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Zearalenone in Aquatic Species - When Problems in Fish Start to Surface

Roy Rosen

In Brief

Zearalenone (ZEN) toxicity and its influences on animals have been described in the literature since the 1920s. This nonsteroidal, estrogenic mycotoxin is produced by species of fungi from the Fusarium genus and often found on field crops and other raw materials used in production of human food and both terrestrial and aquatic animal feed.

Almost 100 years ago ZEN caught the eyes of the scientific community largely due to its estrogenic properties which are attributed to its molecular similarity to the female hormone estradiol (Figure 1). The consequences of these properties were observed in pathologies of farm animals. Later, it was also described in game and wild animals in their natural environments and in relation to pathologies in humans. 

In most aquaculture practices, a close contact between the natural aquatic environment and the cultured animals exist, for example sea cages, lake hapas and coastal pond systems, populated by fed fish (Figure 2). In addition, studies revealed that ZEN can enter natural aquatic environments via pasture and agricultural run-off water. Moreover, the toxin was also found in water and sediment originating from aquaculture. Understanding this agricultural-ecotoxicological relationship is an important step in making aquaculture a sustainable and responsible industry.

Chemical structures of zearalenone, and estradiol. These compounds induce similar responses due to binding and stimulation of estrogenic receptors
Figure 1. Chemical structures of zearalenone, and estradiol. These compounds induce similar responses due to binding and stimulation of estrogenic receptors
Farming within nature, fish cages require responsible farming, specific mycotoxin deactivation may help reduce their effect on the natural habitat
Figure 2- Farming within nature, fish cages require responsible farming, specific mycotoxin deactivation may help reduce their effect on the natural habitat. (Photo: Biomin)

Nevertheless, direct introduction of ZEN as feed contaminant in fish and shrimp diets due to increasing use of plant-based protein is the main route by which it exerts its effects on aquaculture species. The Biomin Mycotoxin Survey has revealed that ZEN is present in 48% of the >200 samples of finished-compound aqua feed that were analyzed between 2016 and 2020  (Table 1 and Figure 3). Additional survey data on ZEN contamination in raw materials used in the aquaculture industry such as wheat, corn and corn product (like DGGS, gluten), soy, rice and more, strengthen the conclusion that ZEN is widely present and unnoticeably effecting the aquaculture industry.

Table 1 - ZEN in compound aqua feed between the years 2016-220 (source: Biomin global mycotoxin survey)

ParameterZEN
Number of samples207
% Contaminated samples48%
% Above Risk threshold18%
Average of positives (ppb)78
Median of positives (ppb)36
Maximum (ppb)888
Prevalence of ZEN and its co-occurrence with other mycotoxins in finished shrimp and fish feed
Figure 3 - Prevalence of ZEN and its co-occurrence with other mycotoxins in finished shrimp and fish feed between the years 2016-2020. (source: Biomin global mycotoxin survey)

Recognising and understandind ZEN in aquaculture

But what actually does ZEN do? How is it affecting the aquatic animals? Which species are affected and what can be done about it? 

ZEN in-vivo effects could be divided into three aspects, direct toxicity, general endocrine effects and reproductive disruption. Direct toxicity of ZEN was evaluated in species of Artemia brine shrimp, lethality was observed in concentration starting as low as 10 µg/kg and was increasing with time and toxin concentration. In vitro work on fish cell-lines (originating from Atlantic salmon, rainbow trout and common carp) demonstrated direct cytotoxicity of ZEN and recognized glucuronidation as the main cellular metabolic pathway. However, this cellular metabolization of ZEN did not prevent its cytotoxic and genotoxic potential that was accompanied by oxidative stress.

Endocrine and reproductive disruptions, like many biological and physiological processes are interconnected, here, through the interaction between ZEN and estrogen receptors (ERs). Many studies describe those ligand-receptor interactions. In fish, ZEN and ZEN metabolites readily bind to ERs, this binding leads to a cascade of various responses, majority of which affects the reproductive system, however, other important effects are also observed.

For example, rainbow trout fed ZEN containing diet, showed Immune and blood changes such as increased respiratory burst, leukogram alterations, biochemichal changes and changes in cytokine expression. Most of these parameters did not normalize after ZEN contaminated feed was withdrawn, suggesting irreversible effects of ZEN on immune functions.

ZEN can cause irreversible effects on immune functions

The connection between the generalized and reproductive effects of ZEN can be seen in fish larvae. ZEN induced developmental defects like pericardial edema, hyperemia, yolk sac edema, spine curvature and a heart rate reduction in zebrafish (Danio rario) .Neurotoxicty, DNA damage, cell apoptosis and structural histological changes were also reported in embryos of this model fish species. These, shed light, not only on ZEN’s toxic effects and mechanisms but also on the importance of the toxin expousre timing.

Similarly to other animals, the system and the stage most notibly influenced by ZEN are the reproductive system in breeding animals and early organ-system development in the offspring during embryonic/larval and juvenile life stages.

Frequently reported developmental and reproductive pathologies in aquatic species include:

I) Mosaic, intersex and feminization 

Simultaneous / merged presence of male and female gonadal tissue and the occurence of feminized males exhibiting a feminine phenotype in a fish of masculine genotype was observed in trout fed diets with ZEN below eurpean guidance value.

II) Gonadal abnormalities 

Macroscopic-morphological changes in males include fragmented testicular lobes and abnormal testicular development. Changes in females include asymetric ovarian develoment, early ovarian maturation and ovarian hypertrophy. Consequantly differences in the gonadosomatic index can have direct economical importance as higher percantage of the fish total weight is coming from the viscera rather than the fillet.

III) Effects on sperm

Reduction of quantity and quality of spermatozoa and pathological increase in sperms relative concentration, as a possible result of reduction in seminal fluid production.

IV) Female fecundity, fertilization success, survival rates and developmental pathologies

Reduced hatchability, abnormally and un-timed reproductive development and beahvour, malformations and mutations. Furthermore, offsprings of parents chronichally exposed to ZEN had significantly higher risk of early mortality.

V) Induction and expression of estrogen-responsive genes and proteins 

Dose-dependent induction of estrogen-responsive egg yolk genes and consequant production of vitellogenin and zona radiata proteins. These protein also serve as biomarker for ZEN and other xenoestrogens expousre and are used in number of OECD test guidelines.

From what research revealed so far ZEN in aquatic animals can be a serious problem going unnoticed in some of the most important commercial aquaculture species of salmonids, cyprinds, crustaceans and others.

ZEN in aquatic animals can be a serious problem going unnoticed
  • It is clear that this kind of exogenous sex-horomone like ditsurbance throughout the animal’s life should be avoided. There are different motives for taking action. Wether it is for welfare, environmental or commercial resons, we should be motivated to understand, monitor and minimize the effects zearalenone have on aquatic animals and aquaculture. Monitoring fish and shrimp health status through biomarker analysis, sampling feed and raw marterials and applying mycotoxin mitigation are the answer.

    The newly developed enzyme-based mitigation strategy called ZENzyme® is specifically designed to detoxify ZEN and provides a promising hi-tech solution for the problem.