Evaluation of the efficacy of a live attenuated vaccine (Porcimune PRRS MLV) against porcine reproductive and respiratory syndrome virus in Landrace piglets

Abstract Porcine reproductive and respiratory syndrome virus (PRRSV) represents a constant challenge for the swine industry, both in Mexico and internationally, due to its high genetic variability and immunosuppressive capacity. This study evaluated the comparative efficacy of four modified-live vaccines in 3 to 4-wk-old seronegative piglets challenged intramuscularly with the heterologous PRRSV strain DAP150520 at 35 d post-vaccination (dpv; 0 d post-challenge, dpc). Fifty piglets were randomly allocated into five groups (n = 10/group): G1, ATCC-VR-2332; G2, SJO151012 ≥10⁴ (Porcimune); G3, P129; G4, attenuated Nebraska ≥4.0; and G5, unvaccinated controls. Outcomes included ELISA seroconversion, qRT-PCR cycle threshold (Ct) values (PRRSV RNA detection), daily clinical scoring, BW, and gross lung lesions assessed using the modified Madec scale at 7 dpc. Group 2 (Porcimune) showed complete seroconversion from 21 dpv, controlled post-challenge viremia (cycle threshold [Ct] = 21.84 at 7 dpc), and a 57% reduction in lung lesions compared with controls. Group 1 (ATCC-VR-2332) also reached complete seroconversion by 21 dpv, whereas Group 3 (P129) showed early but fluctuating seroconversion and Group 4 (attenuated Nebraska) showed delayed and inconsistent seroconversion across sampling points, and Group 5 (control) remained seronegative until after challenge. Group 3 achieved a 63% reduction in lung damage, although with a more variable immune response. In contrast, G1, G4, and the control group showed lung lesions in 100% of animals after challenge. Post-challenge BW gain was significantly greater in G1 and G2 than in the unvaccinated control (one-way ANOVA, P = 0.023; Tukey’s HSD, P = 0.039 and P = 0.045, respectively), meanwhile the control group showed a net BW loss after challenge. Overall, under these experimental conditions, Porcimune PRRS MLV was the only vaccine that differed significantly from unvaccinated controls in lung lesions, post-challenge BW gain and ELISA S/P ratio, supporting its protective potential against a heterologous PRRSV challenge.