PCV2d continues to increase in the US swine population

PCV2a is the longest known PCV2 genotype and the current commercial vaccines contain PCV2a. In recent years PCV2b has been the main strain identified in field cases in the US. Since 2012, PCV2d has been associated with apparent PCV2 vaccine failures in the US. In a large study investigating PCV2 sequences from 2012-2013 in the US, 37% of all 143 sequences investigated were classified as PCV2d whereas PCV2a comprised 12.6% of the sequences and PCV2b was identified in 52.4% of all sequences. The objective of the present study was to determine the PCV2d prevalence in US pigs during 2014 and 2015.

A total of 215 ORF2 sequences were obtained through the Iowa State University VDL LIMS system and corresponded to client requests for further characterization of the PCV2 PCR positive samples. In addition, lung tissues were obtained from 578 randomly selected cases submitted to the Iowa State University Veterinary Diagnostic Laboratory from January through March 2015. The DNA of these tissues was extracted and subjected to PCV2 ORF1 real-time PCR. All ORF1 PCV2 PCR positive samples were further tested by a differential real-time PCR based on ORF2 and capable of differentiating PCV2a, PCV2b and PCV2d. Selected PCV2 positive samples were also sequenced.

Among the ISU-VDL sequences, 20.7% (44/213) corresponded to PCV2a, 9.4% (20/213) corresponded to PCV2b, 68.1% (145/213) corresponded to PCV2d and 1.9% (4/213) corresponded to the newly identified genotype PCV2e. Among all 587 lung samples, PCV2 DNA was detected in 22.7% (133/587). Specifically, PCV2a was detected in 10.4% (14/133), PCV2b was detected in 27.8% (37/133) and PCV2d was detected in 70.7% (94/133) of the lung tissue samples. Eighty-five of the 133 samples were sequenced and the sequencing results agreed with the PCR results. When VDL sequence data and lung tissue data were combined, there were a total of 358 PCV2 positive samples and 16.2% (58/358) were PCV2a, 15.9% (15.9/358) were PCV2b, 66.8% (239/358) were PCV2d and 1.1% (4/358) were PCV2e.

Overall the results indicate that the incidence of PCV2d has continued to increase from the first observation period spanning 2012-2013 (37%) to the second observation period spanning 2014-2015 (66.8%) and that PCV2d appears to replace PCV2b. This further highlights that PCV2d may have some advantage over PCV2a and PCV2b in its ability to replicate in pigs. Perhaps this also highlights the need for updated PCV2 vaccines.

Disclosure of Interest: None Declared.