Detection of Porcine circovirus type 2 viremia and seroconversion in naturally infected pigs in a farrow-to-finish barn

Porcine serum was assayed by 2 polymerase chain reaction (PCR) protocols (nested PCR [nPCR] and non-nested PCR) and a competitive enzyme-linked immunosorbent assay to determine when Porcine circovirus type 2 (PCV2) viremia and a rise in the serum level of PCV2-specific antibody occurred in pigs raised in a large Canadian farrow-to-finish barn. Eight serial blood samples were collected from each of 40 pigs from 5 to 156 (+/- 1.5) d of age; 6 pigs were removed from the study for various reasons at various times. Viremia was not detected in the samples collected before 72 d of age but was detected in those collected on or after 72 d: of 33 pigs, 7 (21%) had only 1 serum sample positive for PCV2 DNA by nPCR after day 72; 11 (33%) were intermittently positive by nPCR, non-nested PCR, or both between 72 and 156 d; and the remaining 15 (45%) were repeatedly positive (in 2 to 4 samples). The level of serum antibody against PCV2 declined after weaning and increased between 72 and 107 d of age, only after PCV2 was detected in serum. Our results show that PCV2 viremia persists in the presence of elevated levels of PCV2-specific antibody.