PRRSV Variants Under Monitoring
About the project
Variants of PRRSV continue to emerge, causing issues for the industry. To support proactive decision-making, we developed a national surveillance report to identify and communicate variants for which there are indicators of wider spread. These are Variants Under Monitoring (VUMs). Our approach employs PRRS-Loom variant classifications for ORF5 sequences alongside an algorithm that predicts the future expansion of variants over the next 12 months (i.e., variants that are anticipated to increase by >20% based on sequence counts). These predictions are combined with near-real-time national data from the Morrison Swine Health Monitoring Project (MSHMP) to track the number of currently impacted sites. Key benefits of this project include:
- Proactive monitoring of currently circulating variants.
- Improved situational awareness among stakeholders.
- Promotes faster, more coordinated responses to emerging threats.
What are PRRSV variants and VUMs
PRRSV variants are groups of viruses that are closely related based on their ORF5 gene. Viruses belonging to the same variant classification are typically less than 2.5% different and 5% different from the closest related variant (click here to find more about PRRSV variants). Variants under monitoring (VUMs) are PRRSV-2 variants currently circulating in the U.S. that were predicted to show potential for widespread transmission based on genetic and epidemiological data. This algorithm was developed using retrospective data and uses key early indicators to predict an increase in variant sequencing within the next year, with 77% accuracy (Pamornchainavakul et al., 2024). Sites affected by each VUM are identified through MSHMP by linking genetic sequences to their corresponding Premises ID. VUMs are classified into four categories based on the number of new sites affected over the past six months. Variants that do not show sequence-based potential for widespread transmission are not variants
under monitoring (not a VUM).
Monthly Summary of Variants Under Monitoring (VUMs)
As of November 2025, the following number of VUMs were identified in each category:
Three PRRSV variants remain classified as Variants Under Monitoring (VUMs) category 2 or higher and are described in this month's report: Variants 1C.5.32 (VUM Category 4), 1C.2 (VUM Category 3), and 1C.5 (VUM Category 3). No new variant-specific situation reports were created. Previous reports for all variants ever classified as VUM Category 2 or higher remain available.
Past VUM monthly reports
Variant-specific Situation Reports
General disclaimers
The information presented in this report is based on data shared voluntarily by MSHMP-participating systems and diagnostic laboratories. While efforts have been made to ensure accuracy and timeliness, findings may be subject to reporting delays, variability in sampling strategies, and limitations inherent to diagnostic testing and sequencing. VUM categories are based on MSHMP data, which, while broadly representative of the U.S. swine industry, are not comprehensive. Interpretation of diagnostic laboratory data for pathogen detection should be made with caution, as surveillance efforts differ between breeding and growing sites, with greater sampling effort typically focused on breeding herds. Designations such as Variants Under Monitoring (VUMs) and their assigned categories are intended to support situational awareness and facilitate informed discussions among veterinarians, producers, researchers, and other stakeholders. They do not constitute definitive assessments of risk at the farm level and should not replace site-specific diagnostics, expert consultation, or context-specific decision-making.
Project Team and Institutional Affiliations
This report was developed by a collaborative team of researchers at the University of Minnesota, including Dr. Mariana Kikuti, Dr. Cesar Corzo, Dr. Kimberly VanderWaal, Dr. Igor Paploski, and Dr. Nakarin Pamornchainavakul. It builds upon two key initiatives: the Morrison Swine Health Monitoring Project (MSHMP), which provides ongoing monitoring data across U.S. swine production systems, and PRRS-Loom, a platform for PRRSV-2 variant classification and early trend detection.
Suggested citation
National Surveillance Report. PRRSV Variants Under Monitoring, August 2025 [Internet]. 2025. Available from: https://mshmp.umn.edu/PRRS-variant-monitoring
References and related sources
- Morrison Swine Health Monitoring Project
- PRRSLoom-Variants
- VanderWaal K, Pamornchainavakul N, Kikuti M, Zhang J, Zeller M, Trevisan G, Rossow S, Schwartz M, Linhares DCL, Holtkamp DJ, da Silva JPH, Corzo CA, Baker JP, Anderson TK, Makau DN, Paploski IAD. PRRSV-2 variant classification: a dynamic nomenclature for enhanced monitoring and surveillance. mSphere 0:e00709-24. https://doi.org/10.1128/msphere.00709-24
- Pamornchainavakul N, Kikuti M, Paploski IAD, Corzo CA, Vanderwaal K. Predicting Potential PRRSV-2 Variant Emergence through Phylogenetic Inference. Transboundary And Emerging Diseases, v. 2024, p. 1-15, 2024. https://doi.org/10.1155/2024/7945955
- VanderWaal K, Pamornchainavakul N, Kikuti M, Linhares DCL, Trevisan G, Zhang J, Anderson TK, Zeller M, Rossow S, Holtkamp DJ, Makau DN, Corzo CA, Paploski IAD. Phylogenetic-based methods for fine-scale classification of PRRSV-2 ORF5 sequences: a comparison of their robustness and reproducibility. Frontiers in Virology, v. 4, p. 1, 2024. https://doi.org/10.3389/fviro.2024.1433931