Differences in virulence among porcine circovirus type 2 isolates are unrelated to cluster type 2a or 2b and prior infection provides heterologous protection

Abstract

Porcine circovirus type 2 (PCV2) is divided into two genetic clusters designated PCV2a and PCV2b. The objectives of this study were to determine whether isolates from different clusters vary in virulence and to determine whether infection with PCV2a isolates induces protective immunity against subsequent infection with a recent PCV2b isolate. One-hundred and thirteen conventional specific-pathogen-free (SPF) pigs were assigned randomly to treatment groups and rooms: pigs inoculated with PCV2a cluster isolates (ISU-40895 or ISU-4838), pigs inoculated with PCV2b cluster isolates (NC-16845 or Can-17639) and uninoculated pigs. Necropsies were performed at 16 or 51 days post-inoculation (p.i.). There were no significant differences in PCV2-associated lymphoid lesions between PCV2a and PCV2b clusters; however, within the same cluster, significant differences were found between isolates: ISU-4838-and Can-17639-inoculated pigs had significantly (P,0.05) less severe lesions compared with ISU-40895-and NC-16845-inoculated pigs. To evaluate cross-protection, six pigs within each group were challenged at 35 days p.i. with an isolate from the heterologous cluster and were necropsied 51 days p.i. The severity of PCV2-associated lesions was reduced in pigs with prior exposure to an isolate from the heterologous cluster in comparison with singly inoculated pigs. Results indicate that the virulence of PCV2a and PCV2b isolates is not different in the conventional SPF pig model; however, the virulence of isolates within the same cluster differs. Increased virulence as reported to be associated with PCV2b isolates in the field was not observed under the conditions of this study. Moreover, cross-protection between PCV2a and PCV2b exists. INTRODUCTIONPorcine circovirus (PCV) is a small, circular, nonenveloped, single-stranded DNA virus (Tischer et al., 1982) that belongs to the genus Circovirus of the family Circoviridae (Todd et al., 2005). To date, two types of PCV have been recognized (Allan et al., 1998;Hamel et al., 1998;Morozov et al., 1998) in pigs: the non-pathogenic PCV type 1 (PCV1) and the pathogenic PCV type 2 (PCV2), which is the aetiological agent of porcine circovirusassociated disease (PCVAD). Previously, phylogenetic analyses have shown that PCV2 isolates can be further divided into two main clusters (Larochelle et al., 2002;Mankertz et al., 2000;Olvera et al., 2007) now commonly referred to as PCV2a and PCV2b (Gagnon et al., 2007).Systemic PCV2 infection, which is also known as postweaning multisystemic wasting syndrome (PMWS), is characterized clinically by wasting or decreased weight gain, enlarged lymph nodes and dyspnoea (Harding & Clark, 1997;Opriessnig et al., 2007). The hallmark microscopic lesions of systemic PCV2 infection are lymphoid depletion and granulomatous lymphadenitis associated with the presence of PCV2 antigen or nucleic acids (Sorden, 2000). Systemic PCV2 infection or PMWS was initially observed in a Canadian high-health-statusThe GenBank/EMBL/DDBJ accession numbers for the complete genomic sequ...

Differential Effects of Porcine Circovirus Type 2 (PCV2) Vaccination on PCV2 Genotypes at Japanese Pig Farms

“…It was recently reported that the virulence of PCV2a and PCV2b did not differ in specific-pathogen-free (SPF) pigs [22]. However, we found that PCV2b can raise the mortality rate on conventional commercial farms infected with various pathogens such as PRRSV and M. hyopneumoniae.…”

Singular PCV2a or PCV2b infection results in apoptosis of hepatocytes in clinically affected gnotobiotic pigs

“…While differences in virulence between PCV2 isolates exist (Opriessnig et al, 2006), this appears not to be confined to genotype (Opriessnig et al, 2008). To our knowledge, this is the first study comparing apoptosis in pigs infected with PCV2a or PCV2b.…”

Immune Responses of Piglets Immunized by a Recombinant Plasmid Containing Porcine Circovirus Type 2 and Porcine Interleukin-18 Genes

“…The current commercial vaccines are based on PCV2a genotype. Cross-protection between PCV2a and PCV2b genotypes is further supported by the efficacy of PCV2a-based vaccines under field conditions (5,24,27). However, PCV2-associated diseases (PCVAD) outbreaks in vaccinated herds do occur (25).…”