A total of 40 gilts (initial BW 9.4 ± 0.9 kg) were used in three experiments (Exp) to evaluate the impact of disease induced either by bacterial lipopolysaccharide (LPS) or porcine reproductive and respiratory syndrome virus (PRRSV) on the measures of immune function, as well as protein and amino acid (AA) utilization. An isotope tracer technique was employed in all of the Exp to evaluate the impact of disease on kinetics/utilization of nine AA (Lys, Met, Thr, Trp, Ile, Leu, Val, Phe, Gln) simultaneously. In Exp 1, apparent ileal digestibility (AID) and protein retention were determined in saline-treated pigs. An isotope infusion rate and the best blood sampling frequency for studying AA kinetics in plasma were also determined in Exp 1. In Exp 2 and 3 effects of LPS and PRRSV on measures of immune function, as well as protein and AA utilization (i.e. protein digestibility and retention as well as AA utilization) were evaluated, respectively. Blood chemistry, hematology, and body temperature results indicated that both models of disease induced effective immune system stimulation. However, compared to LPS, PRRSV model elicits a more severe response with a more negative impact on the performance of pigs. Relative to feed-restricted control (550 g/d feed allowance), LPS did not reduce the daily feed intake. However, PRRSV-challenged pigs consumed only 55 % of their daily feed allowance. Compared to control, protein retention was decreased in PRRSV-challenged pigs (37 %), while LPS resulted in a numeric decrease in protein retention (6.4 %). AID of protein was decreased by LPS and PRRSV (16 and 26 %, respectively). Relative to control, LPS reduced the utilization of Lys and Phe by 17.4 and 37.6 %, respectively. The utilization rate of other AA was not affected by LPS. PRRSV significantly increased plasma Lys concentration, while it had no effect on Lys rate of utilization, suggesting a substantial decrease in Lys needs in PRRSV-challenged pigs. Relative to control, the PRRSV challenge increased the utilization rate of Met (111 %), and Thr (55 %). The increased Met and Thr utilization in PRRSV-challenged pigs could be associated with the enhanced use of Met and Thr for synthesis of immune system metabolites and increased catabolism of these AA. These factors may increase dietary Met and Thr requirements of PRRSV-challenged pigs, relative to requirements for other AA (e.g. Lys). Collectively, results of the current project suggest that LPS and PRRSV model of ISS effectively alter the measures of immune function and metabolism. However, it seems that the effect on AA utilization is either disease-specific, or is a function of disease severity.
• Disease alters metabolism of growing pigs and impacts their ability to utilize dietary protein and AA.
• Disease reduces (apparent) ileal digestibility of dietary protein and AA in growing pigs.
• Disease reduces lysine requirements in growing pigs.
• Infection with PRRSV increases requirements for methionine and threonine in growing pigs.