Abstract
In 2012, a mutant porcine circovirus type 2 (mPCV2) strain was identified in cases of PCV-associated disease (PCVAD) in the USA. The mPCV2 had an additional amino acid, lysine (K), in the capsid at position 234. The objectives of this study were to compare the pathogenicity of mPCV2, PCV2a and PCV2b in pigs using biologically pure infectious virus stocks derived from respective infectious DNA clones, and to investigate the importance of genotype-specific ORF2 and the presence of lysine at position 234 of the capsid. A total of 47, 2-week-old, caesarean-derived, colostrum-deprived (CDCD) pigs were assigned to one of seven groups. At 3 weeks of age, the pigs were experimentally inoculated with saline, PCV2a, PCV2b, mPCV2, PCV2b-234-K (lysine addition in ORF2), chimeric PCV2b-ORF1/mPCV2-ORF2 or reciprocal chimeric mPCV2-ORF1/PCV2b-ORF2. All pigs were necropsied 21 days post-infection (p.i.). Gross lesions were limited to visible icterus and loss of body condition in a portion of the mPCV2 pigs. The amount of PCV2 DNA was significantly higher in pigs inoculated with mPCV2 compared with PCV2b in sera at 7 days p.i. and faecal swabs at 14 days p.i. Based on lymphoid lesions, a higher prevalence of PCVAD was seen in pigs infected with PCV2s containing the additional 234-K (64.3 %) compared with those infected with a PCV2 with the regular 233 bp ORF2 (40 %). Results indicated that all PCV2 isolates were capable of inducing severe lesions and disease in the CDCD pig model, and there was no significant difference in virulence.
Highlights
Porcine circovirus type 2 (PCV2) has been associated with a number of diseases in growing pigs, including systemic disease, respiratory disease, enteric disease, and porcine dermatitis and nephropathy syndrome, collectively known as PCV-associated disease (PCVAD) (Opriessnig et al, 2007)
Differential PCR on serum samples collected at 21 days post-infection (p.i.) indicated the presence of the correct ORF2 sequences in each of the groups except for the chimeric PCV2 group where the ORF2 of both PCV2b and mutant porcine circovirus type 2 (mPCV2) was detected
The increasing use of molecular techniques and mathematical modelling has considerably advanced our knowledge on infection dynamics, the determination of differences in virus virulence still relies on utilization of in vivo models
Summary
Porcine circovirus type 2 (PCV2) has been associated with a number of diseases in growing pigs, including systemic disease, respiratory disease, enteric disease, and porcine dermatitis and nephropathy syndrome, collectively known as PCV-associated disease (PCVAD) (Opriessnig et al, 2007). Several PCV2 genotypes have been recognized and designated with consecutive lower-case letters, i.e. PCV2a, PCV2b, PCV2c, PCV2d and PCV2e (Dupont et al, 2008; Gagnon et al, 2007; Guo et al, 2010; Jantafong et al, 2011). Amongst all PCV2 genotypes, PCV2a was the predominant strain prior to 2000 and PCV2b is currently the main genotype in the global pig population (Segales et al, 2008). Initial introduction of PCV2b in PCV2ainfected herds was commonly associated with severe PCVAD outbreaks (Cheung et al, 2007; Gagnon et al, 2007).
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