The Swine Health Information Center is reporting a notable shift in PRRS patterns across the U.S. swine industry, with lineage L1C now emerging as the most prominent strain circulating in 2025. New monitoring data shows PRRS activity continuing to climb in wean-to-finish populations, even as sow farm positivity unexpectedly declines.
According to the latest monthly disease monitoring update, wean-to-market sites are showing positivity rates over 43%, while sow farms have dropped closer to 17%. The divergence is creating new challenges for veterinarians and producers, especially as L1C continues its rapid climb.
L1C Surpasses Other Lineages
Data shows that more than 3,600 PRRS cases attributed to L1C have already been identified in 2025, surpassing the total recorded for all of 2024. The strain has now overtaken both L1A and L1H, solidifying its position as the predominant lineage this year.
L1C’s rise is particularly concerning because the strain is associated with greater virulence compared to other PRRS variants. Its increasing footprint reinforces the importance of tightening biosecurity protocols across production systems and reassessing animal movement, site-to-site risk, and state-level exposure.
Industry Implications
SHIC’s updated analysis underscores several key actions for producers:
• Maintain heightened biosecurity as L1C continues to spread.
• Reassess transportation and cross-site movement to reduce exposure risks.
• Review herd-level intervention strategies, including potential vaccination options.
• Monitor circulating strains closely to align herd health decisions with current risk.
The latest PRRS trends provide producers and veterinarians with a clearer view of what’s circulating industry-wide, helping guide proactive steps to protect herds and limit virus incursions as winter approaches.
Swine Web will continue to track PRRS activity and report on emerging disease trends affecting North American production.
