Co-localization of porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus in nasopharynx-associated lymphoid tissue in pigs with PCV2 subclinical infection.

Porcine circovirus-associated disease (PCVAD) and porcine respiratory disease complex (PRDC) significantly impact the pig industry worldwide. Porcine circovirus type 2 (PCV2) is the primary pathogen associated with PCVAD, whereas PRDC is caused by a combination of pathogens including porcine reproductive and respiratory syndrome virus (PRRSV) and pathogenic bacteria. Despite the availability of a PCV2 vaccine that effectively controls and prevents PCVAD, subclinical PCV2 infection (PCV2-SI) still occurs under field conditions. Various porcine pathogens enter via the nasal cavity, where nasopharynx-associated lymphoid tissue (NALT) plays an important role in nasal mucosal immunity. This study examined the pathogenesis of PRDC in pigs with PCV2-SI and the distribution of PCV2 and PRRSV in NALT using in-situ hybridization (ISH) analyses in PCV2-SI pigs. All six pigs had suppurative nasopharyngitis and suppurative pneumonia. Lymphoid tissue contained no cytoplasmic inclusions or giant cells. ISH showed positive signals for PCV2 in the NALT and lungs of all six pigs and PRRSV in the NALT of three pigs and lungs of five. PCV2-positive macrophages were primarily identified in the follicles of NALT, whereas PRRSV-positive macrophages were found primarily around crypts. This is the first study to suggest that replication of both viruses in NALT may cause suppression of local immunity and activate important risk factors of PRDC in pigs with PCV2-SI.