Gut Health and the Post-Weaning Transition in Piglets

Introduction

A gastrointestinal tract (GIT) that functions in an optimum way clearly is of importance to the overall metabolism, physiology, disease status and performance of pigs of all stages of growth and development, and especially in the sensitive post-weaning production period. Disruptions (dysbiosis) in the GIT after weaning caused by internal and external influences can cause large economic losses in the pork industry, therefore the period after weaning generates much interest. Diseases and conditions of the GIT that can cause losses, for example, post-weaning diarrhoea, have to some extent been controlled traditionally by the use of antimicrobial compounds administered in the feed and (or) water, such as antibiotics and supraphysiological levels of trace elements such as zinc and copper. However, legislation and judgements in various parts of the world together with general efforts to reduce the use of these compounds have caused a reassessment of measures to influence GIT structure and function (i.e., ‘gut health’), and have caused unparalleled interest in alternative strategies (e.g., genetic, dietary, management, environmental, veterinary) to effectively manage the GIT under conditions of external and internal challenge. It is important that the pork industry continues to explore and understand the numerous factors that influence GIT after weaning.

What is gut health?

‘Gut health’ was defined originally in human medicine (Bischoff, 2011) and is now ubiquitous with respect to pig health and production, especially in the post-weaning period (e.g., Lallès et al., 2004; 2007; Lallès, 2008; de Lange et al., 2010; Pluske, 2013; Heo et al., 2013; Lindberg, 2014; Kogut & Arsenault, 2016; Celi et al., 2017, 2019; Jayaraman & Nyachoti, 2017; Moeser et al., 2017; Pluske et al., 2018; Pluske & Zentek, 2019). ‘Gut health’ is also regularly used in the popular press and in on-line articles and reports and is a common point of discussion and debate at meetings, forums and workshops. The term is therefore used in many different contexts and applications. Nevertheless, Bischoff (2011) remarked that ‘gut health’ can be defined as ‘a state of physical and mental well-being in the absence of GI (gastrointestinal) complaints that require the consultation of a doctor, in the absence of indications of or risks for bowel disease and in the absence of confirmed bowel disease’. Bischoff (2011) said that the prevention or avoidance of GIT disease forms an integral part of our understanding of ‘gut health’, and defined five major criteria that could form the basis of a definition of ‘gut health’: (1) effective digestion and absorption of food, (2) absence of GI tract illness, (3) normal and stable intestinal microbiome, (4) effective immune status, and (5) status of well-being. In this regard, Pluske and Zentek (2019) commented that there is sometimes a tendency to associate ‘gut health’ with bacterial and (or) viral pathogens that cause, either clinically or sub clinically, illness to pigs after weaning, and indeed at any stage of the production cycle. However, ‘gut health’ can be compromised in the absence of any diseases in the GIT and expression of clinical disease. Low feed intake after weaning, for example, caused by maternal separation and other psychosocial stressors including transportation, mixing, fighting and the establishment of a new social hierarchy, and (or) immunological stressors such as vaccination, can cause inflammation and dysbiosis in the GIT (Pluske et al., 1997; McCracken et al., 1999; Spreeuwenberg et al., 2001). In turn, GIT barrier function is compromised (Wijtten et al., 2011; Kim et al., 2012; Moeser et al., 2017; Modina et al., 2019) and the microbiome and its functions can change (Guevarra et al., 2018), negatively impacting on the overall ‘gut health’ of the young pig.

Separate to these stressors impacting on ‘gut health’ after weaning, the (usually abrupt) change from sows’ milk to a dry, solid feed offered to pigs that occurs adds additional challenges in the post-weaning period, concerning particularly the digestive and absorptive processes and impacts on the microbiota (microbiome) and immune system, especially the innate immune system (Pluske et al., 2018). At a deeper level, and as summarised by Xiong et al. (2019), weaning significantly down-regulated the expression of proteins involved in the tricarboxylic acid cycle, β-oxidation, and the glycolysis pathway in the upper villus and middle villus of the jejunum in early-weaned pigs, but up-regulated proteins involved in glycolysis in crypt cells. In the post-weaning period, the expression of proteins related to various cellular metabolic or biological processes, such as energy metabolism, protein amino acid glycosylation, ion transport, mTOR signalling pathway, and differentiation and apoptosis, were reduced in jejunal-differentiated epithelial cells (villus upper cells). Proteins involved in the respiratory electron transport chain, Golgi vesicle transport, protein glycosylation, as well as the metabolism of nutrients such as lipids, monosaccharides, and nucleotides, were also downregulated in the jejunal-differentiating epithelial cells of piglets during this period. These results indicate that weaning influenced energy metabolism, cellular macromolecule organisation and localisation, and protein metabolism, thereby further impacting the proliferation of intestinal epithelial cells after weaning. In addition, polyamine metabolism and ornithine decarboxylase expression were also altered by weaning.

Collectively, an understanding of the composition of the microbial community in the GIT before and after weaning and its functional capacity during the post-weaning transition is important for pig production and veterinary and nutritional practices.

The microbiome and gut health after weaning

The resident GIT microbiota provides the pig with many functions including improved energy harvesting capacity, the production of short-chain fatty acids, production of vitamin K, polysaccharide fermentation, and enhanced resistance against pathogenic bacteria. The pig GIT contains a diverse and complex microbial community, with the total number of bacteria in the pig colon been estimated as 10¹⁰ – 1 x 10¹¹ per gram of content; more anteriorly, populations are at a lower density (Gaskins, 2001). As mentioned previously, weaning causes profound physiological changes in the structure and function of the GIT corresponding to the GIT microbiota undergoing a very quick ecological succession upon induction of these various factors during the weaning (transitional) period. This microbial shift (Kim & Isaacson, 2015) is influenced strongly during weaning by the sudden change in diet from simple to more complex nutrient sources, which effects digestion and absorption capacity of the small intestine and hence influences growth and feed efficiency.

The weaning period exposes young pigs to thousands of new bacterial species, which will play an important role in establishing an adult-like microbiota later in life (Kim et al., 2011; Isaacson & Kim, 2015). Early-life microbial exposure is of particular importance to growth, development of immune system and health (Dou et al., 2017), and may be able to be used to predict a disease outcome (‘gut health’). In this particular experiment, the GIT bacterial community was assessed for diversity and composition during the suckling period and then associated with differences in susceptibility of pigs to post-weaning diarrhoea. Using a molecular characterisation of faecal microbiota with CESSCP fingerprint, Next Generation Sequencing and qPCR, diarrhoeic and healthy pigs could mainly be discriminated as early as postnatal day (PND) 7, i.e. 4 weeks before the post-weaning diarrhoea actually occurred. At PND 7, healthy pigs (i.e., healthy after weaning, showing no diarrhoea) displayed a lower evenness and a higher abundance of Prevotellaceae, Lachnospiraceae, Ruminocacaceae and Lactobacillaceae compared to the diarrhoeic pigs. Regression analyses indicated that these bacterial families were strongly correlated to a higher Bacteroidetes abundance observed in PND 30 healthy pigs one week before the diarrhoea. These results emphasise the potential of early microbiota diversity and composition as being an indicator of susceptibility to post-weaning diarrhoea (Dou et al., 2017), and provides potentially tangible, practical ways to positively impact ‘gut health’ after weaning.

Additionally, the establishment of a beneficial microbiota is important during the weaning stage because piglets still have an immature immune system and depend on sow’s milk to prevent colonisation and overgrowth of opportunistic pathogens (Castillo et al., 2007). Therefore, understanding GIT microbial succession during the weaning transition, and how different factors such as diet, stress and housing influence gut microbial shifts in association with enhanced ‘gut health’, growth performance and well-being of pigs, is critical. In turn, such information may be able to be used to assist with decision-making regarding the use of different diet compositions, antimicrobial compounds, and (or) selected feed additives.

In this regard, Guevarra et al. (2018) performed 16S rRNA gene and whole metagenome shotgun sequencing of DNA from faecal samples from healthy piglets during weaning to measure microbiome shifts, and to identify the potential contribution of the early-life microbiota in shaping piglet health with a focus on microbial stress responses, carbohydrate and amino acid metabolism. Major findings were that the faecal microbiome of sucking piglets showed higher relative abundance of bacteria in the genus Bacteroides with abundant gene families related, not surprisingly, to the utilisation of lactose and galactose. Prevotella and Lactobacillus were enriched in weaned piglets (Figure 1) with an enrichment for the gene families associated, again unsurprisingly, with carbohydrate and amino acid metabolism. In addition, functional capacity of the faecal microbiome showed higher abundances of genes associated with heat shock and oxidative stress in the metagenome of weaned piglets compared to nursing piglets.

Inflammation of the GIT can be a negative outcome from the post-weaning milieu, and although the impacts of localised GIT inflammation are well recognised (summarised in Moeser et al., 2017; Pluske et al., 2018), less is known about its impacts on the GIT microbiome after weaning. In a recent important study, the mechanisms by which GIT inflammation contributes to imbalance of the microbiota has been proposed (Zeng et al., 2017). Under intestinal inflammatory conditions, the host responds by producing reactive oxygen species such as nitric oxide (NO) that is rapidly converted to nitrate (NO₃⁻) when released in the lumen. In turn, the nitrate-rich environment is conducive for the growth of Enterobacteriaceae that encodes for nitrate reductase genes (Winter et al., 2013). Some pathogens within Enterobacteriaceae, namely Salmonella enterica serovar Typhimurium and enterotoxigenic E. coli (ETEC), induce intestinal inflammation in pigs which disrupts composition (Héctor et al., 2013). For example, in a piglet model of Salmonella Typhimurium infection, Arguello et al. (2019) concluded that the host response to infection (immune response and metabolic changes) could be a major contributor to the depletion of commensal/beneficial inhabitants of the intestinal tract (Lactobacillus, Bifidobacterium, Prevotella or Megasphaera), and the increase of synergists of infection such as Akkermansia or Citrobacter. The relative abundance of these synergists in the ileal microbial ecosystem of the post-weaned pig was found to be is positively correlated to the degree of epithelial damage in the ileum. Therefore, inflammation of the GIT caused immediately by perturbations linked to weaning begins a cascade of events including adverse alterations to the GIT microbiome, which appears to favour the growth of pathogenic bacteria, especially members of Enterobacteriaceae (Guevarra et al., 2019).

Figure 1. The relative abundance of the significantly different taxa between Nursing and Weaned piglets at the genus level. The interquartile range is indicated by the outer bounds of the boxes, and the median is indicated by the black midline. The whiskers represent the minimum and maximum values. The [P < 0.001], [P < 0.01] and [P < 0.05] were indicated as [***], [**] and [*], respectively (from Guevarra et al., 2018).

The relative abundance of the significantly different taxa between Nursing and Weaned piglets at the genus level. The interquartile range is indicated by the outer bounds of the boxes, and the median is indicated by the black midline. The whiskers represent the minimum and maximum values. The [P < 0.001], [P < 0.01] and [P < 0.05] were indicated as [***], [**] and [*], respectively (from Guevarra et al., 2018).

Diet, feed additives and gut health after weaning

The development of management and feeding strategies to optimise GIT development and health in newly weaned pigs in order to improve growth performance and reduce morbidity and mortality, while either reducing reliance on antimicrobial compounds or not having their availability at all, is essential for the sustainability of the pork industry. It is therefore necessary to find combinations of feed ingredients, either alone or in combination with feed additives acceptable for use, that are effective in amending the post-weaning growth check and reducing the incidence and severity of digestive problems frequently encountered (Pluske, 2013). Given the considerable advances already made in the understanding of intestinal nutrient utilisation and metabolism, de Lange et al. (2010) stated at the time that, “a complimentary goal in post-weaning nutrition should be to formulate young pig diets with the specific task of optimising the growth, function and health of the GIT”. Since then, the scrutiny associated with using antimicrobial compounds has become even more pronounced, meaning there is even more interest in the use of feed-related strategies to assist with optimisation of ‘gut health’ in the post-weaning period.

There is a plethora of reviews, papers and articles describing the effects of post-weaning nutritional interventions and changes on ‘gut health’. Given that the GIT of the young weaned pig is undergoing rapid changes in size, protein turnover rates, microbiota mass and composition, and quick and marked alterations in digestive, absorptive, barrier and immune functions, then it is problematic to suggest that a single dietary-related strategy can be effective in optimising ‘gut health’ health in different groups of pigs that are managed under wide ranging conditions of housing, management, feeding and health status. Therefore, a direct ‘like-for-like’ replacement of antibiotic growth promoters, prophylactic antibiotics and (or) pharmacological levels of zinc oxide (ZnO) with for example, a single feed additive, is unlikely. This emphasises the need to explore underlying mechanisms when evaluating the functional properties of feed ingredients and feed additives, so that a better understand can occur to achieve the optimal response to dietary interventions (Pluske, 2013). Therefore, and currently, it is unlikely that there is any single substance that could reliably and repeatedly replace the function of in-feed or in-water antibiotics. Since the growth benefit found from feeding antibiotics is achieved through many different effects on the GIT, the strategy for replacing them will depend on a combination of nutritional, management, housing, health and (or) husbandry factors. There is also considerable inconsistency in the experimental and (or) commercial outcomes of the many alternatives evaluated, which makes it difficult to judge the efficacy or otherwise of a particular additive (Pluske, 2013).

Amongst the high quality and novel feed ingredients (e.g., insoluble fibre, plasma protein) effective feeding strategies (e.g., lower protein diets) and feed additives that are available, the organic acids have been widely used over time as feed additives for their positive effects on growth efficiency. They are generally considered a valid tool for use in the post-weaning period, more so than in growing-finishing pigs, although expectedly there is a relatively large variation in responses due to various factors such as type and dose of organic acids used, supplementation duration, type of diet and buffering capacity, hygiene and welfare standards, health status, and age of the animals. In a post-weaning meta-analysis study, Tung & Pettigrew (2006) reported that the improvements of growth rates were 12.2% and 6% for the first 2 weeks or 4 weeks post-weaning respectively, while the enhancement was lower for growing (3.5%) or finishing (2.7%). A full and recent review of different organic acids can be found in Tugnoli et al. (2020).

Of course, there are many other feed additives available for use to modify different aspects of ‘gut health’, and these are covered in many reviews. Liu et al. (2018), in their extensive review, commented that there are a number of feed additives that potentially may be used in diets fed to pigs, but the main challenge with all of these additives is the fact that results obtained so far have been inconsistent, and especially in field conditions. The reason for this inconsistency may be that efficiencies of each additive are diet dependent and also dependent on the health status of the animals (e.g., Li et al., 2018). Nevertheless, a meta-analysis conducted by Vanrolleghem et al. (2019) evaluated the use of potential dietary feed additives (pDFA) with antibacterial effects and their impact on the performance of weaned piglets, versus a positive control (a diet containing a therapeutic antibiotic or antibiotics). Twenty-three peer-reviewed in vivo studies, comprising 50 trials, were identified between January 2010 and January 2017. Five classes of pDFA were formed: antimicrobial peptides, chitosan, lysozyme, medium-chain fatty acids/ triglycerides, and plant extracts. Mixed-effect meta-analyses with type of pDFA as the fixed effect were performed for average daily gain and feed conversion ratio. The results of the meta-analysis showed that adding a pDFA at weaning can improve these performance indicators compared to a Negative control, with no overall significant difference to the Positive control. However, this is only a small evaluation and excludes numerous other feed additives that purport to improve ‘gut health’ such as probiotics and prebiotics (Liao and Nyachoti, 2017).

Feed processing and gut health

Finally, feed processing and diet manufacturing also play important roles in ‘gut health’. The benefits of feed processing in terms of animal performance and economics are apparent; however, feed processing optimisation in the future will also likely be impacted by the same issues as mentioned previously about antimicrobials. For example, feed processing should increasingly consider dietary approaches (ingredients and physical characteristics) for maintaining a healthy and functional GIT. The need to achieve high physical quality and to reduce potential levels of feed-borne pathogens such as Salmonella has led to the application of relatively high conditioning temperatures during conventional pelleting processes, but this is a practice that does not favour high nutrient utilisation (Kiarie and Mills, 2019). Furthermore, and with regard to fibre, different processing methodologies may be used to modify ‘gut health’ after weaning.

Molist et al. (2009) showed that wheat bran (WB), a fibrous ingredient, could decrease the number of pathogenic E. coli in the faeces and reduce the incidence of post-weaning diarrhoea. Molist et al. (2010) extended this study by trying to determine whether the effects were due to WB alone and (or) the particle size. Four experimental groups were tested, i.e., (1) a negative control diet (NC) based on corn, wheat, barley and soybean meal (2) NC + 4% coarsely milled WB (WBc, 1088 μm); (3) NC + 4% finely milled WB (WBf, 445 μm); and (4) a positive control diet (PC) consisting of the NC diet supplemented with a commercial feed grade antibiotic mix. Pigs were inoculated with 6.2×10⁹ cfu/mL of E. coli K88⁺ (F4). There were no significant differences in performance attributable to dietary treatment, but the inclusion of WB, either fine or coarse, decreased (P<0.05) E. coli numbers in the ileal digesta. The use of WBc had an additional benefit because the E. coli K88⁺ numbers were lower (P<0.05) compared to WBf (Tab. 1).

Table 1. Effect of wheat bran after 16 days of feeding on the E. coli K88 determination (log₁₀ CFU/g digesta) in the ileal mucosa and the E. coli population (log₁₀ CFU/g digesta), and the faecal score in pigs challenged with enterotoxigenic E. coli K88 at day 9 of trial (from Molist et al., 2010).

ffect of wheat bran after 16 days of feeding on the E. coli K88 determination (log10 CFU/g digesta) in the ileal mucosa and the E. coli population (log10 CFU/g digesta), and the faecal score in pigs challenged with enterotoxigenic E. coli K88 at day 9 of trial (from Molist et al., 2010).

Conclusions

The post-weaning ‘growth check’ and enteric diseases including post-weaning diarrhoea continue to represent a major source of economic loss in some parts of the world’s swine industry. The ‘gut health’ of the young pigs is generally negatively affected, and it can take weeks rather than days for pigs to recover. A healthy GIT should enhance the overall capacity/ability of the host to respond and adapt to challenges/stress and should be concomitant with optimal performance. Much research into the effective uses of feed ingredients, feed strategies and (or) feed additives has occurred to reduce the industry’s reliance on antimicrobial compounds. Fundamental to this research must be inquiry into the GIT of the young pig around weaning. A number of nutritional strategies have been suggested as alternative means of enhancing post-weaning growth performance and ‘gut health’ in piglets.

Published in the proceedings of the International Pig Veterinary Society Congress – IPVS2020. For information on the event, past and future editions, check out https://ipvs2022.com/en.