The link between the growth of fungi in animal and human food stuffs and the occurrence of disease has been recognised for a very long time, but not until the advent of modern large scale animal husbandry did the major health risk of feed containing mycotoxins became apparent. The puzzling deaths of large numbers of turkeys in the 1960's led to the discovery of the aflatoxins. Since then the veterinary importance of mycotoxins has been increasingly recognised, particularly once it became known that even small amounts could influence animal health and cause major economic losses. Much of the earlier interest and work in the animal mycotoxicoses was concerned with those associated with standing crops and the storage of grains or nuts. However from a New Zealand perspective it must not be forgotten that this work was preceded by other work associated with the mycotoxicoses of grassland farming. Foremost among these was facial eczema the importance of which is well known to most North Island farmers and veterinarians. Now many mycotoxicoses are known to be associated with grasslands. Table 59 illustrates some of the important mycotoxicoses of ruminants.
Table 59 Mycotoxicoses and "suspected mycotoxicoses" associated with pasture and crops in New Zealand and elsewhere. +++, major importance; ++, important; +, minor importance; ?, minor or unknown importance (*Clavibacter is a bacterium rather than a fungus.)
| Disease | Fungal Genus | Substrate | Toxin | Importance NZ overseas |
|
| Facial eczema | Pithomyces | Pasture litter | Sporidesmin | +++ | ++ |
| Ryegrass staggers | Neotyphodium | Lolium spp. | Lolitrem B | ++ | ++ |
| Fescue foot | Neotyphodium | Festuca spp. | Ergot alkaloids | + | + |
| Paspalum staggers | Claviceps | Paspalum | Paspalanine | + | + |
| Ergotism | Claviceps | Grasses/Grains | Ergot alkaloids | + | + |
| Fusarium infertility | Fusarium | Lolium? | Zearalenone | ||
| Neotyphodium infertility | Neotyphodium | Zearalenone | +? | +? | |
| Fescue toxicity | Neotyphodium | Festuca spp. | Ergot alkaloids | ||
| Lupinosis | Phomopsis | dead lupins | ? | ? | + |
| Slobbers | Rhizoctonia | Trifolium spp. | Slaframine | ||
| Fusarium toxicoses | Fusarium | Crops/?pasture | Trichothecenes | + | |
| Stachybotryotoxicosis | Stachybotrys | Crops/?pasture | Trichothecenes | + | |
| Myrotheciotoxicosis | Myrothecium | kikuyu grass | Trichothecenes | ||
| Leukoencephalomalacia (ELEM) | F. moniliforme, F. proliferatum | Crops/grains | Fumonisins | ||
| Pulmonary oedema | F. moniliforme | Crops/grains | Fumonisins | ||
| Aflatoxicosis | Aspergillus | mainly grain | aflatoxin | ? | + |
| Annual ryegrass toxicity | Clavibacter* | annual ryegrass | tunicamycin | ? | + |
| Tremor | Penicillium crustosum | walnuts | penitrem | rare | rare |
There are many known diseases of animals caused by metabolites of fungi and these are listed to both emphasise their species diversity and the fact that there may be many more conditions of "unknown aetiology" yet to be elucidated. The range of diseases is shown by the following examples. There is swine nephropathy in Denmark, Ireland and Sweden and in poultry in Denmark caused by ochratoxin, trout hepatoma in the United States and Italy due to aflatoxin, salivation factor in cattle eating fungus infected leguminosis hay in which slaframine is produced; swine, poultry and sheep reproductive problems caused by zearalenone, death of cattle from T 2 toxins, death of farm animals from Fusarium toxins (referred to as alimentary toxic aleukia in Russia); vomition and rejection of corn infected with Fusarium by pigs; acute poisoning and death of poultry fed aflatoxin (turkey "x" disease); tremortin toxin causing death of pigs; stachybotryotoxicosis in horses, sheep and goats and penicillin acid poisoning of pigs.
The mycotoxins are toxic metabolites of the secondary metabolism fungi, mainly those of the genera Aspergillus, Penicillium and Fusarium. Other genera capable of forming highly toxic substances include Claviceps, Stachybotrys, Alternaria, Myrothecium and Pithomyces. Crops of cereals left standing in the field, incorrectly stored feed and mature or dry grass can become contaminated by fungi, particularly when environmental conditions such as substrate humidity, temperature and pH are suitable for their growth and mycotoxin formation. Optimum conditions for fungal growth are not necessarily the best conditions for toxin formation.
Chemically most mycotoxins are aromatic (or rarely aliphatic) hydrocarbons, representing a wide variety of compounds. Since their molecular weight is relatively low they are fairly resistant to environmental factors and they are not immunogenic.
The structural variety of mycotoxins makes it difficult to devise embracing tests for them in organic substrates, and this same variety is also responsible for a wide range of toxic effects. They can be classified according to their predilection sites into hepato , nephro , neuro , myo , cardio , dermato , genito , and gastro intestinal toxins.
In addition certain mycotoxins are strongly carcinogenic or immunosuppressive.
The severity of several mycotoxicoses of sheep which have been discussed are associated with improved pastures and influenced by the closeness of grazing. Sheep, with their method of food prehension are well known for their ability to graze closely to the ground. Research has shown that sheep which are closely grazed on pastures are more likely to suffer certain mycotoxicoses. In addition the toxins of several of these mycotoxicoses are found in higher concentrations either in the litter of the pasture or in the lower portions of the plant, usually before the ligule or in the less palatable stalks. Examples of this are facial eczema, ryegrass staggers, the fescue toxicities, possibly zearalenone associated fusarium toxicity of sheep and lupinosis in Australia. In contrast to this are the toxicities such as paspalum staggers and ergotism, associated with seed head infection and the presence of ergots.
The obvious lesson from this distribution of toxic material is that, especially in times when mycotoxin levels are high, very close grazing of pastures or access to toxic seedheads should be avoided. This fits well with the current philosophies and concepts of pasture utilisation; notably the "controlled grazing system" [CGS]. Among other things CGS is based on the maxim that "grass makes grass" and that for the best utilisation of pastures sufficient length of grass plant should be left after mob stocking to stimulate regrowth after sheep have been removed. Seeding should only be allowed to occur in controlled circumstances such as for hay making or seed production. Not only does seeding of grasses such as ryegrass and fescue allow ingestion of toxins by grazing sheep but it also allows the indiscriminate spread of the mycotoxin producing fungi, Claviceps and Acremonium.
To achieve such a good balance between over and under grazing of pastures is not always easy and requires a high standard of pasture and animal management. However, it should be regarded as a high priority, not only to protect animal health, but also to maximise production and profit.
Bryden, W.L. (1987). Metabolic Effects and Lesions of Mycotoxins in Poultry and Pigs: Proceedings No. 103 Veterinary Clinical Toxicology. The Post Graduate Committee in Veterinary Science, University of Sydney, Australia. 341 376.
Gbodi, T.A. (1988). Mycotoxicoses in Domestic Animals: A Review. Vet Hum Toxicol. 30:235 245.
Seawright, A.A. (1982): Animal Health in Australia. Vol. 2 Chemical and Plant Poisons. Australian Government Publishing Service, Canberra.
Smith, B.L. (1987). Mycotoxicoses of cattle and horses in Australia and New Zealand. Proceedings No. 103 Veterinary Toxicology. The Post Graduate Committee in Veterinary Science, University of Sydney, Australia. 270 276.
Smith, B.L. (1987): Mycotoxicoses in sheep: Proceedings No. 103 Veterinary Toxicology. The Post Graduate Committee in Veterinary Science, University of Sydney, Australia. 279 286.
Surveillance (1974) 1(2): 9 Feedback on fatty liver (hens)
Surveillance (1975) 2(5): 8 Bleeding pig disease
Surveillance (1981) 8(1): 21 The problem was in the feed (turkeys)
Surveillance (1981) 8(3): 3 Mycotoxicosis: from Charles Darwin to Ruakura
Surveillance (1983) 10(2): 19 Mycotoxicosis a lesson to be learnt
Surveillance (1983) 10(4): 27 Mycotoxicosis in poultry