Reducing oxidative stress from mycotoxins

Mycotoxins pose a serious threat to animal production due to their negative impact on animal performance and health. Generally known symptoms are vomiting, decreased feed intake and growth, depressed immunity, fertility problems and increased mortality. Although these symptoms are relevant for farmers who in the end are impacted by the negative effects of mycotoxins, Understanding these effects enables us to effectively reduce and even eliminate the widely recognised harmful effects of mycotoxins. A wide range of research has been performed to elucidate the effects of mycotoxins in the animal. Although the effects depend strongly on the type of mycotoxin, some general effects can be seen in different parts of the animal.

Target organs and cells

Once mycotoxins are taken up orally, the gastrointestinal tract is the first target organ of mycotoxins. Effects on epithelial cells include, among others, changed mucus production, altered cytokine production, decreased cell proliferation and compromised intestinal barrier function. After crossing the epithelial cells, mycotoxins are taken up in the blood and transported to the liver. In the liver, mycotoxins are metabolised into secondary metabolites. Aflatoxin B1 for example is converted into Aflatoxin M1, Zearalenone (ZEA) is mainly converted into α-ZEA and β-ZEA. Although the liver is known to detoxify toxic components, the liver does not always succeed in detoxifying mycotoxins. α-ZEA for example is 100 times more toxic than the initial form. After passing the liver, mycotoxins are systemically distributed throughout the body impacting the immune system and all organs. Exposure to mycotoxins can either result in immune-stimulatory or immunosuppressive effects. Functioning of all types of immunity cells can be impacted including macrophages, neutrophils, T- and B-lymphocyte, and antibodies. Most severely impacted organs are the liver, kidneys and reproductive organs. At cellular level, mycotoxins typically cause inhibition of protein and nucleic acid synthesis, upregulation of pro-inflammatory cytokines as well as apoptosis.

Induction of oxidative stress

An important effect of mycotoxins is the induction of oxidative stress. Under normal conditions free radicals or ‘Reactive Oxygen Species’ (ROS) are produced as part of standard metabolic processes. These highly unstable and chemically reactive molecules are rapidly eliminated by the natural antioxidant system in order to avoid potential damage. When exposed to mycotoxins however, cellular ROS concentrations exceed the level of naturally occurring antioxidants resulting in oxidative stress. Excess ROS will induce a damaging chain reaction causing serious damage to nucleic acids, proteins and lipids. As these components are the basic molecules in all metabolic processes, ROS directly affects viability of cells and consequently animal health and performance. Oxidative stress in animals can be determined using different parameters. The half haemolysis time (HT50) for example is the time required to destroy or rupture 50% of blood cells exposed to a free radical attack as measured by the KRL-test. A lower HT50 indicates that blood cells have been exposed to higher levels of oxidative stress. 2 other parameters are the ratio ‘oxidised over total glutathione’ and the malondialdehyde (MDA) concentration in the blood. Glutathione is a naturally present antioxidant in the animal’s body. In its reduced form glutathione can neutralise free radicals converting glutathione to its oxidised form. Therefore, the higher the ratio ‘oxidised over total glutathione’, the more oxidative stress the animal was exposed to. MDA on the other hand is an end product of lipid peroxidation and is a third biomarker of oxidative stress. Lower MDA blood values indicate less oxidative stress. Table 1 clearly shows the impact of mycotoxins on oxidative stress levels in piglets. Piglets exposed to the maximum allowed EU level of feed contamination with DON (0.9 ppm) experienced more oxidative stress as indicated by the three previously discussed blood parameters. Higher oxidative stress levels resulted in lower feed intake and growth performance and had a negative effect on profitability.

Intestinal barrier function

After absorption or contact with epithelial cells, the gastrointestinal tract is highly impacted by the induction of oxidative stress by mycotoxins. Besides the negative effects on all cellular processes, oxidative stress has an enormous impact on intestinal barrier function (Figure 1). The intestinal barrier is mainly formed by a layer of epithelial cells covered with mucus. Tight junction proteins form a physical barrier between two adjacent epithelial cells preventing paracellular absorption of undesired substances such as toxins or pathogens. Oxidative stress has a negative effect on this intestinal barrier function due to the modification of certain cellular proteins. This modification promotes production of pro-inflammatory cytokines which in turn has a negative effect on the expression of tight-junction proteins. In vitro and ex vivo studies measuring trans-epithelial electrical resistance (TEER) have shown that DON and fumonisin B1 for example are able to increase the permeability of the intestinal epithelial layer of pigs and poultry. As a result, the compromised intestinal barrier function results in increased permeability for toxins, pathogens and feed-associated antigens.

Figure 1 – Oxidative stress results in an impaired intestinal barrier function.

Nutritional strategy

Most common feed additives against mycotoxins consist of clay minerals. These layered structures are perfect materials to adsorb mycotoxins. When bound properly, absorption of mycotoxins in the gastrointestinal is avoided as the mycotoxin-clay complex is excreted in the faeces. Clay minerals can bind mycotoxins with a rather flat structure but are not suited to bind globular mycotoxins such as DON. To counter DON, feed additives able to change the chemical structure of DON into its non-toxic form are advised. Although these strategies have been proven to have a very effective direct effect against mycotoxins, part of the mycotoxins can still escape the action of these additives and cause oxidative stress. For maximum protection, feed additives protecting the animal’s body from oxidative stress after mycotoxins have been absorbed should be used. Plant extracts rich in polyphenols offer a powerful nutritional strategy to counter oxidative stress caused by mycotoxins. Plant extracts have a strong H-donating activity making them really effective antioxidants. Combining polyphenolic antioxidants may even increase effectiveness. It is generally accepted that no antioxidant alone can lead to health benefits, but the combination as found for example in fruits and vegetables, is the principle that leads to synergistic effects. Not all plant components have the same biological antioxidant capacity. To prove the efficacy of plant polyphenols research should not only look to commonly used in vitro tests based on ORAC-values (Oxygen Radical Absorbance Capacity) which is purely a chemical analysis. The antioxidant capacity and the concomitant protection of the intestinal barrier function should be validated by ex vivo tests and in vivo trials as well. This has led to the inclusion of a synergistic blend of polyphenolic compounds in the mycotoxin detoxifying product with a broad range activity. Tests showed that this product gave a restoring of all performance and profitability parameters to the original level of the negative control (Table 1).