Quantification of the cytokine protein levels in serum collected from PRRSV-infected pigs. Serum samples collected at 0, 3, 7, 14, and 28 dpc were analysed to detect porcine cytokines. A IFN-α by ELISA and a correlation with average viremia at 3 dpc. B TNF-α by ELISA. The error bars represent the SEM. Asterisks indicate significant differences in the levels of cytokines induced by PRRSV among the genotypes (*p < 0.05).