An experiment was conducted at the University of Illinois to determine the effects of dietary soybean meal (SBM) concentration on the growth performance and immune response of pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV). The four experimental treatments included a 2 × 2 factorial arrangement with 2 SBM concentrations (17.5% & 29%) and 2 levels of PRRSV infection (uninfected sham or PRRSV infected). Sixty-four weanling pigs (21 days of age; weighing about 7 kg) were individually housed in disease containment chambers. Pigs were provided a common diet for 1 week postweaning before being equalized for BW and sex and allotted to the 4 treatment groups with 16 replicate pigs per group. Pigs were fed experimental diets for 1 wk before receiving either a sham inoculation (sterile PBS) or a 1×105 50% tissue culture infective dose of PRRSV at 35 days of age (0 d postinoculation, DPI). Pig BW and feed intake were recorded weekly, and rectal temperatures were measured daily beginning on 0 DPI. Blood was collected on 0, 3, 7, and 14 DPI for determination of serum PRRSV load, differential complete blood cell counts, and haptoglobin and cytokine concentrations.

Results indicated that the infection with PRRSV increased (P < 0.01) rectal temperatures of pigs throughout the infection period, with no influence of dietary SBM concentration. Pigs in the PRRSV-infected group had lower (P < 0.01) average daily feed intake and feed efficiencies from 0 to 14 DPI compared with uninfected pigs. In the PRRSV-infected group, pigs fed the higher level of SBM tended to have improved average daily gains (P = 0.06) compared with pigs fed lower levels, whereas there was no influence of SBM concentration on growth of pigs in the uninfected group. At 14 DPI, PRRSV-infected pigs fed the higher level of SBM had a lower serum PRRSV load (P < 0.05), a higher (P = 0.02) hematocrit value, and a tendency for greater hemoglobin concentration (P = 0.09) compared with pigs fed the lower level of SBM. Serum haptoglobin and tumor necrosis factor-α concentrations of PRRSV-infected pigs were lower (P < 0.05) in pigs fed HSBM at 3 and 14 DPI, respectively, than in pigs fed LSBM. Overall, the research group concluded that increasing the dietary SBM concentration modulated the immune response and tended to improve the growth of nursery pigs during a PRRSV infection.