The effect of dietary soy daidzein on pig growth and viral replication during a viral challenge

Twelve replications of four littermate pigs from a porcine reproductive and respiratory syndrome (PRRS) naive herd were weaned (11 +/- 2 d of age) and penned individually in isolation rooms. Pigs were randomly allotted within litter to one of four dietary soy daidzein concentrations (0, 200, 400, or 800 ppm) to quantify the effect of daidzein on growth and immune response during a PRRS challenge. Daidzein was provided as the soy aglycone. At 27 +/- 2 d of age (4.9 +/- 1.4 kg BW), pigs were oronasally inoculated with 10(4.3) PRRS virus/mL from strain JA142 in a 2-mL dose. Blood was collected every 4 d from d 0 to 24 after inoculation and analyzed for serum PRRS virus, interferon, and alpha-1-acylglycoprotein (AGP) concentrations. Serum virus and interferon peaked at 10(5.3) virus/mL and 79% protection, respectively, at 4 d after inoculation and then declined steadily. Serum AGP concentration peaked at 12 d after inoculation. Each log increase in serum virus was associated with an increase in serum interferon, which resulted in a decrease of pig ADG and daily feed intake of 0.019 kg and 0.023 kg, respectively, in 5.8-kg pigs and a feed intake reduction of 0.024 kg in 12.5-kg pigs. Dietary daidzein additions did not (P > 0.3) alter the serum concentration after inoculation of PRRS virus (10(1.79), 10(1.94), 10(1.86), 10(1.93) virus/mL of serum) or AGP. Serum concentrations of interferon responded cubically (30.3, 28.9, 29.4, and 31.1% protection) as dietary daidzein concentrations increased; however, the magnitude of the response decreased over time. Dietary daidzein additions resulted in improvements in daily pig gain, daily feed intake, and gain/feed during periods of peak viremia (d 4 to 16 after inoculation), but not in periods when systemic virus concentrations were minimized (d 16 to 24 after inoculation), resulting in a daidzein x days after inoculation interaction. Based on these data, the magnitude of the growth responses that occur in pigs infected with a virus is quantitatively related to the animal's serum concentration of the virus and interferon, and dietary soy daidzein at 200 or 400 ppm is a weak enhancer of body growth in virally challenged pigs.