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 estrogenic effect (Binder et al., 2017; Pierzgalski et al., 2021). Pigs are highly susceptible to ZEN exposure because they biotransform it into the derivative α-zearalenol (α-ZEL), a metabolite with significantly higher estrogenic activity than the parent mycotoxin (Tkaczyk and Jedziniak, 2021; Pierzgalski et al., 2021).The European Food Safety Authority (EFSA) assigned α-ZEL a Relative Potency Factor (RPF) of 60.0, indicating that it has an estrogenic effect 60 times more potent than ZEN (Lorenz et al., 2018; Pierzgalski et al., 2021). This high potency makes the correct estimation of exposure especially critical.
The toxicity of α-ZEL goes beyond its estrogenic action. This metabolite is capable of modifying DNA by altering methylation patterns and histone acetylation in cells (Pierzgalski et al., 2021). This epigenetic impact can alter the gene expression of crucial pathways, suggesting that α-ZEL may contribute to the development of metabolic diseases in pigs.
In in vivo studies in piglets, three main ZEN modified forms have been shown to be completely hydrolyzed in the gastrointestinal tract: the plant metabolites ZEN-14-O-β-glucoside (ZEN-14-Glc) and ZEN-16-O-β-glucoside (ZEN-16-Glc), and the fungal metabolite ZEN-14-sulfate (ZEN-14-S) (Binder et al., 2017; Lorenz et al., 2018). The absence of these intact forms in urine and feces after their oral administration confirms their rapid hydrolysis (Binder et al., 2017). Due to this complete hydrolysis, EFSA has recognized that ZEN-14-Glc, ZEN-16-Glc, and ZEN-14-S should be considered in risk assessment, suggesting that the sum of ZEN and its natural metabolites be established as a group guidance or maximum value to protect animal health (Binder et al., 2017; Lorenz et 2018). The clinical relevance of modified ZEN was clearly evidenced in a field case where suckling piglets developed hyperestrogenism, with swollen and reddened vulvas, and splay legs, after sows consumed hay with a concentration of ZEN-14-S (530 μg/kg) that exceeded that of ZEN (479 μg/kg). It was concluded that this modified form contributed to the total ZEN burden in the animals, confirming its role as a precursor to clinical toxicity (Henning-Pauka et al., 2018).
Modified forms of deoxynivalenol
Regarding deoxynivalenol (DON), there are also modified forms with a high prevalence. DON-3-β-D-glucoside (DON-3G), a plant-derived conjugate, is efficiently hydrolyzed in the gastrointestinal tract of pigs, releasing DON and thus contributing to the total exposure (Pierzgalski et al., 2021; Tkaczyk and Jedziniak, 2021). Similarly, the acetylated DON derivatives (3-AcDON and 15-AcDON) are fungal metabolites that are hydrolyzed to release DON in the intestine and are considered the most toxic metabolites of the parental DON (Pierzgalski et al., 2021; Tkaczyk and Jedziniak, 2021). On the other hand, de-epoxy-deoxynivalenol (DOM-1), a microbial metabolite, has shown remarkably reduced toxicity in piglets: unlike DON, orally administered DOM-1 did not induce vomiting or pathological changes in the liver or intestines (Pierzgalski et al., 2021).
Biomonitoring of modified mycotoxins in pigs
Given the «masked» nature of these mycotoxins and their heterogeneous distribution in feed, the most accurate way to assess individual exposure and toxicokinetics in pigs is through biomonitoring of metabolites in biological matrices (Tkaczyk and Jedziniak, 2021). For ZEN, the metabolite α-ZEL in urine is a suitable biomarker of exposure, showing a positive linear dose-response correlation (Tkaczyk and Jedziniak, 2021; Binder et al., 2017). For DON, the metabolite DOM-1 in urine is a reliable biomarker of exposure, also showing a positive linear dose-response correlation (Tkaczyk and Jedziniak, 2021).
It is important to note that glucuronides and other conjugates, such as α-ZEL-14-GlcA, often require enzymatic digestion (with β-glucuronidase) of biological samples for quantification, as analytical standards are not available for the direct quantification of many conjugates (Tkaczyk and Jedziniak, 2021; Lorenz et al., 2018).
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.
