Introduction of a PRRSV-1 strain of increased virulence in a pig production structure in Spain: virus evolution and impact on production

Background: A strain of Porcine reproductive and respiratory syndrome virus (PRRSV), showing characteristics of enhanced virulence, affected a pyramidal production system from Spain with 7600 sows in 4 genetic nuclei and 13,000 sows in multipliers. Different PRRSV strains circulating in this production system from December 2020 to Octo-ber 2021 were detected and sequenced. The spread of each isolate was examined and their impact on health and production in three of the affected farms was evaluated.Results: The newly emerged PRRSV isolate with enhanced virulence entered the system before the onset of the study (January 2020) and afterwards four significantly different clades were detected during the study period in differ-ent farms, probably because of independent introduction events. The diversification of the enhanced virulence strain was higher for those clades (substitution rates up to 1.1% nucleotides/year) compared to other PRRSV strains present in the production system (up to 0.17%), suggesting a faster spread and adaptation. The impact of the infection in the first affected farm was dramatic, with an average abortion rate above 27% during 17 weeks before returning to the baseline production. Fertile sow mortality reached 6.5% for 39 weeks. In two farms infected later by other clades of this enhanced virulence strain, the impact was less acute; despite the fact that for parameters such as the proportion of stillbirths or mummies, more than 10 months were needed to recover pre-outbreak values. In the examined nurser-ies, mortalities reached peaks between 28 and 50% and several months were needed to return to normality.Conclusion: Introduction of a PRRSV strain of enhanced virulence in a production system where several farms were previously positive for other PRRSV strains, resulted in a fast spread such as would be observed in naive farms. The productive and health impact was very high taking several months to return to normality.