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Pigs are exposed to various types of stress during their life. There is dietary, social and environmental stress but also metabolic stress through high performance. Some stages are considered to be especially critical and can result in a status called oxidative stress.
What is oxidative stress and why is it important?
Normally, there is a balance between oxidizing components called reactive oxygen species (ROS), and antioxidants of the endogenous radical defense system. However, even small changes in energy metabolism or immune response can upset the delicate balance. This condition, characterized by an excess of free radicals and/or insufficient protection by antioxidants, is defined as oxidative stress (Dröge, 2002). Oxidative stress is directly linked with inflammation due to the fact that oxidants are activators of the nuclear transcription factor NF-ĸB, the key regulator of inflammation (Pantano et al., 2006). NF-ĸB regulates proteins such as cytokines which stimulate the production of oxidants by activated neutrophils, again promoting oxidative stress. This finally results in a vicious cycle. Symptoms of oxidative stress are decreased immunity, muscle degeneration, loss of appetite, diarrhea, liver damage and finally cell death. Oxidative stress and related inflammation is suggested to be involved in a number of disorders in pigs, like mulberry heart disease, leaky gut syndrome, pneumonia and also the MMA complex in sows. A quite newly discussed approach is the role of oxidative stress, inflammation and related apoptosis in the development of secondary tail biting. Continuous oxidative stress will result in oxidative DNA damage. In order to assure homeostasis in the complete organism, damaged cells need to be eliminated, this is done by programmed cell death (Kannan & Jaine, 2000). If there are too many dead cells, typically occurring at the ear tips and tails of the piglets, the smell becomes different thus attracting other piglets.
Factors triggering oxidative stress
In the weaning phase piglets often show growth depression and are more susceptible to diseases, a phenomenon known as post-weaning stress syndrome (Campbell et al., 2013). During this period, social and environmental stress occur as piglets are separated from their mother and shifted to other barns whereas dietary stress is a result of replacing easily digestible sow milk with mainly plant based solid feed. Dietary stress is typically caused by poor-quality feed. Due to the immature gut system piglets need highly digestible feed. Undigested protein reaches the hindgut and is available for pathogenic bacteria, which can cause inflammation and oxidative stress (Amarakoon, 2017). Oxidized fat sources, mycotoxins, and various anti-nutritional factors (ANF) in soy, the main plant based protein source used in piglet diets, can additionally induce oxidative stress, either directly or indirectly via triggering an inflammation in the gut. While trypsin inhibitors and oligosaccharides are usually considered when choosing soy protein sources, the antigen beta-conglycinin, is not mentioned, even though it has been shown to induce oxidative stress and inflammatory processes as part of the immune response to the antigen (Chen et al., 2011; Xu et al., 2010).
Another occurrence when pigs are experiencing oxidative stress is as sows during late pregnancy and high lactation. The shift from anabolic to catabolic metabolism is a huge burden for the animal. Similar to what has been observed in dairy cows, there is a dramatic increase in energy utilization needed to support the onset of milk synthesis, especially since piglet numbers are increasing and more milk is needed. Several metabolic reactions are involved in milk synthesis and reactive ROS are formed in the mitochondria during this process as by-products of the electron transport chain reaction (Sordillo & Aitken, 2009). It has been shown that oxidative stress and oxidative DNA damage increase dramatically during late gestation and lactation in sows (Berchieri- Ronchi et al., 2011).
Measures to reduce oxidative stress in pigs
When it comes to measures to be taken to reduce oxidative stress, the focus should be to reduce dietary stress. High quality feed, low in antinutritional factors (ANFs), mycotoxins and oxidized fatty acids is an absolute must. Recent trial findings have shown that piglets given a starter feed with a highly digestible soy protein with low ANFs, in particular low beta-conglycinin, show reduced oxidative stress and an improved inflammatory status compared to pigs fed other soy sources (Ma et al., 2018, Bayer et al, 2019). This was indicated by higher plasma gluthathion peroxidase (GPX), superoxide dismutase (SOD) concentrations and higher plasma vitamin E levels, all components of the endogenous radical defense system.
The inclusion of certain feed additives can be a solution in case the oxidative stress is not related to dietary stress but to high performance and/or environmental or social stressors which cannot be changed. Increased dietary vitamin E levels are often used to overcome oxidative stress. Various polyphenols such as resveratrol, catechins, quercentin or curcumin are also discussed to reduce oxidative stress (Landete, 2013, Menon & Sudheer, 2007). However, while there are studies showing that polyphenols do result in a better oxidative status in production animals (Zhou et al., 2016; Männer et al, 2017), they are also discussed in directly blocking NF- ĸB and thus downregulating inflammatory genes (Gessner et al., 2013).
Nevertheless, whatever way is chosen, the reduction of oxidative stress and related inflammation will not only lead to healthier animals and better animal welfare but also to higher performance. The immune system is less challenged, thus more protein and energy are available to promote piglet growth (Klasing, 2004).