Abstract
Porcine reproductive and respiratory syndrome (PRRS) virus appeared 20 years ago as the causeof a new disease in swine. Today PRRS is the most significant swine disease worldwide in spiteof intensive immunological interventions. The virus showed remarkable genetic variation withtwo geographically distinct genotypes at the time of its discovery, indicating the possibility ofprolonged evolutionary divergence prior to its appearance as a swine pathogen. Since then, both type 1 and type 2 have spread geographically, radiated genetically, and acquired new phenotypic characteristics, especially increased virulence. Here, we explore various hypotheses that mightaccount for rapid expansion and diversification of PRRSV, including mechanisms specific to PRRSV and other arteriviruses, cellular modification processes, and immunological selection. Phylogenetic analysis of PRRSV has provided a broadly applicable means to relate diverseisolates, but it does not explain biological variation in virulence or immunological cross-protection. We present other methods of classification and review their limitations. Major questions about PRRSV remain unanswered despite intensive investigation, suggesting that theinteraction of PRRSV with pigs involves novel biological processes that may be relevant to other RNA virus and host interactions
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