Comparison of viraemia and nasal shedding after PRRSV-1 challenge following vaccination with three commercially available PRRS modified live virus vaccines

The effectiveness of three Porcine Reproductive and Respiratory Syndrome (PRRS) Modified Live Virus (MLV) vaccines against PRRSV viraemia and nasal shedding following experimental challenge was compared. The study comprised a negative control (T01), and three treatment groups (T02, T03 and T04) each vaccinated with a single dose of a commercial PRRS MLV vaccine, given in accordance with the vaccine's Summary of Product Characteristics (SPC). Pigs aged 21 days were vaccinated (day 0), challenged intranasally (day 28) with heterologous PRRSV-1-1 strain Olot/91, then monitored for PRRSV viraemia and nasal shedding for 12 days. After challenge, pigs were viraemic on fewer days in group T04 (0.67) than groups T01 (0.91), T02 (0.81) and T03 (0.97) (P < 0.0296). From day 34, inclusive, serum PRRSV titres were lower in group T04 than negative controls (P ≤ 0.0001) and groups T02 and T03 (P ≤ 0.0047); serum PRRSV titre Area Under the Curve (AUC) for group T04 (42.34) was lower than in T01 (65.49), T02 (60.67) and T03 (67.38) (P < 0.0100); pigs exhibited nasal shedding on fewer days in group T04 (0.40) than T01 (0.78), T02 (0.64) and T03 (0.56) (P < 0.0101); and nasal shedding AUC for group T04 (8.52) was lower than in groups T01 (23.59, P < 0.0001) and T02 (19.37, P = 0.0001). The ability of PRRS MLV vaccines to reduce the duration of viraemia and nasal shedding after intranasal challenge with a heterologous PRRSV-1-1 strain differ significantly.