The toxic effects of mycotoxins is well understood. But what about the metabolites of mycotoxins, also known as masked mycotoxins? It seems that we still only know a little bit.
Mycotoxins could be structurally modified by plants as a defence mechanism, generating plant metabolites of mycotoxins, also known as masked mycotoxins. Masked (or modified) mycotoxins can also result from feed processing, in particular in the processing of cereal-derived products. Masked mycotoxins often exhibit similar toxicity as their parent toxin as they eventually follow the same metabolic pathway, but they might also be less or more toxic than their parent compounds depending on bioavailability. It is important to know the effects of these masked mycotoxins in livestock animals. The EC (European Commission) regulation covers mycotoxins recommendations for aflatoxins, ochratoxin A, patulin, deoxynivalenol (DON), zearalenone (ZEN), fumonisin B1 and B2 and T-2 and HT2, but not for their masked forms. EFSA (European Food Safety Authority) recognised the need for regulating masked mycotoxins, and in 2017, the last Scientific Opinions covering masked forms of DON and ZEN mycotoxins were published.
Regarding the masked mycotoxin of DON, the DON-3-glucoside (D3G) EFSA reported last year that no chronic toxicity, hematoma- and myelotoxicity, neurotoxicity and carcinogenicity caused by D3G is reported. Immune response studies are, for instance, very scarce. Regarding fertility, embryotoxicity, skeletal abnormalities, effects on body weight and relative epididymal weight and postnatal mortality, no data were identified regarding the masked form D3G. Also, no in vivo genotoxicity studies on D3G were identified. In comparison with the parent form, DON, the D3G was considerably less toxic than in vitro and in vivo studies realised. Studies in vivo showing effects of DON, when modified and (or) masked and co-occurring with other mycotoxins, were not yet identified.
Toxicity data for D3G are scarce and in vivo data on chronic toxicity are lacking. Hence the CONTAM Panel could not conclude conclusion on the adverse effects of D3G and could also not compare it with that of DON. The CONTAM Panel assumed then that D3G is metabolised to DON, absorbed at the same extent, and that similar acute and chronic adverse health effects of D3G cannot be excluded.
EFSA recommends, among others suggestions, that well-designed toxicity studies to study toxicokinetics and toxicity of the 4 forms of DON (DON, 3-Ac-DON, 15-Ac-DON, and DON-3-glucoside) that occur predominantly in cereal grains. Also, studies searching co-occurrence and toxicological properties to refine the human and animal risk assessment. Diving into the literature, we find few studies about D3G. Pierron et al., 2015 demonstrated the lack of effect of D3G on the immune response of pigs while confirming the interference of DON, and concluded that D3G is not toxic by itself but may pose a risk of gut health through its reconversion into its parent mycotoxin DON. Nagl et al. 2014 demonstrated in a study using piglets that D3G is cleaved to DON in further parts of the gut compared to DON, hence metabolised and partly absorbed. Based on these data the author presumes that D3G is less bioavailable than its parent toxin in pigs and therefore of lower toxicological relevance. However, the bioavailability of D3G in pigs may increase after chronic exposure via feed. Furthermore, it should be emphasised that D3G may exhibit biological activity on its own, and therefore, Nagl et al. (2014) advised that future studies should address possible emetic effects of the masked mycotoxin or its influence on intestinal gut health.
According to EFSA (2017), very few experiments investigated the adverse effect of the modified forms of ZEN on livestock species, horses, fish and dogs and none of them were suitable to define safety levels. There was no data available on the health risk of ZEN and its modified forms for cattle, horses, rabbit, goat, duck, mink and cats. For poultry, sheep, dogs, fish, piglets, and gilts low risk of health effects from ZEN and its modified forms were observed. EFSA recommends further study on the occurrence of modified forms of ZEN in the feed. Also, more toxicological and toxicokinetic data on ZEN modified forms, particularly for cattle, horses, rabbit, poultry, for companion animals and mink to ZEN, to reduce the uncertainties in the animal risk assessment. Gratz et al. 2017 reported that none of the tested masked zearalenone compounds were hydrolysed under upper GI tract conditions and that none of the tested masked mycotoxins were efficiently hydrolysed or transported through the intestinal epithelial monolayers, indicating limited bioavailability of glucose bound zearalenone compounds. According to Lorenz et al. 2018, an excellent supportive approach to increase the knowledge about masked forms of ZEN to increase current health risk assessment is biomonitoring. This approach in farm animal revealed significant differences in toxicokinetics and metabolism between species, age groups, as well as genders within a species.
Gratz et al. 2017 reported that the masked mycotoxins D3G (DON), T2GLc (T-2 toxin) and ZEN14Glc, – ZEL14Glc, and -ZEL14Glc (ZEN) was stable toward gastro-intestinal digestive juices and were not efficiently transported through intestinal epithelial cell monolayers. However, upon contact with human gut microbiota, all masked mycotoxins were hydrolysed efficiently. D3G were likely to contribute to overall toxicity as free DON. Colonic bioavailability of masked ZEN compounds is high as they are rapidly hydrolysed by gut microbiota, and the released zearalenone compounds are highly reactive with gut microbiota and epithelial cells. Regarding T-2 toxin, microbial hydrolysis of T2Glc may be too slow to increase its bioavailability in vivo. In vivo studies would be helpful describing the contribution of colonic hydrolysis of masked mycotoxins to their absorption and toxicity in the body. In the same direction, Pierron et al. 2017 argued that ingestion of D3G might lead to a release of DON in the distal part of the intestine shifting the toxic effects. Despite this, no data were found on the effect of DON in the large intestine. Last year, the knowledge that D3G can be cleaved to DON by bacteria in the gastrointestinal tract and distributed, metabolised and excreted similarly to DON was published by EFSA.
There is a big knowledge gap when we talk about the toxicity and safe levels of masked mycotoxins. We need more research on what is happening with these masked mycotoxins when they are taken up by the animal. For example, which of the masked mycotoxins are hydrolysed, absorbed and do they influence the immune system, hence animal performance. In addition, it is essential to highlight the importance of studying co-occurring mycotoxins in its various forms. The issue with co-occurrence is that the toxicity of mycotoxins mixtures cannot always be predicted based upon their separated toxicities, once the toxicity could be synergetic (Pierron et al., 2016 and Pinotti et al., 2015), and this is a critical point already highlighted by the EFSA.
References are available on request.
Jaqueline Maas has a Masters degree in Animal Sciences with specialisation in Nutrition and Metabolism from Wageningen University and Research in the Netherlands.