Manipulating the immune system for pigs to optimise performance

Disease and enhanced microbial load are considered to be major factors limiting the performance and overall efficiency of feed use by pigs in Australian piggeries. It is recognised that pigs exposed to conventional housing systems with high microbial loads grow 10-20% more slowly than do gnotobiotic pigs or pigs kept in 'clean' environments. Consequently, a proportion of pigs in any production cycle are continuously being challenged by their immediate environment, which can cause an immune response to be mounted. Such a process is physiologically expensive in terms of energy and protein (comprised of amino acids), with, for example, the enhanced rate of protein turnover associated with the production of immune cells, antibodies and acute-phase proteins increasing energy expenditure by 10-15% of maintenance needs and protein requirements by 7-10%. The requirements for lysine, tryptophan, sulfur-containing amino acids and threonine can be increased by a further 10%. The over-stimulation of the immune response with excess production of pro-inflammatory cytokines causes excessive production primarily of the prostaglandin E-2 (PGE(2)), which contributes to anorexia, fever and increased proteolysis, and a concomitant reduction in pig performance. Prostaglandin E-2 is produced from dietary and cell-membrane phospholipids via secretory phospholipase A(2) (sPLA(2)) to produce arachidonic acid, which is catalysed by the COX-2 enzyme. Negating the negative effects of PGE(2) appears not to adversely affect the ability of the immune system to combat pathogens, but improves pig performance. There are negative outcomes for pig health and productivity through both under-and over-stimulation of the immune response. This review briefly outlines the impact of immune stimulation on pigs and discusses strategies to optimise the immune response for pig health and performance.