Porcine reproductive and respiratory syndrome virus continues to generate massive losses in the U.S. industry. One of the challenges that producers and veterinarians have faced is distinguishing new introductions from older viruses on their farms. To address this challenge, PRRSV classification has changed significantly over the last decade. The industry continues to gradually move away from RFLPs (i.e, 1-4-4, 1-8-4, 1-7-4), which are unreliable indicators of genetic relationships, and has instead classified PRRS into groups known as lineages and sub-lineages.
This is a positive development, as lineages and sub-lineages consistently group viruses that are more closely related to each other; however, these lineage and sub-lineage groups can still be quite large. To address this, a variant nomenclature for PRRSV, which further divides sub-lineages into smaller groups that are more closely related based on their genetic information. Every PRRSV sequence can be assigned to a variant, although this classification tends to be less reliable for sequences collected before 2015. Currently, around 200 PRRSV variants have been described, with roughly 30 circulating in U.S. swine populations today. This is still a lot and may be still overwhelming for practitioners!
Historically the U.S. industry has relied on the number of sequences to monitor epidemiological trends. The issue with this approach is that the number of sequences generated from an outbreak depends on how often the attending veterinarian chooses to collect and sequence samples. In some outbreaks, more than 40 sequences from the same site may be generated, while in others only a single sequence is obtained. This creates an artifact in which the number of sequences within a lineage, sub-lineage, or variant does not necessarily reflect how widespread or important that group of viruses truly is.
To address both the challenge of identifying which variants are most relevant, increase situational awareness about which variants that are rapidly spreading and address the limitations of relying solely on sequence counts, the University of Minnesota has leveraged a unique feature that the Morrison Swine Health Monitoring Project has, where almost every sequence in its database can be linked to the farm where that sequence was found. Combined with the UMN’s work on forecasting which variants are likely to increase in occurrence, obtained from PRRS-Loom, UMN’s Swine Group is publishing reports quantifying how many farms have been infected recently by variants that are predicted to expand. These variants are referred to as “Variants Under Monitoring.”
Information on Variants Under Monitoring is updated monthly and can be found on the University of Minnesota’s VUM website . While the system monitors a large number of variants, the report includes only variants predicted to be rapidly expanding, and they are classified into categories of increasing importance, similar to how hurricanes are classified in the U.S. (i.e., category 4 is spreading more rapidly than category 2). This analysis is based on how many new farms were infected in the past six months. This honed-in view of PRRSV occurrence provides a valuable resource for practitioners and industry stakeholders by offering a clearer picture of which variants deserve the most attention. In addition, the University of Minnesota team behind the VUM report also summarizes current knowledge on production impact and disease severity when such data are available for monitored variants.
In short, while our understanding of PRRSV-2 continues to improve, the virus remains complex and constantly evolving. Classification systems such as lineages, sub-lineages and variants help organize that diversity, but they can still feel overwhelming when many variants are circulating at once. The VUM approach helps diminish that complexity by focusing on what matters most in the field: how many farms are affected and which variants appear to be spreading rapidly.
By combining near real-time farm-level data from the Morrison Swine Health Monitoring Project with predictive analytics from PRRS-Loom, the VUM report highlights which variants currently deserve the most attention. For producers and veterinarians, this provides a clearer and more practical snapshot of the PRRSV situation, helping guide awareness, monitoring and decision-making across the industry.
This article was written by Igor Paploski, Kimberly VanderWaal, Cesar Corzo, Mark Schwartz and Mariana Kikuti of the University of Minnesota for the National Hog Farmer.