A descriptive study on spatial and temporal distributions of genetic clusters of porcine reproductive and respiratory syndrome virus infecting pig sites in Quebec, Canada, between 2010 and 2019.

BACKGROUND: The wide diversity of porcine reproductive and respiratory syndrome virus (PRRSV) strains combined with incomplete heterologous cross-protection complicates the management of the disease at both the herd and the regional levels. The objectives of this study were to describe the spatial and temporal distribution of various PRRSV genetic clusters infecting pig sites in Quebec, Canada, and to compare PRRSV regional diversity of wild-type sequences over the years.

MATERIALS AND METHODS: A retrospective surveillance-based study was conducted on all pig sites which had PRRSV ORF5 sequences from field submissions transferred into the Laboratoire d'épidémiologie et de médecine porcine database from January 1, 2010 to December 31, 2019. A maximum likelihood phylogenetic tree inferred from multiple sequence alignment was used to identify genetic clusters. For each wild-type cluster gathering ≥ 15 sequences, the number of pig sites in which the cluster was detected per administrative region and per year were displayed on bubble charts and the spatiotemporal distribution of pig sites was illustrated using pie chart maps. A molecular analysis of variance was performed to compare PRRSV wild-type sequence diversity according to the administrative region for each year.

RESULTS: A total of 32 wild-type clusters gathering 1653 PRRSV2 sequences from 693 pig sites were described. Each cluster was detected on up to 132 pig sites and 7 administrative regions over the 10-year period. Annually, the mean (min-max) number of wild-type clusters detected in at least one pig site reached 24 (17-29). Some clusters remained localized on a few sites over time whereas others were widespread over the territory during a few or many years. For each year, regional differences were also observed in PRRSV diversity of wild-type sequences.

CONCLUSIONS: The differences observed in both the spatiotemporal distributions of PRRSV clusters and in the regional diversity of wild-type sequences highlight the importance of ongoing provincial surveillance to improve collective PRRS management strategies.