Retrospective Study Reveals Decades of PCV4 Circulation in Spain.

Porcine circovirus 4 (PCV4) was first identified in China in 2019 and retrospectively traced back to 2008. However, unlike other circoviruses, its distribution appeared to be largely confined to Asian countries until recent reports from Spain and the United States. This study aims to enhance knowledge of the past and present circulation of PCV4 in non-Asian countries, particularly in Spain and in wild boars, where it was previously detected at a significant prevalence. By genetically characterizing the strains, the contextualization within both national and international epidemiological frameworks was attempted. A total of 302 lymph node samples were tested, with 62 testing positives by quantitative real-time polymerase chain reaction (qPCR), predominantly from the 2011 to 2015 period, although five positives were detected in the 2022-2024. Complete open reading frame 2 (ORF2) sequences were obtained from 10 strains. Phylogenetic analysis revealed two major clusters: one comprising only Chinese sequences and another containing strains from multiple continents, including Spain and the United States. The Spanish strains formed a distinct monophyletic clade whose introduction in the country was estimated through phylodynamic analyses around 2000, suggesting long-term undetected circulation. Within Spain, a progressive geographical spread and strain exchange between wild boars and outdoor reared domestic pigs could be inferred. On the other hand, the lack of detection in intensively raised pigs, at least in Europe, remains unclear. The present findings extend the current knowledge of PCV4 history outside Asia and challenge the paradigm of a recent international spreading. Recognizing the current uncertainties in PCV4's international distribution and epidemiology, further efforts are needed, including the use of archived samples in diagnostic activities as well as the sharing of negative results.