A lot of threats to pig health can be easily seen and identified. In the case of mycotoxins, however, problems can be more complex and diffuse. Useful information about mycotoxins is sometimes hard to find – and the problem is often underestimated. In this series, Pig Progress will explore mycotoxin types, causes and of course – remedy strategies.
Part 1. The mycotoxin mystery It is well known that pigs are sensitive to combinations of feed-borne Fusarium mycotoxins and hence the feeding of contaminated materials should be minimised. There are, however, numerous different types of mycotoxins and a lot is still unknown about them. What types can be found and how can these be classified?
By Prof Dr Trevor K. Smith, Department of Animal and Poultry Science, University of Guelph, Canada
Mycotoxins are metabolites produced by fungi (moulds) that can infest crops pre-harvest and can continue to flourish under sub-optimal storage conditions. Grains with a high moisture content are particularly unstable and prone to mould 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.
Grains with a high moisture content are particularly unstable and prone to mould proliferation and possible mycotoxin production. Keeping foodstuffs in top quality is essential.
The most significant species of mycotoxin-producing fungi that have an impact on pig production would include Aspergillus and Fusarium. In turn, the most significant mycotoxin produced by Aspergillus fungi are the aflatoxins. The fungi that synthesise aflatoxins, A. flavus and A. parasiticus, are considered to be tropical or semi-tropical moulds that thrive under conditions of high moisture and temperature.
The effects of feed-borne aflatoxin on pig production have been extensively studied and we have a good understanding of the tolerance of various classes of pigs. This is partly due to concern for human health and food safety issues arising from contamination of pork 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.
Another important mycotoxin is the nephrotoxin ochratoxin A. This compound is produced by Aspergillus ochraceus and Penicillium verrucosum. As with aflatoxin, there is concern that residual ochratoxin A in pork products could pose a threat to human health due to the possible carcinogenic nature of this compound.
Fusarium fungi flourish in more temperate climates. Our understanding of Fusarium mycotoxicoses in pigs 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, that have been chemically characterised, which makes complete analysis of feedstuffs for Fusarium mycotoxins impractical, if not impossible.
The most commonly recognised 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 oestrogenic compound; fumonisins and fusaric acid.
Analysing mycotoxins in pig 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 minimise this source of error. However, even with such protocols, error is unavoidable as mycotoxins are not evenly distributed within a batch but occur in hotspots.
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 (Liu et al., 2005).
Although conjugated forms of dexoynivalenol (DON, vomitoxin) were identified many years ago (Sewald et al., 1992), little information is available about the relative significance of conjugated and free mycotoxins in pig feeds. Schneweis et al. (2002) identified glucose conjugated zearalenone in samples of wheat. Naturally-contaminated wheat and corn samples from Slovakia have been found to contain glucose-conjugated DON with up to 29% of deoxynivalenol in a glucose conjugated form (Berthiller et al., 2005).
Zhou et al. (2007) reported an increase in DON concentrations of up to 88% when barley samples from North Dakota were treated with trifluoroacetic acid prior to analysis. Such acid treatment would hydrolyse all different conjugates of DON. Similar acid treatment of different barley samples showed up to 21% of total DON found in conjugated forms (Zhou et al., 2008). Recently, Zachariasova et al. (2008) have found even higher levels of bound DON in barley and beer using a variety of analytical techniques. The frequency of bound fumonisin routinely exceeded free fumonisin in samples of European corn and corn-based foods (Dall’Asta et al., 2008). It is not yet clear if the conjugated forms of mycotoxins are as harmful to pigs as the parent compounds, but it has been shown that some conjugated mycotoxins can be hydrolysed in the pig digestive tract (Gareis et al., 1990).
It must be concluded that until we have a better understanding of the frequency, toxicity and nature of conjugated mycotoxins, current mycotoxin analysis of pig feeds should often be considered to be an underestimate of correct values. To further complicate matters, there exist a number of different analytical techniques (for example ELISA and HPLC) which vary in accuracy and can be sensitive to interference from some dietary components (such as in DDGs).
It should also be noted that typically feeds are only analysed for the presence of certain ‘indicator’ mycotoxins. It is well established that mycotoxins rarely occur in isolation and that mycotoxins when present in combination can act synergistically to produce more pronounced detrimental effects in the animal. 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 pigs.
Effects of feeding blends
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 pigs. This was done in an effort to mimic conditions seen in commercial pig 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. Different modes of action of the mycotoxins fed have been identified including:
1. Reduced cellular protein synthesis
2. Impaired reproduction
3. Reduced immunity
4. Alterations in brain neurochemistry
Reductions in cellular protein synthesis result in lesions of the gastrointestinal tract including necrosis, increased frequency of ulcers, haemorrhaging, and malabsorption of nutrients.
Zearalenone, the oestrogenic Fusarium mycotoxin has been shown to have a profound effect on pig reproduction. Hyperoestrogenic symptoms are seen in prepubertal gilts characterised by vuvovaginitis, uterine enlargement, ovarian atrophy and rectal and vaginal prolapse. Infertility in sows, small litter size, abortions and weak piglets are all characteristic of zearalenone toxicosis. Infertility is also seen in boars.
Many Fusarium mycotoxins, as well as aflatoxin and ochratoxin, have been shown to be immunosuppressive. This results in increased susceptibility to disease, lingering health problems in the herd and possible failure of vaccination programmes. 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 herd health very difficult.
Combinations of feed-borne Fusarium mycotoxins can be 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, vomiting and increased lethargy.
Starter pigs are very sensitive to combinations of feed-borne Fusarium mycotoxins. The feeding of a blend of ingredients naturally-contaminated with a combination of Fusarium mycotoxins resulted in reduced feed intake and reduced growth in the starter phase (Swamy et al., 2002). Changes in brain neurochemistry were also seen with decreased concentrations of norepinephine in the pons region and an increased ratio of 5-hydroxyindoleacetic acid (5-HIAA) to serotonin (5-HT) in the hypothalamus and pons. The decreased concentrations of norepinephrine are characteristic of fusaric acid toxicity with the physiological effect of reduced blood pressure. The elevated 5-HIAA:5-HT ratio is also characteristic of fusaric acid toxicosis in pigs (MacDonald and Smith, 1991). A toxicological synergy between fusaric acid and DON has been demonstrated in pigs (Smith et al., 1997) with the basis being that both mycotoxins elevated brain serotonin concentrations triggering behaviours such as feed refusal, loss of muscle coordination, vomiting and lethargy.
Many of the mycotoxin-induced neurochemical changes seen in the brains of starter pigs can be prevented by the simultaneous feeding of a polymeric glucomannan mycotoxin adsorbent (GMA; Alltech) derived from the cell wall of yeast (Figure 1). It was observed that the feed refusal seen in starter pigs correlated with elevations in brain serotonin concentrations while in broiler chickens, which will readily consume diets refused by piglets, the elevations in brain serotonin are counterbalanced by elevations in norepinephrine concentrations (Swamy et al., 2004). Despite reduced feed consumption, starter pigs exposed to the combination of feed-borne mycotoxins exhibited some changes in immune status (Swamy et al., 2003).
Gestating and lactating sows
There is much less information available regarding the effects of feeding combinations of Fusarium mycotoxins to gestating and lactating sows. Blends of naturally-contaminated wheat and corn were fed for three weeks before farrowing and for three weeks of lactation (Diaz-Llano and Smith, 2006). It was observed that gestating sows readily consumed contaminated diets that would have been refused by starter piglets. This resulted in significantly reduced weight gain in late gestation compared to controls and reduced efficiency of feed conversion. Bodyweight of piglets born alive, however, was not affected by diet.
The feeding of contaminated diets to lactating sows resulted in significant weight loss compared to controls (Diaz-Llano and Smith, 2007). Nutrient content of milk and growth rates of suckling piglets, however, were not affected by diet. In both gestation and lactation, sows fed contaminated diets sacrificed body reserves to maintain gestation and lactation and this resulted in an increase in the number of days from weaning to oestrus. The feeding of contaminated diets also significantly increased the incidence of stillborn piglets and this was prevented by the feeding of GMA.
It appears that the frequency of mycotoxin contamination of pig feeds is increasing. This may be due in part to adverse weather conditions pre-harvest arising from global climate change. The complex nature of modern pig rations including the increasing use of potentially contaminated by-products such as distillers’ dried grains adds to the possibility of toxicological synergy between combinations of mycotoxins thereby increasing the severity of the response of pigs to contaminated feeds.
Many of the adverse effects seen in the studies reviewed above could be prevented by the simultaneous feeding of GMA. The use of an appropriate mycotoxin adsorbent is likely the best short term strategy available for minimising the adverse effects of feed-borne mycotoxins in pigs. 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 minimise mycotoxin challenges to pork industries in the future.