Porcine circovirus type 2 (PCV2) is of major economic importance in the pig industry globally. PCV2 has a high rate of evolution among DNA viruses and four distinct genotypes have been identified: PCV2a, PCV2b, PCV2c and PCV2d1,2. This study aims to determine the occurrence of these genotypes in Australia.
Lymph node samples were collected at slaughter from 200 pigs with pleurisy derived from 44 farms and blood samples were collected from 30 live pigs from another farm. Serum and lymph nodes were stored at -20°C until kit-based DNA extraction. PCV2 status of samples was determined in a qPCR assay3. Of the lymph node and serum samples, 85 (42.5%) and 25 (83.3%) were PCV2-positive, respectively. Eleven PCV2-positive DNA samples with high virus load were subjected to conventional PCR to amplify the whole genome of PCV2 (1768 bp), followed by sequencing. The DNA samples were derived from 9 lymph node samples from 6 farms and 2 serum samples from one farm. The sequences were aligned using the Clustal W and phylogenetic analysis was performed using Neighbouring Joining method in the ClwstalW on MEGA7 program, with reference sequences from NCBI and GenBank databases.
The majority (9 out of 11) of the analysed-PCV2 sequences were PCV2a or PCV2b genotypes and clustered with other Australian sequences. Interestingly, the 2 remaining sequences clustered with PCV2d genotype, which has not yet been reported in Australia. In 4 farms each with 2 samples analysed, sequence variation within farm was very low.
This study is in agreement with previous studies that found PCV2a and PCV2b are the predominant genotypes in Australia, but has shown that PCV2d is also present in this country. PCV2d is currently a fast-spreading genotype globally, with reported high virulence. The extent of the distribution of the PCV2d genotype in Australia is unknown. The potential implications of these findings with respect to pathogenicity and vaccine efficacy require further investigation.