From PCV1 to "PCV5": Genetic Diversity, Evolutionary Dynamics, and Host Adaptation of Porcine Circoviruses

Porcine circoviruses (PCVs) are among the smallest animal viruses that replicate autonomously. What began in 1974 as a single cell culture contaminant, PCV1, now counts five members, PCV1 to PCV5. The newest of these, PCV5, was reported in 2025. Its genome is larger than those of the other four (2904 nt), and it bears almost no sequence resemblance to any swine-origin circovirus described so far, with only 26.2% amino acid identity for Rep and 20.8% for Cap. Instead, it clusters with fur seal feces-associated circular DNA virus, a lineage that has been proposed for reassignment to the genus Macochavirus. In this review we set the genomes of PCV1 to PCV5 side by side, together with the domain organization of their Rep and Cap proteins and their phylogenetic relationships, and we then turn to evolutionary rates, genetic diversity, host adaptation, and cross-species transmission, read against host immune pressure, intensive swine production, and the global live-pig trade. Our reading of the evidence is that these viruses do not share one ancestor. PCV1 and PCV2 descend from a single lineage long adapted to the genus Sus scrofa; PCV3 reflects an independent spillover event; the origin of PCV4 is still unresolved; and PCV5 clusters with a fur seal feces-associated circular Rep-encoding single-stranded (CRESS) DNA virus, a phylogenetic affinity consistent with-but at this stage only a testable hypothesis of-an origin at the marine-terrestrial interface. If that is correct, the PCVs are a series of independent spillovers that happened to converge on the same host species, and we argue on that basis for proactive surveillance at the human-livestock-wildlife-environment interface.