BackgroundPorcine circovirus type 2 (PCV2) is considered to be the primary causative agent of postweaning multisystemic wasting syndrome (PMWS), which has become a serious economic problem for the swine industry worldwide. The major genotypes, PCV2a and PCV2b, are highly prevalent in the pig population and are present worldwide. However, another newly emerging PCV2b genotype mutant, which has a mutation in its ORF2-encoded capsid protein, has been sporadically present in China, as well as in other countries. It is therefore important to determine the relative virulence of the newly emerging PCV2b genotype mutant, compared with the existing PCV2a and PCV2b genotypes, and to investigate whether the newly emerging mutant virus induces more severe illness.Methodology/Principal FindingsTwenty healthy, 30-day-old, commercial piglets served as controls or were challenged with PCV2a, PCV2b and the newly emerging mutant virus. A series of indexes representing different parameters were adopted to evaluate virulence, including clinical signs, serological detection, viral load and distribution, changes in immune cell subsets in the peripheral blood, and evaluation of pathological lesions. The newly emerging PCV2 mutant demonstrated more severe signs compatible with PMWS, characterized by wasting, coughing, dyspnea, diarrhea, rough hair-coat and depression. Moreover, the pathological lesions and viremia, as well as the viral loads in lymph nodes, tonsils and spleen, were significantly more severe (P<0.05) for piglets challenged with the newly emerging mutant compared with those in the groups challenged with PCV2a and PCV2b. In addition, a significantly lower average daily weight gain (P<0.05) was recorded in the group challenged with the newly emerging PCV2 mutant than in the groups challenged with the prevailing PCV2a and PCV2b.ConclusionsThis is believed to be the first report to confirm the enhanced virulence of the newly emerging PCV2 mutant in vivo.

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A total of 450 samples from domestic pigs in China were tested for porcine parvoviruses (PPVs) and co-infections with porcine circovirus type 2 (PCV2), and their complete capsid genes were sequenced. The prevalence of PPV1, PPV2, PPV3, PPV4, and PCV2 was 5.56 %, 39.56 %, 45.11 %, 21.56 %, and 47.33 %, respectively, and co-infection with PCV2 occurred in 4 % (PPV1), 22.44 % (PPV2), 24 % (PPV3), and 12 % (PPV4) of the samples. Phylogenetic analysis revealed two main lineages for each virus, and residues that differentiated these viruses were identified. The co-infections of emerging PPVs and PCV2 were prevalent, indicating their cooperative roles in porcine circovirus-associated diseases.