How Zearalenone Impacts Reproductive Performance in Pigs

In Brief

In this article, you’ll learn how ZEN can damage the reproductive potential of your pigs, signs and symptoms of problems in the herd and actions you can take to prevent or remedy them.

Reproductive performance is fundamental to the economic success of swine production operations, but zearalenone, a common mycotoxin, is a frequent cause of reproduction problems.

Reproductive performance is fundamental to the economic success of swine production operations. If reproductive problems occur, farms suffer tremendous losses due to a reduced number of piglets. Additionally,  both determining the cause of a decline in reproductive performance and implementing corrective measures can incur high costs.

Zearalenone (ZEN) can severely impact reproductive performance in pigs due to its estrogenic effects (Zinedine et al., 2006).  Hence, ZEN should always be considered as a differential diagnosis in case of reproductive failure. Understanding how ZEN works to undermine reproduction in your animals, being aware of what symptoms to look for and knowing how to prevent problems before they occur is critical to maximizing performance—and profit—in swine operations.

This means that feed should be monitored for ZEN and other relevant mycotoxins systematically. Having profound knowledge about toxin burden in the respective feed lot is the basis to review the implemented mycotoxin risk management strategy. If applicable, changes of dosage or used product can be performed based on the results.

Pigs are the livestock species most affected by the estrogenic effects of ZEN. ZEN binds to specific estrogen receptors and simulates the effects of this hormone (Osweiler, 2000). Subsequently the endocrine system is disrupted, which results in different outcomes in dependence of sex, age and reproductive status. The high susceptibility of swine to ZEN may be at least partly due to its metabolic pathways, which is driven to the more estrogenic metabolite α-ZEL (Biehl et al., 1993; Malekinejad et al. 2006). In fact, the first published reports of hyperestrogenism, retrospectively supposed to be caused by ZEN, were in swine (MacNutt et al., 1928; Pullar and Lerew, 1937; McErlean, 1952).

The European Commission (EC) identified the risk originating from ZEN and defined recommended thresholds for complementary and complete feeding stuffs (2006/576/EG). The differences of susceptibility of age groups are reflected by the guidance levels, which are 100 (gilts and piglets) and 250 ppb (sows and fatteners), respectively.

Figure 1a) Piglet was exposed to ZEN

Figure 1b: Piglet received uncontaminated diet

Clinical presentation in female pigs is extremely variable depending on the stage of the reproductive cycle in which ZEN is ingested, dosage and duration of exposition (Osweiler, 2000). Especially, but not exclusively, the reproductive system of sexually immature gilts is affected by this estrogenic substance as well, with effects including:

• Swelling and reddening of the vulva (Christensen et al., 1972; Edwards et al., 1987; Obremski et al., 2003; Zwierzchowski et al., 2005; Teixeira et al., 2011; Oliver et al., 2012; Grenier et al., 2019)
• Increase of size and weight of the genital tract (Figure 1a and 1b) (Christensen et al., 1972; Oliver et al., 2012; Chen et al., 2015; Zhou et al., 2018)
• Change of the histological structure of the genital tract (uterus and ovaries) (Gajecka et al., 2012; Chen et al., 2015;  Zhou et al., 2018)
• Disturbance of physiological development of follicles (Obremski et al., 2003; Zwierzchowski et al., 2005; Gajecka et al., 2011; Yang et al., 2018)
• Negative impact on oocyte maturation and embryo development (in vitro) (Alm et al., 2006)

Figure 2: Juvenile hyperestrogenism of the offspring of sows exposed to ZEN
Copyright: Traunkreis Vet Clinic, Ried im Traunkreis

Detrimental effects of ZEN can also be seen in sexually mature gilts and sows. Clinical presentation depends on the phase of reproductive cycle, in which ZEN is ingested, dosage and duration of exposition, but includes:

• Delay of first estrus (Edwards et al., 1987)
• Prolongation of weaning-to-estrus interval (Edwards et al., 1987; Young et al., 1990)
• Infertility (Chang et al., 1979)
• Constant estrus (Chang et al., 1979)
• Pseudopregnancy (Chang et al., 1979; Etienne and Jemmali, 1982; Young and King, 1986)
• Juvenile hyperestrogenism of the offspring (Figure 2) (Chang et al., 1979)
• Decreased weight of offspring (Chang et al., 1979)
• Reduction of numbers of fetuses or live embryos by sow (Young et al., 1990, Kordić et al., 1992)
• Increased embryonic mortality (Young et al., 1990)
• Degeneration of blastocytes (Long et al., 1992)
• Abortion (Kordić et al., 1992)
• Inhibition of preovulatory follicle maturation (Osweiler, 2000)

In addition to ZEN’s impact on females, negative effects of ZEN on boars´ reproductive performance have been reported. Observable effects include:

• Reduced libido (Berger et al., 1981)
• Reduction of testosterone level in plasma (Berger et al., 1981)
• Reduction of sperm motility (Young and King, 1986; Bielas et al., 2017)
• Enlarged mammary glands (feminization) (Reviewed by Liu and Applegate, 2020)
• Atrophy of testes (Christensen et al., 1972; Reviewed by Liu and Applegate, 2020)

The impact is possibly due to reduction of testosterone production by the hormone producing Leydig cells in the testicles (Yang et al., 2007).

Figure 3: Spermatozoa exposed to ZEN: Differentiation based on fluorescence in alive (arrow) and dead (star sign) spermatozoa (Tassis et al., 2020)

In addition to observable effects in boars, direct effects of ZEN on spermatozoa could be proven in vitro (Figure 3), such as:

• Reduction of sperm motility (Tsakmakidis et al., 2006; Tassis et al., 2020)
• Increase of head abnormalities of spermatozoa (Tassis et al., 2020)
• Decrease of viability of spermatozoa (Figure 3) (Tsakmakidis et al., 2006; Benzoni et al., 2008; Tassis et al., 2020)
• Impairment of the sperm membrane functionality (Tassis et al., 2020)
• Impairment of chromatin structure stability (Benzoni et al., 2008)
• Negative effect on acrosome reaction (Tsakmakidis et al., 2006)

This may be mediated by binding of ZEN to estrogen receptors located on spermatozoa as they were found by Rago et al. (2007).

The dsm-firmenich World Mycotoxin Survey reveals that ZEN is one of the most prevalent well-known mycotoxins found worldwide, following Deoxynivalenol (DON) and Fumonisin (FUM) (Gruber-Dorninger et al., 2019). Within the last 10 years, prevalence of ZEN-positive corn kernel samples ranged between 45-55%. Average of concentration within positive samples was between 123 and 444 ppb in the respective period of time. Wheat can also significantly contribute to toxin burden within the feed. 32-52% of wheat samples have been identified as ZEN-positive within the last 10 years with average concentrations ranging from 55-102 ppb.

Figure 4: Prevalence of main mycotoxins in finished swine feed 2014-June 2023. Source: dsm-firmenich World Mycotoxin Survey

It is important to know that contamination levels below the regulatory limits can negatively affect the performance of pigs. This can be due to:

• Combined occurrence with other mycotoxins
• Underestimation of toxin burden
• Multifactorial disorders
• Phytoestrogens

ZEN regularly co-occurs with other mycotoxins with potential impact on health and reproduction. Frequently, ZEN co-occurs with Deoxynivalenol (DON) as both toxins are  produced by Fusarium spp.  In contrast to ZEN, DON influences reproductive performance mainly indirectly for example by reduced feed intake (Tiemann and Dänicke, 2007). Recently it could be shown, that ZEN and DON reveal additive and synergistic effects on particular parameters, which are indicative for viability and velocity of spermatozoa (Tassis et al., 2020). As well ZEN as DON can inhibit maturation of  pig oocyte maturation and therefor impair fertility. Co-occurrence probably leads to an additive effect (Malekinejad et al. 2007).

Fungi, plants and animals can modify mycotoxins. The resulting metabolites are not necessarily of less toxicity or estrogenicity than the parental substance. For example, the metabolite α-ZEL is known to be 60 times as estrogenic as ZEN (Lorenz et al., 2019). Some metabolites of mycotoxins  may elude analytics and are therefore called masked mycotoxins (Berthiller et al., 2013). Masked mycotoxins lead to underestimation of toxin burden. It has been shown that during digestion in swine, toxic substances were set free from masked ZEN-metabolites  (Gareis et al, 1990). Hence, if the different metabolites of ZEN are not covered by the applied diagnostic methods, overall toxin burden may be underestimated.

Underestimation of toxin burden can also be caused by a simple reason: mycotoxins are not evenly distributed in feed and therefor analyzed samples are not necessarily representative for the whole batch (Figure 5).

Figure 5: Schematic showing inhomogeneous distribution of mycotoxins (dark orange)

In swine production, health problems are often caused by multiple factors. It can be assumed that severity of mycotoxin-induced effects could be influenced by deficiencies regarding  general health and immune status, herd management and stress exposure. Of course, the situations on individual farms are difficult to simulate in a scientific model and therefor the effects of different factors are difficult to assess. However, experience and a recent case report indicate significance of low levels of ZEN and its metabolites under practical conditions (Hennig-Pauka et al. 2018).

Besides the Fusarium-derived ZEN, diets can also contain phytoestrogens. These substances occur naturally in soybeans, clover and other legumes (Nikov et al., 2000). In cases of hyperestrogenism in animals, which cannot be explained by exposure to ZEN, rations should be controlled for phytoestrogens.

Conclusion

ZEN is a frequently occurring contaminant, and its effect on reproductive performance can be severe. Hence, a proper mycotoxin risk management strategy should be implemented on every farm to avoid negative impacts on swine health and performance. Be aware, that impairment of reproduction is the predominant outcome of ZEN ingestion, but other organ systems can be affected as well.

Tips

• Act preventively and test feed regularly.
  ZEN is one of the most prevalent mycotoxins worldwide.

• Do not underestimate ZEN.
  Even concentrations below regulatory limits can severely effect reproduction.

• Reduce pigs’ toxin burden.
  Lower levels of toxins can help your animals reach their full potential.

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