PCV2 Genetic Selection Seems to Eventually Favour “The Underdog” from Greg Wideman from South West Ontario Veterinary Services

Porcine circovirus 2 (PCV2) emerged as a  clinical issue presenting as Post-Weaning Multisystemic Wasting Syndrome (PMWS) in some isolated regions during the 1990’s. Although most DNA viruses have a reputation of being more genetically stable than their RNA cousins the PCV2 virus exhibits a relatively high evolutionary potential due to both high mutation and recombination rates. Consequently, different genotypes including PCV2a , b, c, d, e, f, g, h, i  have emerged over time. It is now thought that PCV2a has been kicking around since the early 20th century. PCV2a was eventually displaced as the dominant genotype by PCV2b around 2000 to 2003. Finally PCV2d became the predominant genotype starting around 2010 to 2014 depending on region. While PCV2d was initially considered a PCV2b-mutant  it appears that it is more probable that PCV2d originated along similar timelines as PCV2b but it’s peak at a later time is speculated to be linked to the widespread application of PCV2a-based vaccines. Although PCV2d is currently the most common genotype the other genotypes continue to circulate.

These European researchers wanted to look at the epidemiological and evolutionary patterns of PCV2 genotypes using phylodynamic analyses. Some 8,283 sequences from around the globe were included in the final dataset and this constitutes an almost 10-fold increase in ORF2 sequence availability when compared to previous epidemiological  studies. Because the sequences were categorized by region, the phylogeographic analysis was able to shed light on viral dispersal patterns and frequency. Selective pressures acting on the capsid protein (immune recognition) were estimated and compared among genotypes. PCV2 “clades” from different continents were interspersed throughout the phylogenetic tree and this indicated that frequent strain exchanges occurred between different regions of the world and that after establishment of a new strain within a region the future virus evolution was independent within that new location. Some very small clades did end up as “epidemiological dead ends”.