ABOVE: Several nutritional strategies have been identified as potential governors of the interaction between growth and health in pigs. Photo: Mark Stebnicki
Today’s swine production systems are mostly intensive, and pigs are continuously exposed to pathogens and immune stimulatory antigens that may negatively impact productivity.
It is known that immune system stimulation may reduce appetite, growth and nutrient use efficiency compared to healthy animals.
On the other hand, there is progressive pressure towards the reduction in antibiotic usage in livestock production, which highlights the need for furthering our understanding of the relationship between nutrition and the immune response.
Swine nutritionists will need to consider the role of nutrition on health in order to develop programs that support production and robustness of pigs under a variety of stressful conditions.
Among the nutritional strategies whose utilisation may directly impact on the immune status of pigs, feeding low protein, amino acid-supplemented diets, supplementation of functional amino acids, dietary fibre level and source, diet complexity, organic acids and plant secondary metabolites are at the forefront.
As such, this review by Canadian researchers will discuss the impact of immune status on swine production and the interaction between nutrients and animal health, focusing on the roles of each nutritional strategy during times of immune challenge.
Our understanding of nutrition has been evolving to support both performance and immune status of pigs, particularly in disease-challenged animals, which experience repartitioning of nutrients from growth towards the immune response.
In this sense, it is critical to understand how stress may impact nutrient metabolism and the effects of nutritional interventions able to modulate organ – as an example, gastrointestinal tract – functionality and health.
This will be pivotal in the development of effective diet formulation strategies in the context of improved animal performance and health.
Therefore, this review will address qualitative and quantitative effects of immune system stimulation on voluntary feed intake and growth performance measurements in pigs.
Due to the known repartitioning of nutrients, the effects of stimulating the immune system on nutrient requirements, stratified according to different challenge models, will be explored.
Finally, different nutritional strategies – that is low protein, amino acid-supplemented diets, functional amino acid supplementation, dietary fibre level and source, diet complexity, organic acids, plant secondary metabolites – will be presented and discussed in the context of their possible role in enhancing the immune response and animal performance.
Figure 1: Pigs subjected to stimulation of immune system experience increased production of acute-phase proteins – C-reactive protein, serum amyloid A, haptoglobin and pig-MAP. Additionally, there is a marked syntheses of endogenous antioxidants to mitigate oxidative stress, including the production of immune-signalling compounds, the release of superoxide dismutase by phagocytic cells and the increased turnover of glutathione, which is a key non-enzyme antioxidant in the body. Furthermore, a higher concentration of circulating pro-inflammatory cytokines decreases the contractility of gastrointestinal smooth muscle, thereby reducing its motility and sparing energy and nutrient expenditure for digestion. In attempt to minimise metabolic costs, pigs mobilise nutrients, particularly amino acids, from body reserves. This is mediated by proinflammatory cytokines and reactive oxygen species, which stimulate the IκB/NF-κB signalling pathway, eventually activating the ubiquitin proteasome system and further triggering muscle proteolysis, with the skeletal muscle being the primary source. The reduced feed intake, which is mainly a result of upregulation of feed intake-lowering – anorexigenic – peptides and downregulation of orexigenic peptides, limits the exogenous nutrient supply and aggravates muscle proteolysis.
Nutrition and health – the merge
Current nutrient requirement estimates are largely based on growth performance response, however nutrient effects on non-proteinogenic functions, such as intestinal development, immune status and response, has been increasingly demonstrated.
With legislation introduced in some countries to eliminate use of antimicrobial growth promoters and increasing pressure to reduce overall antibiotic use in livestock production, it will become increasingly important to understand how immune status affects nutrient requirements and how feed formulations can be adjusted to support animal robustness in addition to growth performance.
The impact of disease-challenge can be substantial.
For example, pigs kept in low sanitary conditions – lack of vaccination, environmental hygiene and biosecurity protocol, and preventive antibiotic injection – having a 55g per day reduction in daily gain during the complete fattening period compared to counterparts housed in high sanitary conditions.
This indicates a high cost associated with the activation of the immune system, including a higher production of immune cells and signalling molecules, as well as losses in efficiency of affected tissues, which will decrease the efficiency of nutrient utilisation for body protein deposition.
It is reasonable to infer that nutritionists must consider the components associated with building a strong immune system when formulating swine diets in commercial operations.
Of note, while nutritional strategies may support animal robustness during a disease and recovery from illness, attention should also be given to dietary strategies that trigger excessive immune activation and may impair the efficiency of animal production.
Additional evidence for the importance of nutrition for immune status lies in the endocrine regulation of appetite and growth.
Specifically, the reduction in performance due to immune system activation is not entirely and not always associated with a reduction in feed intake.
Upon exposure to stressful situations, mononuclear myeloid cells of the innate immune system synthesise and release proinflammatory cytokines, such as interleukin-1b, IL-6 and tumor necrosis factor alpha.
These cytokines will orchestrate a redirection of nutrients from growth towards the immune system, a specific response of the central nervous system with fever, decreased feed intake, and activation of hypothalamic-pituitary-adrenal axis, a suppression of gastric emptying and motility, and an increased concentration of glucagon, insulin and leptin in blood.
These events combine to affect feed intake and weight gain, and guide how inflammation impacts growth.
In order to meet growing demand, pork production will need to increase its sustainability and efficiency, which will encompass the development of nutritional programs that support animal growth performance and robustness under commercial conditions.
Several nutritional strategies have been identified as potential governors of the interaction between growth and health in pigs, including feeding low protein diets, functional amino acid supplementation and provision of fibre sources and natural – as an example, phytochemicals – compounds.
As such, the present review will discuss the impact of different immune stimulating conditions on nutrient digestion and metabolism and how different nutritional interventions may help nutritionists formulate diets for improved health status of pigs.
One important aspect approached by the present review is the differentiation between situations where stressors decrease feed intake and consequently impair growth performance versus the effects of immune system stimulation on nutrient metabolism.
Researcher and co-author Lucas A Rodrigues.
Immune system stimulation, nutrient efficiency and requirements
Immune stimulation and inflammation result in a number of effects in the pig, most notably a reduction in voluntary feed intake and efficiency of nutrient utilisation, even in the absence of clinical manifestation of disease, though this may not always be the case.
While both feed intake and nutrient utilisation efficiency influence pig performance, the extent to which reductions in these factors are related to the reduction in performance is dependent on the specific immune challenge.
For example, Rodrigues et al showed that the reduction in growth in challenged compared with control pigs associated with a reduction in feed intake was 14, 4, 45, 1 and 1 percent in pigs challenged with enteric pathogens, environmental stressors, bacterial lipopolysaccharide, respiratory pathogens and sanitary conditions, respectively.
This is important to note as in experimental environments, several challenge models are used to investigate stimulation of the immune system and the associated effects on outcomes such as growth performance impairment, nutrient requirements and immune status.
These include challenge with enteric and respiratory pathogens, housing pigs under heat stress, high stocking densities or suboptimal sanitary conditions and triggering systemic immune stimulation using LPS.
Overall, it is likely that the effect of any nutrition intervention in challenged pigs will be, at least in part, due to the specific challenge model used and results may not be transferable to other conditions.
An important factor differentially impacting performance response in challenged pigs is the stage of production.
As an example, the meta-analytical approach of Rodrigues et al demonstrated that recovery of performance in post-weaned pigs was dramatically influenced by the recovery of feed intake, which was not observed in nursery and grower phases.
The authors – Lucas Rodrigues, Bonjin Koo, Martin Nyachoti and Daniel Columbus – also reported a more abrupt and greater decrease in both growth and feed intake in younger lighter pigs compared to older heavier pigs.
This may be explained by the impact of gut fill, where newly weaned pigs take longer to reach maximum feed intake and decreased growth potential due to limited feed intake in the immediate post-weaning phase.
The nutritionist needs to understand the many stressors concomitantly reducing post-weaning feed intake, including introduction to solid feed, abrupt depletion of passive immunity transfer from the dam and the building of a new hierarchy with unfamiliar littermates.
Under this scenario, reestablishment of nutrient intake through higher nutrient supply or recovery of feed intake will be one of the most important governors for restoring weight gain.
Since different stressors may impact productivity at different stages of production, it is pivotal to understand how the exposure to pathogens and other stressors changes nutrient utilisation and the associated decrease in performance.
According to Pluske et al, it is possible to manipulate the immune system through nutrition and other supportive strategies, including suppressing the presence and action of pathogens, breeding for improved resilience and controlling the immune system to prevent overt response.
With the increased trend for reduction in in-feed antimicrobial growth promoters’ usage and the need for a more sustainable production system, a multidisciplinary approach will be needed to attenuate the negative impact of stressful agents, which are expected to be more harmful as pork production becomes more efficient.
Part 2 in next month’s issue will feature immune challenges in pork production.
Note that the article in its entirety can be viewed at ncbi.nlm.nih.gov/pmc/articles/PMC9598274/
