Porcine circovirus update
Josepha DeLay, Jim Fairles, Davor Ojkic
The significance of porcine circovirus 2 (PCV-2) to the swine industry is recognized worldwide. Vaccination strategies have successfully controlled PCV-2-associated disease (PCVAD) in many herds. However the emergence and evolution of new genotypes of the virus raise concern about the level of protection offered by current PCV-2 vaccines as genotype prevalence shifts within the North American swine population. Currently, 5 PCV-2 genotypes have been recognized (PCV-2a, PCV-2b, PCV-2c, PCV-2d, and the newly proposed PCV-2e). PCV-2a, PCV-2b, and PCV-2d are associated with clinical disease, and PCV-2d is further classified as PCV-2d-1 and PCV-2d-2. PCV-2c is rarely identified and may be of limited or no clinical significance. PCV-2e was recently identified, and the clinical significance of this genotype is currently undetermined.
To evaluate the prevalence of PCV-2 subtypes identified in Ontario herds, gene sequence analysis was completed for all PCV-2 PCR-positive samples received at the AHL between June 2015 and October 2016 (Table 1). PCV-2a and PCV-2b were the predominant genotypes among these samples, and were present in approximately equal frequency. PCV-2d was detected in a single sample. This is in contrast to a recent report in which a change from PCV-2b to PCV-2d-2 predominance was identified among US swine. Most commercial vaccines use PCV-2a, however cross-protection against PCV-2b and PCV-2d has been demonstrated in experimental infections. Despite this, monitoring of PCV-2 genotype dynamics within swine populations is important for anticipating genotype shifts that may occur.
A novel circovirus, designated PCV-3, was recently identified by metagenomic sequencing from 2 separate case series in the US. Clinical syndromes among these cases included sow death with lesions compatible with porcine dermatopathy-nephropathy syndrome (PDNS) and concurrent abortion; and myocarditis, myocardial arteriolitis, and systemic inflammation in nursing and weaned pigs. PCV-3 antigen or nucleic acid was detected in association with lesions in these pigs. The exact role of PCV-3 regarding these disease syndromes is unknown, and Koch’s postulates remain to be fulfilled. Routine testing is not currently available for PCV-3. AHL
Table 1. PCV genotypes detected among PCV2 PCR-positive cases at the AHL, June 2015-October 2016.