Divergence time of porcine reproductive and respiratory syndrome virus subtypes.

Porcine reproductive and respiratory syndrome virus (PRRSV) recently emerged in domestic pigs of Western Europe and North America. Although time of emergence was identical on the two continents, genetic composition was markedly different with a clear geographical subtype structure, indicating that subtypes diverged in separate reservoirs prior to emergence. Genetic analyses have shown that the most recent common ancestor (MRCA) of Western European isolates existed around 1980 and that these originate from Eastern European pigs. These findings are challenged by a study of Hanada et al. who place the MRCA of all PRRSV isolates around 1980 and find that no significant subtype divergence occurred before emergence. Here, I discuss problems of information content, methodology, and biological plausibility associated with this study. Using alternative methodology, I reanalyze the existing data and conclude that the MRCA of all PRRSV isolates existed around 1880, 100 years before the date estimated by Hanada et al.