Zearalenone (ZEN) in Livestock and Poultry: Dose, Toxicokinetics, Toxicity and Estrogenicity

One of the concerns when using grain ingredients in feed formulation for livestock and poultry diets is mycotoxin contamination. Aflatoxin, fumonisin, ochratoxin, trichothecene (deoxynivalenol, T-2 and HT-2) and zearalenone (ZEN) are mycotoxins that have been frequently reported in animal feed. ZEN, which has raised additional concern due to its estrogenic response in animals, is mainly produced by Fusarium graminearum (F. graminearum), F. culmorum, F. cerealis, F. equiseti, F. crookwellense and F. semitectums, and often co-occurs with deoxynivalenol in grains. The commonly elaborated derivatives of ZEN are α-zearalenol, β-zearalenol, zearalanone, α-zearalanol, and β-zearalanol. Other modified and masked forms of ZEN (including the extractable conjugated and non-extractable bound derivatives of ZEN) have also been quantified. In this review, common dose of ZEN in animal feed was summarized. The absorption rate, distribution (“carry-over”), major metabolites, toxicity and estrogenicity of ZEN related to poultry, swine and ruminants are discussed.


Introduction
Grains are major ingredients for farm animals raised in integration production systems, which primarily contributes energy to the diet. One of the concerns when using grain ingredients in feed formulation is mycotoxin contamination [1]. Previous publications have suggested that about 70% of cereal based feeds are contaminated with at least one mycotoxin [2].
Mycotoxins are secondary metabolites produced by different fungi and are defined as "natural products produced by fungi that evoke a toxic response when introduced in low concentration to higher vertebrates and other animals by a natural route" [3]. The term "mycotoxin" is derived from "mykes" meaning "fungi" and "toxicon" meaning "poison" [4]. Among the approximately 300 to 400 mycotoxins that have been identified, aflatoxin, fumonisin, ochratoxin, trichothecene (deoxynivalenol, T-2 and HT-2) and ZEN are frequently reported mycotoxins due to safety concerns and economic impact [4,5]. ZEN can interact with estrogen receptors in animals and has been defined as an estrogenic mycotoxin and raised additional attention because of its toxicokinetics, toxicity and estrogenicity [6][7][8].
ZEN also often co-occurs with other Fusarium mycotoxins, mainly deoxynivalenol [31]. After harvest, the production of deoxynivalenol is favored by grain moisture over 20% and temperatures ranges between 21 to 29 °C [32]. Because both ZEN and deoxynivalenol could be produced by F. graminearum or F. culmorum [12], which means the most suitable moisture and temperature Recently, the metabolites and toxicity of modified and masked forms of ZEN have been frequently discussed [16,[24][25][26]. Plants can alter the chemical structure of mycotoxins as part of their defense against xenobiotics [24]. However, research data on extractable conjugated and non-extractable bound derivatives of ZEN have not been well established [16,24]. One explanation for the missing adequate number of publications is the undetectable characteristic of these masked derivatives via previous routine analytical procedures [27,28]. The calibrants and reference materials for conjugated forms are still not commercially available [16]. To date, more than 30 modified forms of ZEN have been described, including correspondent cis-forms (due to the sunlight exposure) and zearalenone-14-sulfate [16,26,27]. Masked forms derivatives have been often mentioned in literatures including: zearalenone-14-glucoside, α-zearalenol-14-glucoside, and β-zearalenol-14-glucoside [16,24,28,29]. Zearalenone-16-glucoside was another recently reported ZEN glucoside in wheat and barley [30].
ZEN also often co-occurs with other Fusarium mycotoxins, mainly deoxynivalenol [31]. After harvest, the production of deoxynivalenol is favored by grain moisture over 20% and temperatures ranges between 21 to 29 • C [32]. Because both ZEN and deoxynivalenol could be produced by F. graminearum or F. culmorum [12], which means the most suitable moisture and temperature conditions for Fusaria growth and production of ZEN are the same that favor deoxynivalenol production [32].

ZEN Guidance and Concentration in Animal Feed
Mycotoxin contamination in grain ingredients can happen both pre-and post-harvest. The production of mycotoxin by fungi occurs during plant growth, maturity, harvesting, and processing of grains [33]. Multiple factors influence fungal growth and mycotoxin formation, including season, geographical location, drought, harvest time, processing, storage, and distribution [4]. ZEN is mainly formed pre-harvest, however continued fungal growth and ZEN synthesis might continue during poor storage conditions [34].
The first review on the occurrence of mycotoxins dates back to 1977, when it was presented at the first FAO/WHO/UNEP conference on mycotoxins [35]. The contamination of ZEN in grain and animal feed can range from 4 to 11192 µg /kg [36,37]. The regulatory guideline for ZEN varies among different national authorities and regulatory organizations. In this review, the focus remains on the regulatory limits of ZEN in livestock feed (mainly poultry, swine and ruminants). Table 1 shows the regulatory limits for ZEN concentration in complete feed from the European Commission Guidance (EU) and US Food and Drug Administration Guidance [16,31]. Currently, modified and masked forms of ZEN are not considered in the regulations and within the EU guidance [16]. The contamination level of ZEN varies distinctly by the region, country, climate, etc. A recent publication [19] summarized mycotoxin concentrations from more than 74,000 samples from 100 countries over 10 years. Based on the survey results, ZEN is one of the top three mycotoxins in complete animal feed and feedstuffs, with occurrence in 45% of all samples and 55 µg/kg median concentration among the positive samples. ZEN contamination is specific to common feed ingredients used in livestock animal feed ingredients, occurring in corn (44%), corn-DDGS (75%), soybean meal (61%), wheat (33%) and barley (20%). Meanwhile, ZEN was the most prevalent mycotoxin in both soybean and soybean meal samples.
Furthermore, the co-occurrence of mycotoxins in grain samples (containing more than one mycotoxin) has been frequently reported [38][39][40][41]. As ZEN and deoxynivalenol are both produced by F. graminearum or F. culmorum, the co-occurrence of ZEN and deoxynivalenol was detected in 48% of finished feed, 39% in corn, and 28% in wheat samples (ZEN and deoxynivalenol was also the most frequently observed mycotoxin combination in wheat samples), respectively [19]. This underlines the importance of considering the synergistic effects of multi-mycotoxin contamination. The survey on occurrence of modified forms of ZEN in feed ingredients are less extensive [16]. The co-occurrence of ZEN and ZELs glucosides has been reported, with the sum of modified forms exceeding the ZEN concentration alone by as much as 1.5 times in barley samples and 0.5 times in oat and wheat samples [42].
Various analytical methods have been well-established to characterize ZEN and its modified forms in feed [43][44][45] HPLC or UPLC methods are coupled to UV or FL detectors. These HPLC-UV or HPLC-FL methods are based on the use of a combination of acetonitrile, methanol and water as the extraction solvent combined with the used of specific immunoaffinity columns or solid-phase extraction cartridges as clean-up steps [46]. The LC-MS and LC-MS/MS provides information about molecular mass and structural features of components, which are considered more sensitive than other methods in terms of separation, identification and quantification of ZEN [45]. The extraction of modified forms of ZEN is based on the same protocol that have been used to extract ZEN [16,47,48]. Of note, there is a need of calibrants and reference materials for development of validated and sensitive routine methods for modified forms in feed samples, especially with the highly sensitive analytical methods for α-ZEL [16]. A practical option is to conduct quick ELISA test kit for ZEN screening in field situations and to subsequently further validate test results with LC-MS/MS within a laboratory setting.
co-occurrence of ZEN and ZELs glucosides has been reported, with the sum of modified forms exceeding the ZEN concentration alone by as much as 1.5 times in barley samples and 0.5 times in oat and wheat samples [42].
Various analytical methods have been well-established to characterize ZEN and its modified forms in feed [43][44][45], including ELISA, LC-MS/MS, HPLC, LC-MS, GC-MS ( Figure 2). The common HPLC or UPLC methods are coupled to UV or FL detectors. These HPLC-UV or HPLC-FL methods are based on the use of a combination of acetonitrile, methanol and water as the extraction solvent combined with the used of specific immunoaffinity columns or solid-phase extraction cartridges as clean-up steps [46]. The LC-MS and LC-MS/MS provides information about molecular mass and structural features of components, which are considered more sensitive than other methods in terms of separation, identification and quantification of ZEN [45]. The extraction of modified forms of ZEN is based on the same protocol that have been used to extract ZEN [16,47,48]. Of note, there is a need of calibrants and reference materials for development of validated and sensitive routine methods for modified forms in feed samples, especially with the highly sensitive analytical methods for α-ZEL [16]. A practical option is to conduct quick ELISA test kit for ZEN screening in field situations and to subsequently further validate test results with LC-MS/MS within a laboratory setting. The occurrence of ZEN has been widely reported in a variety of different countries, regions, years, etc. As mentioned previously, the current review specifically focuses on the concentrations of ZEN and its modified forms in poultry, swine and ruminant feeds. The literature search was conducted using the University Libraries Database at the University of Georgia, PubMed, and Google Scholar search engines with the key words: mycotoxin, zearalenone, feed, occurrence, livestock animal, swine, pig, sow, gilt, poultry, broiler, laying hen, ruminants, dairy, and cattle. Table 2 summarizes the ZEN concentration in selected peer-reviewed surveys, studies and review publications. To date, the research on evaluation of modified and masked forms of ZEN in poultry, swine and ruminant animals are not adequate. The occurrence of ZEN has been widely reported in a variety of different countries, regions, years, etc. As mentioned previously, the current review specifically focuses on the concentrations of ZEN and its modified forms in poultry, swine and ruminant feeds. The literature search was conducted using the University Libraries Database at the University of Georgia, PubMed, and Google Scholar search engines with the key words: mycotoxin, zearalenone, feed, occurrence, livestock animal, swine, pig, sow, gilt, poultry, broiler, laying hen, ruminants, dairy, and cattle. Table 2 summarizes the ZEN concentration in selected peer-reviewed surveys, studies and review publications. To date, the research on evaluation of modified and masked forms of ZEN in poultry, swine and ruminant animals are not adequate.

ZEN Absorption Rate, Distribution (Carry-Over), Metabolites and Excretion in Livestock Animals
ZEN is a heat-stable compound despite its large lactone ring (ZEN in ground corn was stable at 150 • C for 44 h), and degradation was only observed at extremely high temperatures or within an alkaline environment, which makes ZEN thermostable during storage, milling, processing and distribution [16,49,50].
Once ingested by the animal, ZEN and/or its modified forms are rapidly and extensively absorbed by intestine and modified by liver [12,18,25]. The metabolites ( Figure 3) include ZEN, ZELs, ZAN, ZALs and its corresponding conjugates [16,18,23,25,51]. In farm animals, the reductive biotransformation predominates and the resulting metabolites mainly are α-ZAL and β-ZAL, with limited amount of α-ZEL, β-ZEL and other metabolites being produced [16]. The metabolic profile in urine and feces are significantly different among species. For example, higher proportions of an administered ZEN dose were metabolized to α-ZEL in pigs than in cows, whereas ZEN was mainly found as glucuronide conjugates of ZEN and α-ZEL in pig urine [52]. The concentrations of α-ZEL in plasma in some studies may be higher than others; some studies only detected the conjugated form metabolites, while others found both free and conjugated forms of ZEN [18,53,54]. Briefly, among ZEN treated animals, the α-derivatives seem to be the most prevalent in pigs and turkeys, versus β-derivatives which appear to be the abundant metabolites in cattle, goats, broilers and laying hens based on the levels measured in plasma, urine or bile [16]. Very limited amounts of ZEN were detected in liver, kidney, and muscle in animals [25]. In general, there are potentially two major biotransformation pathways for ZEN in animals [54,55]: (1) hydroxylation to form α-ZEL, β-ZEL, and catalyzed by 3α-and 3β-hydroxysteroid dehydrogenase; (2) conjugation with glucuronic acid and catalyzed by uridine diphosphate glucuronyl transferase. Additionally, there is a general consensus that sulfation presents an additional conjugation route for both ZEN and its metabolites [16]. However, limited in vitro data were reported both on the structures and the enzymology of such sulfate derivatives [56]. In the in vivo pig study, zearalenone-14-glucoside, zearalenone-16-glucoside, and zearalenone-14-sulfate were found to be completely hydrolyzed and absorbed in pigs [57].
The high degree of re-absorption in the intestinal tract influences ZEN excretion via enterohepatic circulation [25,54]. Recent publications have relayed investigations on ZEN, α-ZEL and β-ZEL binding to bovine and porcine serum albumins [58,59]. Serum albumin is the most abundant plasma protein in the circulation, which effects tissue distribution and elimination of xenobiotics [58]. The results showed that ZEN can bind to bovine serum albumin with strong intermolecular forces [59]. ZEN binds with higher affinity than α-ZEL and β-ZEL to albumins [58]. ZEN and its metabolites are mostly excreted via the fecal route as glucuronides [16,18].
Previous research demonstrate that ZEN and its reductive metabolites can be detected in both animal tissues and products (Tables 3-5). Systemic circulation (Figure 3) of ZEN and its modified metabolites in animals are essentially related to their distributions in different tissues and potential carry-over into animal products [16].

Poultry
For poultry, researchers have found both ZEN and reductive ZELs metabolites (α-ZEL and β-ZEL) can be detected in blood, liver, kidney, muscle, intestine and excreta [88,89]: ZEN administered via both intravenous injection and oral administration at a dose of 1.2 mg/kg b.w. was measurable from 5 min to 2 h in plasma and was rapidly transformed into α-ZEL and β-ZEL in plasma of broilers. Other research [90] also revealed the metabolites of ZEN in blood in different poultry species, with a higher production of β-ZEL than the α-ZEL in broilers and layers, versus turkey poults, which were more efficient at bio-transformation of ZEN to α-ZEL. ZEN was measurable up to 1 h in the liver, kidney, and small intestine. α-ZEL and β-ZEL were detectable up to 12 h in the liver, kidney and small intestine, whereas both were only detectable up to 1 h in muscle following oral administration. The concentration of α-ZEL and β-ZEL in different tissues is as follows: small intestine > liver > kidney > muscle. In poultry, bile has been reported to play an important role as an excretory route for ZEN and its metabolites [91]. Besides, ZEN, α-ZEL and β-ZEL were detected in excreta up to 12 h after oral administration, with the concentration α-ZEL > β-ZEL [89]. Another study fed birds diets contaminated with 0.4 mg/kg ZEN feed from d 29 to 84 and did not find ZEN carry-over into the liver, whereas α-ZEL was detected on the last sampling day [87]. The carry-over of α-ZEL may be possible due to the fact that ZEN is mostly and rapidly eliminated in excreta [88]. This was supported by the excreta results: when they measured the excreta ZEN concentration collected within the last two days, the ZEN concentration was 0.27 mg/kg. In a laying hen study [92], researchers found that α-ZEL was detectable in the liver, whereas ZEN was not detected in either breast meat or the liver. Dailey et el. [93] found after a single dose exposure of ZEN (10 mg/kg) in laying hens, it is possible that significant levels of ZEN lipophilic metabolites might accumulate in egg yolk if the exposure time was prolonged. Another study was conducted to assess the carry-over of ZEN into eggs [92], and researchers reported that no detectable ZEN was detected in eggs from commercial production [12]. The metabolites and carry-over effects of dietary ZEN in poultry from previous publications are shown in Table 3.

Swine
Biehl et al. [54] reported that the absorption rate of ZEN in pigs was estimated to be 80-85% following a single oral dose of 10 mg ZEN/kg b.w. The estimated biological half-life of radiolabeled ZEN was 87 h in the intact pig, whereas it was reduced to 3.3 h when the bile of the pig was removed. Additionally, 45% of the administered dose was recovered in urine and 22% in feces within the first 48 h. After absorption, ZEN and its metabolites could be detected in the liver, bile, plasma, urine, digesta and feces [94]. Earlier study showed that ZEN and its metabolites can be detected in plasma around 30 min after oral administration [54,55]. In swine, the major metabolites are glucuronide conjugates of ZEN and α-ZEL [54]. Gajęcka et al. [95] surmised that α-ZEL is the predominant metabolite of ZEN in pigs and the low dose of ZEN could alleviate inflammation in the digestive tract (especially in the proximal and distal intestinal tract), and could increase body weight gains in gilts. The ZEN was reduced to α-ZEL and β-ZEL when incubated with homogenized intestinal mucosa from sows in the presence of nicotinamide adenine dinucleotide phosphate. Additionally, the rate of glucuronic acid conjugation of ZEN was about 30-fold higher than that of reduction in the presence of UDP glucuronic acid [96]. The β-ZEL was detected only in bile of gravid sows, and when fed a high (0.42 mg/kg) concentration of ZEN in female pigs [97,98]. In addition, researchers have reported that the main route for excretion of ZEN and its metabolites in swine is through urine, which was twice as high as the amount eliminated through feces [54,57]. Research found that in gilts the cumulative recovery of ZEN and α-ZEL in duodenal digesta and urine was 35% and 70% after 72 h, respectively (as a percentage of total ZEN administrated). Additionally, 14 days after the bolus injection, both ZEN and α-ZEL were lower than the detection limit in the bile, liver and urine. The elimination half-time of ZEN in excreta was 2.63 h. This is to say that ZEN is completely eliminated from gilts within this period of time with a massive single bolus [94]. The metabolites and carry-over effects of dietary ZEN in swine from previous publications are shown in Table 4. ZEN is mainly converted to α-ZEL in pigs, and exposure Toxins 2020, 12, 377 9 of 21 risk to humans by consumption of edible product is negligible compared to direct consumption of grain-based food [99].

Ruminants
For ruminants, prior reports noted that β-ZEL was the predominant ZEN metabolite in urine, and the free and conjugated ZEN can both be detected in cows' milk [52]. In a trial with one single cow, researchers found that 0.7% of ZEN could carry-over into the milk when feeding up to 200 mg ZEN/day in the feed for 7 days [100]. This carry-over into milk normally occurs when animal ingests a high dose of ZEN in feed. Other researchers [78] have also demonstrated that ZEN is mainly metabolized to β-ZEL, and less extensively to α-ZEL in bovine species. The authors could detect the ZEN, α-ZEL and β-ZEL in bile. The concentrations of ZEN, α-ZEL, β-ZEL, ZAN, α-ZAL and β-ZAL in serum, urine and milk were lower than 1, 1, 4, 100, 50 and 200 ng/g, respectively. The concern for ZEN carry-over in ruminants may be minor based on their endogenous ruminal detoxification [101]. In addition, the health status and blood-milk barrier would also affect the transfer of ZEN into milk [102]. Seeling et al. [67] conducted a trial and found that different feed intakes could affect the sum of all ZEN metabolites and the proportion of β-ZEL in dairy cows. In a sheep study with two adult ewes (30-35 kg), researchers noted that sheep were capable of metabolizing ZEN with further reduction of the C11-C12 double bond, which led to the α-ZEL and β-ZEL [103]. In another study with goats [104], blood plasma, urine, and feces samples were collected consecutively after intravenous injection of ZEN at doses of 2.4 mg/kg and 1.2 mg/kg b.w. The distribution half-life and elimination half-life of ZEN were 3.15 and 28.58 h, respectively. ZEN, α-ZEL, and β-ZEL were detected in both urine and feces, with β-ZEL being the predominant metabolite. Additionally, ZEN and its metabolites were largely in their glucuronide and/or sulphate conjugated forms in urine, while they were largely in free forms in feces. The metabolites and carry-over effects of dietary ZEN in ruminant animals from previous publications are shown in Table 5. ZEN and its metabolites can be detectable in liver and bile, but in most studies are not detected in the milk [67,78].     Residues of ZEN + α-ZEL + β-ZEL was positively correlated in liver and bile (Liver showed 0.9% carry-over ratio) [99]

Toxicity
Generally, ZEN has low acute toxicity to animals at low concentrations. However, previous publications elaborated the sub-acute, sub-chronic, chronic immunotoxicity, genotoxicity, productive and developmental toxicity, as well as endocrine disturbance effects caused by ZEN [12,51,59]. The fate and adverse effects of ZEN are partly determined by the processes of elimination, which is closely related to the biliary excretion and enterohepatic cycling [54,119]. Based on JECFA, the safety of ZEN can be evaluated based on the dose that had no hormonal effects in pigs, which is known as the most sensitive animal species to ZEN [120].
Poultry seem to be quite tolerant to ZEN, which may be explained by the naturally high concentration of estrogen in poultry blood. Natural estrogens are considered to have higher receptor affinity compared to the Fusarium toxins [101]. ZEN had no effect on feed intake or body weight gain on young male turkey poults. In contrast, feeding ZEN-contaminated diets to turkey poults lead to strutting behavior, increased size and coloration of caruncles and dewlaps, and swollen vent tissue [105]. An additional study indicated that purified ZEN may increase oviduct development in growing female chickens and delay growth of the testes in young male chickens [121].
Feeding female pigs with 1.3 mg/kg ZEN diet can reduce platelets, haemoglobin, globulin, triglycerides and high-density lipoproteins in serum; increase enzyme activities; and lead to degeneration of the liver and kidney [43]. In addition, the difference in susceptibility to the estrogenic effects of ZEN was also not related to the circulation difference of ZEN and its metabolites [1,54].
For ruminants, ZEN may lead to lower conception rates in heifers [122]. However, the contribution of ZEN to the susceptibility difference is unknown, because most of the related research has been conducted in pigs [89]. One possible explanation for the differences related to species' susceptibility may be related to the variation in the number and affinity of estrogen receptors. In general, pigs and sheep are more susceptible species than poultry [16].
Last but not least, the toxicity of modified and masked forms of ZEN (both extractable conjugated and non-extractable bound forms) has not been adequately evaluated [16,26,123]. These modified forms of ZEN are not detected by routine analytical procedures [16,27,28]. Toxicological data related to these undetectable forms of ZEN are scarce, which implies that analysis of samples containing these compounds leads to an underestimation [24]. Study has found zearalenone-glucoside can be hydrolyzed during digestion in 27 kg female pigs [28]. A more recent study has found that the estimated oral bioavailability of ZEN was 61 to 85%. The α-ZEL and β-ZEL were completely absorbed after the oral administration. The absorbed fraction of zearalenone-14-glucoside was estimated to be 61%, which suggested complete hydrolysis and absorption of zearalenone-14-glucoside. The authors indicated that α-ZEL, β-ZEL, zearalenone-14-glucoside, zearalenone-14-sulfate contribute to the total systemic toxicity of ZEN in pigs and should be taken into consideration for the risk assessment [124]. It is important to account for the total concentration of ZEN and its modified forms [16].

Estrogenicity
The estrogenic factor has been recognized as early as late 1920s [125]. Since then, the association between the consumption of moldy grains and hyper-estrogenism in pigs has been frequently observed and reported [126][127][128]. ZEN is constituted from phenol derivatives and passively crosses the cell membrane [16]. The special hormonal-like response can mimic the endogenous steroidal sex hormone 17-β-estradiol actions after binding to estrogen receptors and effect the estrogen signaling pathway in animals [129]. These responses could result in permanent pathologic alterations of the reproductive tract, which can cause infertility at high intake levels, particularly in prepuberal gilts, leading to pseudopregnancy, infertility, increased embryo lethal resorptions, swollen edematous vulva, vaginal/rectal prolapse, and reduced litter size (due to fetal resorption and implantation failure) [1,12,16,54,55,107,113,130]. The α-ZEL and β-ZEL are two common metabolites of ZEN that relate to the hormonal and estrogenic effects. The estrogenic activity of α-ZEL was 3 to 100 times higher than ZEN [89]. Le Guevel and Pakdel [131] found that α-ZEL was 17 times stronger versus α-ethinyl estradiol with three different bioassays using estrogen receptor gene activation. ZEN binds to estrogenic receptors and has a stronger affinity to αthan to β-estrogenic receptors [16]. Based on the "uterotrophic activity" assed in rodents, the estrogenic activity of ZEN and its modified forms are classified in order: α-ZEL > α-ZAL > ZEN ≈ ZAN ≈ β-ZAL > β-ZEL [16]. Other researchers claimed that the major metabolites known to have affinities for estrogenic receptors are in the following order: α-ZAL > α-ZEL > β-ZAL > ZEN > β-ZEL [87]. The risk of hyper-estrogenic effects for α-ZEL is underestimated, because it is neither often determined or regulated [24]. Little is known about the metabolic fate of α-ZAL, which is used legally as a growth promoter in some countries with the name zeranol [16]. Poultry only respond to the presence of ZEN at extremely high concentrations. Cattle are more resistant to the estrogenic effect of ZEN because they bio-transform ZEN more into β-ZEL than α-ZEL [16].
Recently, the estrogenic activity of metabolites hydrolyzed zearalenone and decarboxylated hydrolyzed zearalenone formed by hydrolysis of ZEN has raised attention as a potential ZEN degradation strategy in animal feed [132]. Additionally, ZEN and its reduced forms are competitive substrates for 3α-hydroxysteroid dehydrogenase and 3β-hydroxysteroid dehydrogenase enzymes, which effect the synthesis of steroids [133].
Concentrations of 1-5 mg/kg of dietary ZEN have been reported to be sufficient to cause clinical signs [134] and hyper-estrogenic clinical signs at 1 mg/kg in pigs [135]. In a preliminary trial [53], researchers reported that prepubertal female pigs fed 0.25 mg/kg of ZEN resulted in redness and swelling of the vulva, swelling of the mammaries and numerous vesicular follicles and some cystic follicles on the ovaries, versus pigs fed 0.05 mg/kg ZEN. However, after ingestion of diets with 0.05 or 0.25 mg/kg ZEN, both treatments showed higher numbers of vesicular follicles on the ovaries when compared to pigs fed the control diets without the mycotoxin. Gilts fed with 1.1 mg/kg ZEN diet showed increased vulva length, vulva width, vulva height and vulva area compared with gilts fed a control diet [71]. Similar results were found by feeding gilts with 0.2, 0.4, or 0.8 mg/kg ZEN contaminated diets and noted that dietary ZEN linearly increased vulva size (width, length and area) [75]. Patience et al. [32] demonstrated that feeding gilts with 1-3 mg/kg ZEN for 3 to 7 days, can lead to hyper-estrogenism and prolapses in prepubertal gilts; with 3 to 10 mg/kg ZEN in the middle of the estrus cycle leading to anestrus and pseudopregnancy; with 15 to 30 mg/kg during the first trimester of pregnancy leading to early embryonic death and reduced litter size. Additionally, the prevalence of ZEN may also cause infertility in boars, with atrophied testes and enlarged mammary glands [32,130]. These symptoms can normally be alleviated after the replacement with clean feed in practice.

Conclusions
The concern of mycotoxin ZEN in relation to livestock animals is vital: (1) The occurrence of ZEN and co-occurrence of ZEN with other mycotoxins in grain and complete feed is still relatively high; (2) The amount of ZEN that carries over into final animal products (meat, egg, milk) is very limited under normal farming systems. However, ZEN and its modified metabolites can be detected in blood, liver, gut, urine and feces; (3) The special structure of ZEN mimics the effects of estrogen, which closely relates to the reproductive functionality of livestock, with swine being the most sensitive species. (4) Modified and masked forms of ZEN should be included and taken into consideration for the risk assessment of ZEN for farm animals.
In summation, sub-clinical doses of ZEN may not significantly influence on the productive performance of livestock and poultry, however, the continuous consumption of ZEN contaminated feed could lead to a detectable amount of ZEN and/or its metabolites in the blood, liver, intestine content, urine and feces.
Author Contributions: Writing-original draft preparation, J.L.; writing-review and editing, T.A.; supervision, T.A. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.