Phylogenetic Lineages of PRRSV-2 from Canada Reveal Patterns of Transboundary Spread and Two Novel Sub-Lineages in North America.

PRRSV-2 represents a major threat to the swine industry. Canada is one of the world's leading pork producers and a major trading partner of live pigs with the United States, yet PRRSV-2 evolutionary dynamics in these two countries are often studied independently, partly due to limited publicly available sequence data from Canada. We analyzed more than 3000 PRRSV-2 ORF5 sequences collected between 2000 and 2024 from five Canadian provinces. Thirteen previously described sub-lineages were detected in Canada, while approximately one-third of the sequences could not be assigned to any known sub-lineage. Phylogenetic analyses incorporating global reference sequences revealed that most unclassified sequences clustered into four distinct monophyletic clades, exhibiting genetic distances greater than 9.5% from recognized sub-lineages. We propose two new sub-lineages, 1K and 1L, corresponding to clades that were prevalent and persistent over time, whereas the remaining two clades were rare and last detected in 2021. We reconstructed cross-border transmission histories and found that sub-lineages 1C, 1H, 1I, 1K, and 1L originated in Canada, whereas 1A, 1B, 1E, and 1F originated in the United States. Transmission patterns varied across sub-lineages, ranging from unidirectional to bidirectional movement. Our findings refine PRRSV-2 classification and provide insights to inform targeted surveillance, particularly at national borders.