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.
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.

Infection of pig farms with porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) causes substantial economic losses globally. However, little epidemiological data of PRRSV and PCV2 in the Hong Kong Special Administrative Region (HKSAR) were available. This pilot study aimed to provide baseline information of the prevalences of PPRSV and PCV2 in the HKSAR. A complex survey was conducted from 3 February 2020 to 11 March 2021 on 29 of the 40 pig farms in the HKSAR, with five pigs each from seven age groups (representing key production stages) tested using a real-time PCR. Evidence of presence of PRRSV European strain (PRRSV-1), PRRSV North American strain (PRRSV-2) and PCV2 was confirmed on 48%, 86% and 79% of farms, with overall prevalences of 7.6% (95% CI: 4.8-10.3%), 12.2% (95% CI: 9.6-14.7%) and 20.3% (95% CI: 14.3-26.2%) in the HKSAR pig population based on pooling results from all pigs across all farms. PRRSV-1 and PRRSV-2 were more prevalent in younger pigs, with the highest prevalences of 32.1% (95% CI: 20.8-45.0%) and 51.5% (95% CI: 38.9-64.0%) for 8-week-old pigs. In contrast, the distribution of PCV2 prevalence across age groups appeared to be more symmetrical, with higher prevalences reported in pigs from 12 weeks old to 24 weeks old but lower prevalences in younger pigs and sows. The results of this study demonstrate that PRRSV-1, PRRSV-2 and PCV2 are widely spread across pig farms in the HKSAR, which indicates that the current farm management and control protocols should be improved. We recommend the implementation of on-farm intervention strategies combined with ongoing surveillance to reduce these viruses, and their consequences, in the HKSAR pig population.