simultaneously (Placinta, D'Mello and Macdonald, 1999). Concentrations of DON and 15-ADON in Fusarium-damaged kernels ranged from 4 to 320 mg/kg and 3 to 86 mg/kg, respectively, but the axial stems of the cobs were more heavily contaminated, at 9 to 927 mg/kg (Table 2) and 6 to 606 mg/kg, respectively. Oat grains produced in Norway by commercial growers were found to be more heavily contaminated with DON than barley or wheat kernels (Table 2). In addition to NIV (Table 2), other contaminants included 3-ADON and fusarenon-X. For example, 56 percent of certain oat samples contained detectable quantities of 3-ADON at 0.03 mg/kg or more. Other notable examples of DON contamination include wheat and barley samples from Japan and the United States (Table 2). It should be stated, however, that even in samples with lower levels of contamination, high incidence rates have been recorded. Thus, 90 and 79 percent of cereal samples in the Netherlands were positive for DON and NIV respectively (Placinta, D'Mello and Macdonald, 1999).

  Widespread contamination of maize and animal feed with fumonisins has recently been reported (Table 3). In most instances the predominant fumonisin was FB₁. The highest values for FB₁ were recorded from maize samples in China, where AFB₁ co-occurred in 85 percent of samples, and in Thailand. Multiple contamination of maize with fumonisins, DON, NIV and AFB₁ was also observed in northern Viet Nam. For FB₂, the highest values in maize were found in samples from Argentina. In the Philippines, Thailand and Indonesia, FB₁ and FB₂ occurred in more than 50 percent of maize samples, and these mycotoxins co-occurred with aflatoxins in 48 percent of samples (Placinta, D'Mello and Macdonald, 1999).

  The Fusarium mycotoxins induce a wide range of effects in farm livestock (D'Mello, 2000). DON is a potent feed intake inhibitor in pigs; ZEN is associated with reproductive abnormalities in pigs and ruminants. Fumonisins have been linked with specific syndromes, namely porcine pulmonary oedema and equine leukoencephalomalacia. Fumonisin contamination of maize in South Africa has been correlated with the occurrence of oesophageal cancer in humans.

Endophyte alkaloids

The endophytic fungus Neotyphodium coenophialum occurs in close association with perennial tall fescue, while another related fungus, N. lolii, may be present in perennial ryegrass (D'Mello, 2000). Ergopeptine alkaloids, mainly ergovaline, occur in N. coenophialum-infected tall fescue, while the indole isoprenoid lolitrem alkaloids, particularly lolitrem B, are found in N. lolii-infected perennial ryegrass. The ergopeptine alkaloids reduce growth, reproductive performance and milk production in cattle, while the lolitrem compounds induce neurological effects in ruminants.

Phomopsins

In Australia, lupin stubble is valued as fodder for sheep, but infection with the fungus Phomopsis leptostromiformis is a major limiting factor because of toxicity arising from the production of phomopsins by the fungus. Mature or senescing parts of the plant, including stems, pods and seeds, are particularly prone to infection. Phomopsin A is considered to be the primary toxin, causing effects such as ill-thrift, liver damage, photosensitization and reduced reproductive performance in sheep (D'Mello and Macdonald, 1998).

Sporidesmin

Pithomyces chartarum is a ubiquitous saprophyte of pastures and has the capacity to synthesize sporidesmin A, a compound causing facial eczema and liver damage in sheep.