Algae extract supplementation boosts humoral immunity induced by attenuated PRRSV-1 vaccination.

BACKGROUND: Porcine reproductive and respiratory syndrome (PRRS) remains a major concern for the swine industry worldwide due to its significant economic impact and its association with respiratory co-infections. Vaccination is a key strategy for PRRS control; however, the most effective commercial anti-PRRSV vaccines to date-live attenuated vaccines, also named modified live virus (MLV) vaccines-provide only partial protection. This study investigates whether oral supplementation with formulated Ulva-derived seaweed extracts can enhance immune responses to PRRSV-1 MLV vaccination. METHODS: Two supplementation protocols using commercial seaweed extracts formulated with vitamin A and D (Searup®) were tested: one starting one day post-vaccination for five consecutive days (post-vaccination), and another beginning one day before vaccination and continuing for two days post-vaccination (pre-vaccination). In this exploratory farm-based experimental setting, pigs were divided into six groups: three vaccinated groups (supplemented with seaweed either pre- or post-vaccination or not supplemented), and three non-vaccinated control groups receiving the same supplementation regimes. Serum cytokine levels, PRRSV viral loads, and anti-PRRSV antibody responses were measured to assess immune modulation. RESULTS: Post-vaccination supplementation had no significant effect on vaccine viral load or humoral response, though a modest reduction in viral persistence was noted at 12 days post-vaccination (dpv). In contrast, pre-vaccination supplementation significantly increased vaccine virus levels at 4 dpv (p = 0.048) and enhanced anti-PRRSV IgG levels (p = 0.029). This protocol also induced an early and sustained decrease in pro-inflammatory cytokines, suggesting an immunomodulatory effect of algal extract supplemented with Vitamins A and D. We present here for the first time, in field conditions and in the target species, significant immune modifications upon realistic oral supplementation with Searup®. These findings are consistent with previous studies demonstrating the potential of pre-vaccination seaweed supplementation as a cost-effective, practical strategy to enhance humoral responses to PRRSV vaccination. Further studies are needed to evaluate its impact on neutralizing antibody production, cellular responses and protection against heterologous field PRRSV strains.