Porcine reproductive and respiratory syndrome virus variant emergence: Example and considerations for prospective monitoring.

New PRRSV variants are constantly emerging due to the rapid evolution of this virus. We aimed to describe the emergence of a new PRRSV variant within sub-lineage 1 C, its space-time distribution, and its impact on affected herds. Additionally, we discuss considerations on how to monitor emerging PRRSV variants. This newly emerging variant was first detected in June 2022 on a sow herd undergoing a mild PRRS outbreak. Cases were defined by ORF5 nucleotide identity of ≥98 % between samples using the first detected case as a seed. A total of 382 case sequences were identified in sixteen production systems. Although most sequences originated from breeding sites (58.4 %) compared to grow-finishing sites (33.3 %), they corresponded to 118 individual sites (73 grow-finishing, 37 breeding, and 8 with no farm type information). Two spatial-temporal clusters in the Midwest were detected, but only when system was not accounted for. 63.6 % (21/33) of breeding herds reached stability in a median of 87 weeks (57 weeks in herds in which only the studied variant was detected, and 91 weeks when multiple PRRSV variants were involved). The average mortality in growing pig sites affected by this variant was not statistically different from the one found in L1C1-4-4 variant-affected sites. Altogether, these results pinpoint this as a variant of interest for continued surveillance due to increased time to stability than previously reported in the literature. Prospective monitoring of emerging variants should acknowledge the complex relationship between data limitations and multi-variant outbreaks, amongst other factors.