Since the beginning, two PRRS virus genotypes were established: European or Type 1 (prototype Lelystad virus) and North American or Type 2 (prototype VR-2332).
Interestingly, similarity between both prototype nucleotide sequences is as low as 55%.
In fact, they have been recently considered as two different viral species: PRRSV1 and PRRSV2 (International Committee on Taxonomy of Viruses (ICTV).
More importantly, intraspecific pairwise nucleotide sequence variation exceeds 20% (up to 30% in PRRSV1 and around 21% in PRRSV2).
A significant quantity of subtypes or clades has been identified in both species.
Within PRRSV1, there are at least 4 different subtypes; subtype I is predominant in Western Europe, whereas subtypes II-IV are isolated only in countries eastwards of Poland.
It seems that genetic variability within subtypes II-IV is higher than within subtype I (18.2% vs. 11.9% using ORF5, respectively).
Regarding PRRSV2, although subtypes have not been clearly defined, several lineages exist in two well-defined clades; seven lineages made up by North American isolates and two lineages exclusively consisting of isolates from South East Asia.
It has been described that the allocation of genetic diversity is not constant along the PRRS virus genome. Thus, ORF1a, ORF3 and ORF4 seems to be more diverse.
It is important to note that the phylogenetic analyses aimed to determine subtypes and lineages have generally been based on ORF5, which represents a low percentage of the viral genome. It cannot be ruled out that this classification may change in the future when considering the whole genome.
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