This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.
Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) continues to evolve, posing ongoing challenges for swine health management. This report, brought to you by the MSHMP team, provides an update on several monitored PRRSV variants of interest—1H.18, 1C.2, and 1C.5—along with two recently split variants, 1C.5.32 and 1C.5.36. These updates reflect current detection patterns as of early May 2025, geographic distribution, and clustering activity, helping stakeholders stay informed about trends in variant circulation and transmission dynamics.
1H.18 variant:
PRRSV variant 1H.18, commonly associated with RFLP patterns 1-12-2 and 1-8-4, continues to be monitored. Since its emergence in 2018, 198 sequences have been identified across 111 sites from 12 production systems, primarily in Iowa and Minnesota. Most detections have occurred in growing pig herds. No active space-time clusters have been observed, and the low number of recent cases, combined with limited geographic spread, suggests that the transmissibility of this variant remains low.
1C.2 variant:
The 1C.2 variant, typically linked to RFLP pattern 1-2-4, has been associated with prolonged time to stability, averaging 87 weeks (1). Since 2018, 949 sequences have been reported from 383 sites across 23 production systems, mainly in growing pig herds from Iowa, Colorado, and Illinois. A current space-time cluster involving 40 cases across eight systems has been identified in the Midwest.
1C.5 variant:
Formerly referred to as the L1C 1-4-4 variant, over 3,100 sequences have been identified since its emergence. However, the 1C.5 variant has undergone further differentiation due to genetic diversification (2).See more information on this classification update, at https://stemma.shinyapps.io/PRRSLoom-variants/. While 1C.5 viruses continue to circulate, seven new subvariants—1C.5.32 through 1C.5.38—have been designated. Two of these, 1C.5.32 and 1C.5.36, are being actively monitored.
1C.5.32 variant:
First detected in 2023, variant 1C.5.32 gained attention in January 2024 due to signs of clonal expansion (3). Since then, 979 sequences have been identified through MSHMP originating from 562 sites in 21 production systems, primarily in growing pig herds from Iowa and Minnesota. No ongoing space-time clusters have been observed as of second quarter 2025.
1C.5.36 variant:
Since it was first detected in 2022, 143 sequences from 94 sites across 13 production systems have been identified. Most cases are from growing pig herds in Iowa and Minnesota. One active space-time cluster has been identified in the Midwest, involving nine cases across two production systems.
MSHMP will continue to prospectively monitor these and other variants of interest, and additional reports will be issued as the situation unfolds. Do not hesitate to contact MSHMP in case you have questions.
References
1. Kikuti M, Melini CM, Yue X, Paploski IAD, Pamornchainavakul N, Baker JP, et al. Porcine reproductive and respiratory syndrome virus variant emergence: Example and considerations for prospective monitoring. Vet Microbiol [Internet]. 2024 Nov;298:110293. Available from: https://linkinghub.elsevier.com/retrieve/pii/S0378113524003158
2. VanderWaal K. PRRSV-2 Lineage-Variant Nomenclature: March 6, 2025 Update [Internet]. 2025. Available from: https://stemma.shinyapps.io/PRRSLoom-variants/
3. Swine Health Information Center. SHIC Monitoring Emerging PRRSV-2 Lineage 1C.5 Clonally Expanded Clade [Internet]. 2025. Available from: https://www.swinehealth.org/shic-monitoring-emerging-prrsv-2-lineage-1c-5-clonally-expanded-clade/