Vaccination with a PRRSV-1 modified live vaccine provides protection against a highly virulent PRRSV-1.1 Spanish strain challenge in piglets

In 2020, atypical outbreaks of porcine reproductive and respiratory syndrome virus type 1 strain (PRRSV-1) were reported in northeastern Spain, characterized by high abortion rates, severe respiratory signs, and elevated mortality in affected nurseries. Sequencing analyses identified a highly virulent PRRSV-1.1 strain (Rosalia). This study evaluated the efficacy of a commercial live attenuated PRRSV-1 vaccine against this strain. Seventy PRRSV-naïve, 3-week-of-age piglets were randomly assigned to vaccinated (intradermal) and non-vaccinated groups. Five weeks post-vaccination, all piglets were intranasally challenged. Clinical signs, mortality, and weight gain were monitored for 28 days post-challenge; dpch. Viraemia, nasal shedding, tissue viral loads, lung lesions, and serological response were evaluated. Following challenge, 80% of the non-vaccinated piglets died within the first 22 days, compared to only 8.8% in the vaccinated group (p < 0.0001). Vaccinated animals showed significantly lower clinical scores, greater weight gain, and reduced lung lesions, with a relative decrease of 57.66% interstitial pneumonia and 77.67% lung area affected by bronchopneumonia. Viral loads in serum (3, 7, 14, and 17 dpch) and nasal secretions (10 and 14 dpch) were significantly lower in vaccinated animals, with fewer positive samples. Lung and tonsil viral loads were also significantly reduced. All vaccinated piglets seroconverted before challenge, and PRRSV-specific antibodies were detected in all animals by day 14 pch. Neutralizing antibody titres were significantly higher in vaccinated animals following challenge. These findings support the use of vaccination to confer significant protection against clinical disease and virological replication caused by the Spanish highly virulent PRRSV-1.1 (Rosalia) strain.