Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically-important disease and ingestion of soy isoflavones (ISF) may benefit PRRSV-infected pigs due to demonstrated anti-inflammatory and anti-viral properties. The objective of this study was to determine long-term effects of feeding ISF on growth performance and both acute and recovery immune responses following PRRSV infection.

In total, 96 weaned barrows were group-housed in a BSL-2 containment facility and allotted to 1 of 3 experimental treatments that were maintained throughout the study: non-infected pigs received an ISF-devoid control diet (NC, n = 24), and infected pigs received either the control diet (PC, n = 36) or that supplemented with total ISF in excess of 1,600 mg/kg (ISF, n = 36). Following a 7-day adaptation, weanling pigs were inoculated intranasally with either a sham-control (PBS) or live PRRSV (1×105 TCID50/mL, strain NADC20). After inoculation, individual blood samples (n = 8-12/treatment) were routinely collected to monitor viral clearance, hematological parameters, and anti-PRRSV antibody production, and pen-based oral fluids were used to monitor PRRSV clearance at later growth stages. A 2-way ANOVA (including time effect) was performed to compare NC and PC groups (infection status) and PC and ISF groups (ISF effect within infected pigs).

In general, PRRSV infection decreased performance during early growth phases, resulting in 5.4% lower final BW for PC vs. NC pigs (P < 0.05). Dietary ISF elicited inconsistent effects on growth performance, reduced (P < 0.05) longitudinal serum viral loads, and increased (P < 0.05) the rate of PRRSV clearance from oral fluids. Dietary ISF also elicited earlier, more robust anti-PRRSV neutralizing antibody production as compared with PC pigs. Additionally, PC pigs experienced ~50% greater infection-related mortality vs. ISF pigs (P < 0.05), which could have significant economic implications for producers.