PCV2 a b c - 1 2 3: Taxonomic Conundrum

There are three broad groups of porcine circovirus 2 (PCV2), which differ in their ability to cause PMWS. In its June 2008 newsletter, the EU PCVD Consortium explains its new scientific definition of the genotypes, which was needed to standardise nomenclature.
calendar icon 30 June 2008
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Scientific literature on PCV2 molecular biology and phylogeny has systematically recognized at least two phylogenetic groups. Initial studies had indicated a lack of association of these groups with disease status or geographic area. A p-distance/frequency histogram of 235 PCV2 ORF2 sequences was clearly bimodal (see figure).

ORF2 PCV2 Sequences

A study on PCVDs in Sweden in 2005 suggested that a particular genogroup (SG3) was usually associated with PMWS occurrence, while others (SG1 and SG2) were usually not. In Canada, a restriction fragment length polymorphism (RFLP) pattern (321-like) of PCV2 was associated with severe disease outbreaks, while the RFLP pattern 422-like was linked to sporadic occurrence of PCVD.

Further studies on potential pathogenicity differences among PCV2 isolates by the EU Consortium on PCVDs in Denmark and Spain also support such differences. Research group have been using their own nomenclature for the different phylogenetic groups, sometimes designated as genotypes.

The EU Consortium has developed a scientific definition for PCV2 genotypes in order to standardize nomenclature. The methodology is based on the proportion of nucleotide sites at which two sequences being compared are different. It is obtained by dividing the number of nucleotide differences by the total number of nucleotides compared. Using this system, three genotypes can be defined based on sequences in the GenBank database.

The first two were already known and coincide with previously defined phylogenetic groups. To standardise PCV2 genotype nomenclature, the EU Consortium proposes naming those genotypes as PCV2a, PCV2b and PCV2c, following the existing proposal from Canada. The first sequences recorded in GenBank will be regarded as the prototype viruses - for PCV2a Accession Number AF055392 (Stoon-1010); and for PCV2b Accession Number AF055394 (48285). The PCV2c prototype virus, GenBank Accession Number EU148503, is one detected in Denmark during the 1980s, when PMWS was not present or at least was undetected. Genotypes PCV2a and PCV2b have both been isolated from PMWS cases and both are pathogenic under controlled experimental conditions.

October 2008

Further Reading

- Find out more information on PMWS by clicking here.
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