ABSTRACT Porcine circovirus types 2 (PCV2) and 3 (PCV3) are single-stranded circular DNA genome viruses belonging to the genus Circovirus of the family Circoviridae. These viruses are widespread throughout the world and are known to cause a variety of syndromes, collectively referred to as porcine circovirus-associated diseases (PCVADs), which can cause significant morbidity and mortality in pig populations. It is therefore essential to screen the pig population for these viruses. In this study, we investigated the presence of PCV2 and PCV3 in piglets, weaners and adult pigs in three farms in the western part of Portugal. Of the 62 pigs analysed, 12 tested positive for PCV3 (19.4% prevalence), representing the first reported detection of PCV3 in Portugal. Interestingly, all positive cases belong to pigs less than 10 weeks old (piglets and weaners). In the past, PCV2 had a high prevalence in Portugal (44.6% PCV2 PCR-positive samples tested between 2003 and 2010), but after the introduction of vaccination in the country, the prevalence decreased drastically, with only 2.6% prevalence detected in boars between 2018 and 2020. The results of the current study confirmed these findings, as only two of the 62 pigs analyzed tested positive for PCV2 (3.2% prevalence). Phylogenetic analysis was performed using partial nucleotide sequences for both viruses and showed that the circulating PCV3 and PCV2 strains belong to the PCV3c and PCV2d clades, respectively. This latter finding is also consistent with a prior study suggesting the emergence of the PCV2d genotype in the country. A previously undescribed mutation in the PCV3 genome was identified in the samples of this study, which appears to be beneficial for virus replication in pig cells. As mentioned above, to the best of our knowledge, this is the first report of PCV3 in Portugal.
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