Introduction
Mycotoxins are fungal metabolites that present a significant risk in animal feed. Among livestock species, pigs are the most susceptible to the adverse effects of these toxins, which are classified as prevalent, emerging, and modified mycotoxins.
Modified mycotoxins are the chemically or biologically altered forms of the parent mycotoxin (the mycotoxin directly produced by the fungus) (Lorenz et al., 2018). They are secondary metabolites of fungi, plants, or animals, which, as a mechanism of defense, detoxification, or metabolism, chemically alter the original structure. Although the toxicity of the modified forms varies, these mycotoxins represent a hidden risk, either due to their own toxicity or their capacity to regenerate the parent mycotoxin from which they originate (Nešić et al., 2023; Lorenz et al., 2018).
The toxicological relevance of these modified forms in pigs lies in their metabolic fate: often, the polar group of the conjugate is cleaved (hydrolyzed) in the intestinal tract, releasing the primary mycotoxin (Nešić et al., 2023). This release of the primary mycotoxin can lead to the underestimation of exposure because modified mycotoxins are typically not included in routine analyses and, however, they contribute to the animal’s total toxic exposure (Binder et al., 2017). It is fundamental to assess these modified forms for a comprehensive risk evaluation.
Modified forms of zearalenone
Zearalenone (ZEN) is a macrocyclic mycotoxin known for its potent oestrogenic activity (Binder et al., 2017; Pierzgalski et al., 2021). Pigs are highly sensitive to ZEN exposure because they primarily biotransform it into the derivative α-zearalenol (α-ZEL), a metabolite with significantly greater oestrogenic activity than the parent compound (Tkaczyk and Jedziniak, 2021; Pierzgalski et al., 2021). EFSA has assigned α-ZEL a Relative Potency Factor (RPF) of 60, indicating that its oestrogenic effect is 60 times greater than that of ZEN (Lorenz et al., 2019; Pierzgalski et al., 2021). The high relative potency of α-ZEL makes accurate estimation of exposure in this species particularly important.
The toxicity of α-ZEL extends beyond its oestrogenic activity. This metabolite can modify DNA by altering methylation patterns and histone acetylation in cells (Pierzgalski et al., 2021). Such epigenetic effects may alter the expression of key genes, suggesting that α-ZEL could contribute to the development of metabolic disorders in pigs.
In vivo studies in piglets have shown that three modified forms of ZEN—the plant metabolites ZEA-14-O-β-glucoside (ZEA-14-Glc) and ZEA-16-O-β-glucoside (ZEA-16-Glc), and the fungal metabolite ZEA-14-sulfate (ZEA-14-S)—are completely hydrolysed in the gastrointestinal tract (Binder et al., 2017; Lorenz et al., 2019). The absence of intact conjugated forms in urine and faeces following oral administration confirms their rapid hydrolysis (Binder et al., 2017).
As a result of this complete hydrolysis, EFSA has recognized that ZEA-14-Glc, ZEA-16-Glc, and ZEA-14-S should be included in risk assessment, recommending that the total of ZEN and its naturally occurring metabolites be used as a group guidance or maximum value to safeguard animal health (Binder et al., 2017; Lorenz et al., 2019).
The clinical relevance of modified ZEN has been clearly demonstrated in a field case in which suckling piglets developed hyperoestrogenism, with swollen and reddened vulvas and splay leg syndrome, after sows consumed hay containing ZEA-14-S at 530 μg/kg, exceeding the ZEN concentration of 479 μg/kg. This indicates that the modified form contributed to the overall ZEN burden in the animals, confirming that it contributes to clinical toxicity (Henning-Pauka et al., 2018).
Modified forms of deoxynivalenol
With respect to deoxynivalenol (DON), several highly prevalent modified forms have also been identified. Deoxynivalenol-3-β-D-glucoside (DON-3G), a plant-derived conjugate, is efficiently hydrolysed in the gastrointestinal tract of pigs, releasing DON and thereby contributing to overall exposure (Pierzgalski et al., 2021; Tkaczyk and Jedziniak, 2021). Similarly, the acetylated derivatives of DON (3-AcDON and 15-AcDON) are fungal metabolites that are hydrolysed in the intestine to release DON and are considered among the most toxic metabolites of the parent compound (Pierzgalski et al., 2021; Tkaczyk and Jedziniak, 2021). In contrast, de-epoxy-DON (DOM-1), a microbial metabolite, exhibits markedly reduced toxicity in piglets, as it does not induce vomiting or cause pathological changes in the liver or intestines following oral administration (Pierzgalski et al., 2021).
Biomonitoring of modified mycotoxins in pigs
Given the «masked» nature of these mycotoxins and their heterogeneous distribution in feed, biomonitoring of metabolites in biological matrices represents the most accurate approach to assess individual exposure and toxicokinetics in swine (Tkaczyk and Jedziniak, 2021).
For ZEN, the metabolite α-ZEL in urine is a suitable biomarker of exposure, showing a positive linear dose-response relationship (Tkaczyk and Jedziniak, 2021; Binder et al., 2017). For DON, the metabolite DOM-1 in urine is a reliable biomarker of exposure, also exhibiting a positive linear dose-response relationship (Tkaczyk and Jedziniak, 2021).
It is important to note that glucuronides and other conjugates, such as α-ZEL-14-GlcA, often require enzymatic hydrolysis (e.g., with β-glucuronidase) of biological samples for quantification, as analytical standards for the direct measurement of many conjugates are not available (Tkaczyk and Jedziniak, 2021; Lorenz et al., 2019).
Conclusion
Toxicological research on mycotoxins in pigs underscores that modified mycotoxins are active toxic precursors that significantly increase the risk levels to which the animals are exposed. Therefore, for effective swine health management, it is essential that risk assessments include the quantification of these modified forms and the monitoring of key biomarkers such as α-ZEL and DOM-1 (Tkaczyk and Jedziniak, 2021).
Despite the evidence confirming their toxicity, more research is urgently required to obtain specific toxicological data on the effects and kinetics of these modified mycotoxins in swine. A more robust and comprehensive database is essential for establishing more accurate safety thresholds and improving prevention and mitigation strategies in pig production.
