Feed borne mycotoxins: the threat to poultry?

Professor Trevor K. Smith, Ph.D., P.Ag., Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada

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Mycotoxins are metabolites produced by molds (fungi) that can infest crops pre-harvest and can continue to flourish under sub-optimal storage conditions. Grains with high moisture content are particularly unstable and prone to mold proliferation and possible mycotoxin production. Excess rainfall at harvest and at key periods during the growing season can be a major promoter of mycotoxin contamination of feedstuffs.

The most significant species of mycotoxin-producing fungi that have an impact on poultry production would include Aspergillus and Fusarium. The most significant mycotoxins produced by Aspergillus fungi are the aflatoxins.

The fungi that synthesize aflatoxins, A. flavus and A. parasiticus, are considered to be tropical or semi-tropical molds that thrive under conditions of high moisture and temperature. The effects of feed-borne aflatoxin on poultry production have been extensively studied and we have a good understanding of the tolerance of various classes of poultry. This is partly due to concern for human health and food safety issues arising from contamination of poultry products with aflatoxin since aflatoxin is a potent hepatocarcinogen. Analytical techniques for aflatoxin analysis in feeds are very practical due to the small number of different compounds which allows their simultaneous analysis.

Fusarium fungi flourish in more temperate climates including much of the American Corn Belt. Our understanding of Fusarium mycotoxicoses in poultry is much less complete than our understanding of aflatoxicosis. This is in part because of the very large number of Fusarium mycotoxins, more than one hundred have been chemically characterized, which makes complete analysis of feedstuffs for Fusarium mycotoxins impractical, if not impossible. The most commonly recognized Fusarium mycotoxins include the trichothecenes, a large family of structurally-related compounds including deoxynivalenol (DON, vomitoxin), T-2 toxin, nivalenol, diacetoxyscirpenol (DAS) and over 100 others; zearalenone, an estrogenic compound; fumonisins and fusaric acid.

Analysing mycotoxins in poultry feeds
A major source of error in mycotoxin analysis is inadequate sampling of feedstuffs. Proper sampling protocols have been developed and published in an effort to minimize this source of error. Another source of error is the potential presence of different chemical forms of mycotoxins which may escape routine analysis. Attention has been focused on the presence of conjugated forms of mycotoxins that are produced by plants. This may be the result of detoxification of mycotoxins by plant metabolism and it has been suggested that the presence of conjugated mycotoxins might be used in making genetic selection of plant resistance to fungal invasion.

Although conjugated forms of dexoynivalenol (DON, vomitoxin) were identified many years ago, little information is available about the relative significance of conjugated and free mycotoxins in poultry diets. It is not yet clear if the conjugated forms of mycotoxins are as harmful to poultry as the parent compounds, but it has been shown that some conjugated mycotoxins can be hydrolyzed in the digestive tracts of animals.

It must be concluded that until we have a better understanding of the frequency, toxicity and nature of conjugated mycotoxins, current mycotoxin analysis of poultry feeds should often be considered to be an underestimate of correct values. It is necessary at this time, therefore, to consider mycotoxin analysis of feeds as offering only an approximation of the true hazard posed by the feeding of contaminated materials to poultry.

The effects of feeding blends of grains naturally contaminated with Fusarium mycotoxins to poultry
A series of studies has been conducted to determine the effects of feeding blends of naturally-contaminated feedstuffs, largely corn and wheat, to different types of poultry. This was done in an effort to mimic conditions seen in commercial poultry production where diets contain multiple vectors of mycotoxin contamination. The mycotoxins in such diets were determined to be mainly DON with lesser amounts of zearalenone and 15-acetyl DON in addition to fusaric acid. Four different modes of action of the mycotoxins fed were identified: (1) reductions in cellular protein synthesis (2) reduced immunity (3) alterations in brain neurochemistry (4) damage to the intestinal epithelium.

Reductions in cellular protein synthesis result in lesions of the gastrointestinal tract including necrosis, gizzard erosion, haemorrhaging, and malabsorption of nutrients. Reduced hepatic protein synthesis can decrease utilization of dietary amino acids resulting in increased uric acid synthesis as amino acids are oxidized for energy purposes.

Many Fusarium mycotoxins, as well as aflatoxin, have been shown to be immunosuppressive. This results in increased susceptibility to disease, lingering health problems in the flock and possible failure of vaccination programs. The disease symptoms arising from immunosuppression, moreover, are not symptoms characteristic of mycotoxins. They are only indirectly caused by mycotoxins and this makes certain identification of mycotoxins as the causative agent of reduced flock health very difficult.

Combinations of feed-borne Fusarium mycotoxins are pharmacologically active. This means they have drug-like properties due to their effects on brain neurochemistry. The most reproducible effects observed are elevations in brain regional concentrations of serotonin. Such changes alter behaviour including reductions in feed intake, loss of muscle coordination and increased lethargy.

Broilers
The feeding of a blend of ingredients naturally-contaminated with a combination of Fusarium mycotoxins resulted in reduced growth in the grower phase, elevations in blood uric acid levels, discoloration of breast meat and immunosuppression. Changes in brain neurochemistry were also seen.

Broiler Breeders
The feeding of a similar combination of Fusarium mycotoxin contaminated materials to broiler breeders significantly reduced hatchability due to reduced shell thickness of fertile eggs. Changes in brain neurochemistry were also observed. There were no effects of diet on sperm quality. In a parallel study with broiler breeder pullets, immunosuppression was observed.

Layers
Laying hens were very sensitive to the feeding of combinations of Fusarium mycotoxins. Egg production and feed efficiency were reduced while major increases in blood uric acid concentrations were seen. The elevations in blood uric acid levels were likely due to a reduction in hepatic fractional protein synthesis rates. Immunosuppression was also observed.

Turkeys
Turkeys were very sensitive to the feeding of high levels of Fusarium mycotoxin-contaminated feeds. Growth rates were significantly reduced even in the starter phase and some indices of immunosuppression were. The feeding of lower concentrations of Fusarium mycotoxins also reduced growth rates, elevated blood uric acid levels and caused immunosuppression. This was coupled with morphological changes in the small intestine and changes in brain neurochemistry.

Conclusion
It can be concluded that poultry are sensitive to combinations of feed-borne Fusarium mycotoxins and that the feeding of contaminated materials should be minimized. It appears that the frequency of mycotoxin contamination of poultry feeds is increasing. This may be due in part to adverse weather conditions pre-harvest in many parts of the world arising from global climate change.

The complex nature of modern poultry rations including the increasing use of potentially contaminated by-products such distillers’ dried grains adds to the possibility of toxicological synergy between combinations of mycotoxins thereby increasing the severity of the response of poultry to contaminated feeds.

The use of an appropriate mycotoxin adsorbent is likely the best short-term strategy available for minimizing the adverse effects of feed-borne mycotoxins in poultry feeds. It is hoped that long term strategies such as improved quality control measures arising from advances in analytical methodology and plant breeding strategies to reduce the susceptibility of plants to fungal invasion will help to minimize mycotoxin challenges to the poultry industries in the future.

References are available on request