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Current concepts in feed-borne mycotoxins and the potential for dietary prevention of mycotoxicoses |
 
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Author: Trevor K. Smith - Ewen J. MacDonald - Swany Haladi - ALLTECH INC.
The most commonly recognized feed-borne mycotoxins are the aflatoxins
and the Fusarium mycotoxins. Our understanding of the etiology of
aflatoxicosis is far more complete than our corresponding understanding of
Fusarium mycotoxicoses. This is perhaps because of the large volume of
literature fueled by the acutely carcinogenic nature of aflatoxins. Analytical
methodology for aflatoxins in feedstuffs is also simple, sensitive and
reproducible. This is due, in part, to the natural fluorescence of aflatoxin
molecules. Even though Fusarium mycotoxins are likely the most
economically significant grain mycotoxins (Wood, 1992), the complex mixture
of chemically-unrelated compounds has slowed advances in the study of
Fusarium mycotoxicoses. Complete analysis of feedstuffs for Fusarium
mycotoxins is a daunting, time-consuming and prohibitively expensive task.
It is also now clear that toxicological synergism between different
Fusarium mycotoxins can increase the toxicity of a given diet (Smith et al.,
1997) and incomplete analysis can, therefore, give false security as to the
potential hazard posed by the feeding of contaminated grains. It is important,
also, to remember that many different components of a complete feed
can be vectors for mycotoxin contamination. The studies of Smith and
Sousadias (1993, Table 1) indicated that the fusaric acid content of complete
feeds can exceed that found in individual feedstuffs. This has since
been attributed to fusaric acid contamination of soybean meal. Fusaric acid
has been found in soybean plants and is considered to be a phytotoxin in
various vegetable species (Matsui and Watanabe, 1988). Since almost all
strains of Fusarium fungi produce fusaric acid, it has been suggested that
this compound be used as a marker compound for Fusarium contamination
of grains (Bacon et al., 1996). It is therefore important that complete feeds,
and not just suspect individual ingredients, be analyzed when mycotoxin
contamination is suspected.
Current concepts in feed-borne mycotoxins
Several trends have tended to increase the severity and economic importance
of mycotoxicoses in animal and poultry agriculture in recent years. One key
factor in minimizing fungal infestation in field crops and mycotoxin
accumulation in feedstuffs is moisture content during the growing and
harvesting periods. Stored grains should contain less that 15% moisture to
ensure fungal stability. Recent global weather patterns, however, have been
irregular with heavy rainfall and flooding in some areas coupled with drought
and unusual frosts in other regions. Drought stress can also lead to increased
fungal penetration of grains. The result has been an increased frequency of
reports of mycotoxin contamination of feed grains. Some tropical and semitropical
countries are reporting Fusarium contamination of crops where
only aflatoxin was previously detected. It is not clear, however, if this is
also
the result of increasingly frequent testing for compounds that were
erroneously assumed to be absent in the past.
Another trend contributing to the frequency of mycotoxin contamination
of feeds is improved global grain transportation systems and global trading
of agricultural commodities. This allows more extensive shipping of grains
and other feed components throughout the world. The result is that complete
feeds are likely a more complex blend of feedstuffs with more widely
varying geographic origins than was seen in the past. The potential for
aflatoxins and mixtures of Fusarium mycotoxins to be co-contaminants in
feeds is, therefore, enhanced.
The concept of mycotoxin binders
Mycotoxin binders are large molecular weight polymers that, when added
to feed, are capable of forming irreversible complexes with mycotoxin
molecules in the intestinal lumen. Such complexes are not digestible, pass
down the digestive tract and are excreted in the feces. The net effect is to
reduce the dose of absorbed toxin to the point that it is below the biological
threshold. This allows contaminated feed to be fed with minimal losses in
performance. The challenge is to identify compounds capable of effectively
binding a mixture of mycotoxins with widely varying molecular structures
and polarities. The binder must also be effective at low levels of inclusion
since these non-nutritive additives are diluents that will reduce the nutrient
density of the diet. An example of a toxin binder that has been widely used
in veterinary medicine to treat accidental acute poisonings is activated
charcoal.
Mycotoxin binders can be silica-based inorganic polymers or carbon-based
organic polymers. The inorganic polymers currently on the market include
natural clay products as well as synthetic polymers. The advantage of the
clay-based products is their low price. Unfortunately these products also
offer low specificity and so must be used at a relatively high level of inclusion
to be effective. We found this to be the case for both bentonite (Carson
and Smith, 1983a) and spent canola oil bleaching clays (Smith, 1984) when
overcoming T-2 toxicosis. Synthetic inorganic polymers have usually been
designed to effectively bind one specific mycotoxin. This is usually aflatoxin.
Such specific products are, therefore, much less effective against a
mixture of mycotoxins of varying molecular weight and polarity. Synthetic
products are also inevitably more expensive than naturally-produced materials.
Organic toxin binders are derived from plant or microorganism fibers.
Studies in our laboratory indicated that lignin-rich alfalfa fiber was quite
effective at overcoming the toxicity of T-2 toxin (Carson and Smith, 1983b)
and zearalenone (James and Smith, 1982; Stangroom and Smith, 1984). An
advantage of using organic fibers as mycotoxin binders is that feedstuffs
such as dehydrated alfalfa meal have some dietary energy and protein content
and do not act as dietary diluents in the manner of inorganic polymers.
Alfalfa fiber, however, like clay-based products, is an effective mycotoxin
binder at only high levels of dietary inclusion. This makes such materials
impractical when added to livestock and poultry diets.
An innovation in mycotoxin binders is the concept of organic polymers
derived from yeast cell wall fractions. This material has a high surface area
and enough specificity to allow effective mycotoxin binding at a low level of
dietary inclusion. This is the basis of the product Mycosorb (Alltech, Inc.).
A series of experiments was therefore conducted to determine the effectiveness
of yeast cell wall polymers in overcoming Fusarium mycotoxicoses
in broiler chickens and swine.
Response to Mycosorb
FEEDING TRIALS WITH BROILER CHICKENS
Broiler chicks of a commercial strain (Cobb, Big Four Hatchery) were fed
soybean meal, corn and wheat-based diets for eight weeks at the Arkell
Poultry Research Station of the University of Guelph. The diets included:
(1) control (2) same diet formulated with a low level of contaminated corn
and wheat (3) a diet containing higher levels of contaminated corn and
wheat (4) the highly contaminated diet + 0.2% Mycosorb. Three replicate
pens of 30 birds were fed each diet. Weight gain and feed consumption
Current concepts in feed-borne mycotoxins
were determined weekly. Diets were adjusted for protein levels
corresponding to starter (0-3 weeks), grower (4-6 weeks) and finisher (7-8
weeks) phases. Diets were analyzed for deoxynivalenol, fusaric acid,
zearalenone and T-2 toxin. Blood samples were taken at the end of the
starter and finisher phases and a clinical screen of serum metabolites was
conducted. At the end on the experiment, samples of breast, thigh and leg
tissue were tested for discoloration using a Minolta colorimeter.
Results of the trial are given in Tables 2 and 3. Growth rate, feed consumption,
feed efficiency and serum parameters were largely unaffected
by diet with the exception of the finisher phase. In the finisher phase, eeding
of increasing levels of contaminated grains significantly depressed in
growth rates. This effect was overcome by the feeding of 0.2% Mycosorb.
It was also observed that red blood cell counts and concentrations of hemoglobin
and uric acid increased with the feeding of increasing amounts of
contaminated grains. An increasing redness of breast meat was also observed,
as has been reported for turkey poults fed Fusarium culture material
(Wu et al., 1994). The discoloration may be due to the effect of fusaric
acid, which acts as a hypotensive agent. It was concluded that Fusarium
mycotoxicoses can be observed in broiler chickens with the feeding of naturally-
contaminated grains. Such toxicoses can be reversed with the feeding
of 0.2% Mycosorb.
Table 2. Effect of feeding blends of Fusarium mycotoxin-contaminated grains on
growth
performance of broiler chickens.
Table 3. Effect of feeding blends of Fusarium mycotoxin contaminated grains on
blood
metabolites and breast meat coloration of broiler chickens.
FEEDING TRIALS WITH STARTER PIGS
In the summer of 2000, an experiment was conducted at the University of
Guelph Arkell Swine Research Centre to determine the potential for
Mycosorb to overcome the toxicity of diets containing blends of grains
naturally contaminated with deoxynivalenol and fusaric acid. Purebred
Yorkshire pigs (average initial weight 8.1 kg) were fed diets formulated to
contain 3.0 mg deoxynivalenol/kg and 20.0 mg fusaric acid/kg for 21 days.
Diets included: (1) control (2) contaminated grains and (3) contaminated
grains + 0.2% Mycosorb. There was a highly significant reduction in weight
gain and feed intake of pigs fed contaminated grains compared to controls
(Table 4). These differences were too large, however, to be prevented with
0.2% dietary Mycosorb. Higher levels of the product might have been more
effective; however adding the standard dosage of Mycosorb to the diet did
prevent the significant depression in liver weight seen with the feeding of
contaminated grains.
Table 4. Effect of feeding blends of Fusarium mycotoxin-contaminated grains on
growth
performance and liver weights of starter pigs.
At the end of the study, a subgroup of 12 pigs fed each diet was euthanized
and brains were excised and dissected into frontal cortex, pons and
hypothalamus. Brain regional neurochemistry was determined by high
performance liquid chromatography with electrochemical detection. Effects
of diet were seen in the hypothalamus and pons (Figures 1 and 2). Feeding
contaminated grains reduced pons norepinephrine concentrations. This is
likely due to the inhibitory effect of fusaric acid on the activity of dopamine
betahydroxylase, the enzyme that catalyzes the synthesis of norepinephrine
from dopamine. Also seen was an increase in the ratio of 5-
hydroxyindoleacetic acid to serotonin (HIAA/5HT). This ratio can be used,
with caution, as an index of serotonergic neuronal firing and related behaviors
such as loss of appetite, lethargy and loss of muscle coordination. In the
hypothalamus, the HIAA/5HT ratio was also increased by feeding
contaminated grains while concentrations of dopamine were reduced.
Addition of 0.2% Mycosorb to the diet prevented all of the neurochemical
changes seen when contaminated grains were fed.
Figure 1. Effect of deoxynivalenol (DON) and Mycosorb on norepinephrine
and dopamine concentrations in starter pigs (Means without common letters differ,
P<0.05).
Figure 2. Effect of deoxynivalenol (DON) and Mycosorb on 5-hydroxyindoleacetic
acid:serotonin ratios in pons and hypothalamic tissues of starter pigs (Means
without
common letters differ, P<0.05).
Conclusions
The active component of choice in commercial preparations for overcoming
mycotoxin contamination of feeds is a mycotoxin binding agent. In the
absence of an effective binding agent, feeding grains contaminated with
Fusarium mycotoxins results in harmful metabolic changes that reduce
production efficiency of broilers, pigs and other species. Our studies have
shown that Mycosorb effectively prevents such metabolic changes by
preventing intestinal absorption of a mixture of mycotoxins of widely varying
molecular weights and charges. It is an efficient and cost effective solution
to the problem of feeding mycotoxin-contaminated feeds and forages.
TREVOR K. SMITH1, EWEN J. MACDONALD2 AND SWAMY
HALADI1
1Department of Animal and Poultry Science, University of Guelph,
Ontario, Canada
2Department of Pharmacology and Toxicology, University of
Kuopio, Kuopio, Finland
References
Bacon, C.W., J.K. Porter, W.P. Norred and J.F. Leslie. 1996. Production of
fusaric acid by Fusarium species. Appl. Environ. Microbiol. 62:4039.
Carson, M.S. and T.K. Smith. 1983a. Role of bentonite in the prevention of
T-2 toxicosis in rats. J. Anim. Sci. 57:1498.
Carson, M.S. and T.K. Smith. 1983b. Effect of feeding alfalfa and refined
plant fibres on the toxicity and metabolism of T-2 toxin in rats. J. Nutr.
113:304.
James, L.J. and T.K. Smith. 1982. Effect of dietary alfalfa on zearalenone
toxicity and metabolism in rats and swine. J. Anim. Sci. 55:110.
Matsui, Y. and M. Watanabe. 1988. Quantitative analysis of fusaric acid in
the cultural filtrate and soybean plants innoculated with Fusarium
oxysporum var. redolens. J. Rakuno Gakuen Univ. Nat. Sci. 13:159.
Smith, T.K. 1984. Spent canola oil bleaching clays: potential for treatment
of T-2 toxicosis in rats and short-term inclusion in diets for immature
swine. Can. J. Anim. Sci. 64:725.
Smith, T.K., E.G. McMillan and J.B. Castillo. 1997. Effect of feeding blends
of Fusarium mycotoxin-contaminated grains containing deoxynivalenol
and fusaric acid on growth and feed consumption of immature swine. J.
Anim. Sci. 75:2184.
Smith, T.K. and M.G. Sousadias. 1993. Fusaric acid content of swine feedstuffs.
J. Agr. Food Chem. 41:2296.
Stangroom, K.E. and T.K. Smith. 1984. Effect of whole and fractionated
dietary alfalfa meal on zearalenone toxicosis in rats and swine. Can. J.
Physiol. Pharmacol. 62:1219.
Wood, G.E. 1992. Mycotoxins in foods and feeds in the United States. J.
Anim. Sci. 70:3941.
Wu, W., D. Jerome and R. Nagaraj. 1994. Increased redness in turkey
breast meat induced by Fusarial culture materials. Poultry Sci. 73:331.
Author: Trevor K. Smith - Ewen J. MacDonald - Swany Haladi - ALLTECH INC.
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