Dietary soy isoflavones reduce pathogen-related mortality in growing pigs under porcine reproductive and respiratory syndrome viral challenge

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 antiviral properties. The objective of this experiment was to recreate immunological effects previously observed in young pigs infected with PRRSV receiving ISF and determine how those effects influence growth performance during the entire growth period from weaning to market. In total, 96 weaned barrows were group housed in a biosafety level-2 containment facility and allotted to 1 of 3 experimental treatments that were maintained throughout the study: noninfected pigs received an ISF-devoid control diet (NEG, n = 24), and infected pigs received either the control diet (POS, n = 36) or that supplemented with total ISF in excess of 1,600 mg/kg (ISF, n = 36). Following a 7-d 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 to 12/treatment) were routinely collected to monitor viral clearance and hematological parameters, including serum neutralizing anti-PRRSV antibody production. Pen-based oral fluids were used to monitor PRRSV clearance at later growth stages. A 1- or 2-way ANOVA was performed to compare experimental treatments depending on whether the outcome was repeatedly measured. In general, PRRSV infection decreased performance during early growth phases, resulting in 5.4% lower final BW for POS vs. NEG pigs (P < 0.05). Dietary ISF elicited inconsistent effects on growth performance, increased (P < 0.05) neutrophil cell counts and the relative proportion of memory T-cells, and decreased (P < 0.05) the time to full PRRSV clearance from oral fluids. Dietary ISF also elicited earlier, more robust anti-PRRSV neutralizing antibody production when compared with POS pigs. Additionally, and most notably, POS pigs experienced ~50% greater infection-related mortality rate vs. ISF pigs (P < 0.05), which may have significant economic implications for producers. Overall, dietary ISF ingestion supported immune responses and reduced mortality in PRRSV-infected pigs when fed to growing pigs though the biological mechanism of these effects remains unclear.