Porcine reproductive and respiratory syndrome virus is a major threat to pig production worldwide. The virus is constantly changing, with new variants appearing every year. Some of these new variants can spread more easily or have a greater impact on your herd’s health and productivity. An example is the emergence of a new PRRSV variant named 1C.5 (previously referred to as L1C 1-4-4, or 1-4-4) in late 2020. In the past, monitoring systems have had a difficulty identifying new variants like this quickly enough to allow for a rapid response, and they often focused on describing epidemics that had already happened.
To get ahead of PRRSV variants that could cause significant problems for the swine industry, the PRRSV Variants Under Monitoring initiative was created. This program works to systematically identify and monitors variants that have a high potential for widespread transmission in the U.S. The VUM initiative uses two key tools: the PRRS-Loom classification tool and the Morrison Swine Health Monitoring Project.
Using data from the MSHMP, PRRS-Loom used a machine-learning algorithm predicts which variants are most likely to grow and spread quickly. These variants are called Variants Under Monitoring. These high-risk variants are then classified into four categories based on how many new farms they have affected in the past six months. Categories 1 and 2 are variants that are circulating at lower levels, while categories 3 and 4 are those more heavily impacting the industry right now (Figure 1).
As of August 2025, 22 out of 176 variants were classified as VUMs. Altogether, they represent 23% of all PRRSV ORF5 sequences submissions from MSHMP participants, which includes sequences from both breeding and growing pig herds of most of the top 40 U.S. swine producers. Six variants were classified as VUMs category 2 or higher. Variants 1C.5.32 (VUM Category 4), 1C.2 (VUM Category 4), and 1C.5 (VUM Category 3) have been detected in over 10 states, while variants 1H.18 (VUM Category 2), 1C.5.37 (VUM Category 2), and 1C.5.36 (VUM Category 2) have been detected in five to six states.
For each VUM, charts that illustrate the number of new affected sites per month are generated. These charts are part of the enhanced monitoring efforts and provide a clearer picture of the current intensity of transmission. Additionally, for any variant classified as a Category 2 or higher, more details, such as the total number of sequences and affected sites, and the states where the variants have been found are available. This detailed information is crucial for understanding the potential impact of a variant on your herd and for making informed decisions. This initiative also gathers known information about on VUMs in Category 2 or higher in one place, making it easier to access what is known about these variants.
This information is shared in near real-time through monthly reports that are publicly available on the MSHMP website. By providing this timely data, our goal is to help you detect new variants early and be better prepared. This proactive approach enhances preparedness, helps guide where to allocate resources, and improves situational awareness for producers, veterinarians, researchers and other key stakeholders in the swine industry.
This article was written by Mariana Kikuti, Kimberly VanderWaal, Cesar Corzo, Igor Paploski and Nakarin Pamornchainavakul of the College of Veterinary Medicine, Department of Veterinary Population Medicine, University of Minnesota for National Hog Farmer.