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

Twelve replications of four littermate pigs from a herd naive for porcine reproductive and respiratory syndrome (PRRS) were weaned (10 +/- 2 d of age) and penned individually in isolation rooms. Pigs were randomly allotted within litter to one of four dietary soy genistein concentrations (0, 200, 400, and 800 ppm) to quantify the effect of soy genistein on pig growth and virus replication during a viral challenge. Genistein was provided as the soy glycoside, genistin. At 29 +/- 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 postinoculation and analyzed for serum PPRS virus, interferon activity, and alpha1-acylglycoprotein (AGP) concentrations. Serum virus and interferon peaked at 10(5) virus/mL and 57% protection, respectively, at 4 d postinoculation and then declined steadily. Serum AGP concentration peaked at 12 d postinoculation. Each log increase in serum virus was associated with a reduction of daily gain of 0.034 kg in 5.3-kg pigs and 0.004 kg in 11-kg pigs. As dietary genistein concentration increased, serum concentrations of PRRS virus decreased linearly (10(2.46), 10(2.26), 10(2.05), and 10(2.14) virus per milliliter of serum, P < 0.07) and interferon responded quadratically (28.4, 25.7, 22.8, and 30.9% protection, P < 0.06) independent of days postinoculation. The AGP concentrations increased (P < 0.01) quadratically with the magnitude of the response to dietary genistein maximized at 12 to 16 d postinoculation. Effects of dietary genistein on daily pig gain and feed intake were dependent on dietary genistein concentration and stage of viremia. Daily pig gains from d 0 to 24 postinoculation were improved as dietary genistein increased, but the magnitude of the response to dietary genistein concentration lessened as the serum virus concentrations were minimized resulting in a linear genistein x period interaction (P < 0.07). Daily feed intakes also were increased quadratically as genistein concentration increased. These data indicate that soy genistein at dietary concentrations of 200 to 400 ppm is an orally active immune modulator that enhances systemic serum virus elimination and body growth in virally challenged pigs.