At the moment, two main PRRSV genotypes are recognized, the European (EU) and the North American (NA) genotypes. Sequence comparison has shown significant genetic differences between them.
In North America only the NA type is found, although there has been a report of the isolation of a European-like genotype (Ropp at al. 2004).
In Europe, the EU type predominates. To date no field strains of the NA type have been isolated in Western Europe. In those countries where the NA type is present it was most likely introduced in 1996 through the use of a live attenuated PRRSV vaccine (Madsen et al, 1998; Oleksiewicz et al, 1998).
In Asia and South America both genotypes have been isolated.
Within each genotype some differences are seen. Examples of the differences between the various strains are shown in Table 1 and Figure 1.
|Strain||Country of origin||Percentage homology|
Figure 1 shows a phylogenetic tree constructed from the data in Table 1. It is obvious that there are two serotypes, the North American serotype (with the American strain VR2332, the Canadian strain Quebec 807/94 and the Taiwan strain) and the European serotype (with the Dutch Isolate 10 and the Spanish strain Olot).
Figure 1. Phylogenetic tree from 5 PRRSV strains from various countries
Some more recent data offers us the following picture (T. Harder). This analysis shows that the ORF 5 homology between the recent European field strains and 2 vaccine strains is:
ORF 5 homology between the recent European field strains and 2 vaccine strains