The pathogenesis of PCV2 infection and the major cell types that support PCV2 replication are poorly understood. It has been suggested that PCV2 initially replicates in the tonsil. Allan et al. (2000b) reported that in porcine parvovirus (PPV) and PCV2 coinfected pigs, PPV antigen predominates in the tissues of the pigs killed between 3 and 14 DPI with a maximum observed between 6 and 14 DPI. PCV2 antigen was first observed in minimal amounts in mesenteric lymph nodes at 10 DPI with increasing density and distribution of PCV2 antigen at 14, 17, 21, and 26 DPI (Allan et al., 2000b).
Large numbers of PCV2 antigen or nucleic acids are often detected in the cytoplasm of macrophages and dendritic cells by IHC or ISH (Allan and Ellis, 2000; Sorden, 2000). A recent study found that antigen presenting cells in general, and not only macrophages stained positive by IHC for PCV2 antigen (Chianini et al., 2003). In contrast, PCV2 antigen in lymphocytes was only sporadically detected. In thymus, PCV2 was only detected in few histiocytic cells in the medulla suggesting that thymocytes and T cells might be more resistant to PCV2 infection (Chianini et al., 2003).To a lesser extent, PCV2 antigen is also found in epithelial cells in lungs and kidneys, in smooth muscle cells, and in endothelial cells in several tissues in pigs experimentally-infected with PCV2 (Kennedy et al., 2000) as well as in pigs with naturally occurring PCV2-associated PMWS (McNeilly et al., 1999; Rosell et al., 1999). Kennedy et al (2000) demonstrated PCV2-antigen in infiltrating macrophages in the tunica albuginea, in interstitial macrophages and in germinal epithelial cells in the testes, and in infiltrating macrophages in the epidiymides of boars 24 to 29 days after they had been coinfected with PCV2 and PPV at 3 days of age. PCV2 was also found in the parenchyma of the secondary sex glands in a naturally infected boar (Opriessnig et al., 2006). PCV2 targets mainly cardiomyocytes, hepatocytes, and macrophages during fetal life, and mainly monocytes in early post-natal life (Sanchez et al., 2003).
Allan GM, Ellis JA. Porcine circoviruses: a review. J Vet Diagn Invest. 12:3-14, 2000.
Allan GM, McNeilly F, Meehan BM, Ellis JA, Connor TJ, McNair I, Krakowka S, Kennedy S. A sequential study of experimental infection of pigs with porcine circovirus and porcine parvovirus: immunostaining of cryostat sections and virus isolation. J Vet Med B Infect Dis Vet Public Health. 47:81-94, 2000.
Chianini F, Majo N, Segales J, Dominguez J, Domingo M. Immunohistochemical characterization of PCV2 associate lesions in lymphoid and non-lymphoid tissues of pigs with natural postweaning multisystemic wasting syndrome (PMWS). Vet Immunol Immunopathol. 94:63-75, 2003.
Kennedy S, Moffett D, McNeilly F, Meehan B, Ellis J, Krakowka S, Allan GM. Reproduction of lesions of postweaning multisystemic wasting syndrome by infection of conventional pigs with porcine circovirus type 2 alone or in combination with porcine parvovirus. J Comp Path. 122:9-24, 2000.
McNeilly F, Kennedy S, Moffett D, Meehan BM, Foster JC, Clarke EG, Ellis JA, Haines DM, Adair BM, Allan GM. A comparison of in situ hybridization and immunohistochemistry for the detection of a new porcine circovirus in formalin-fixed tissues from pigs with post-weaning multisystemic wasting syndrome (PMWS). J Virol Methods. 80:123-128, 1999.
Opriessnig T, Kuster C, Halbur PG. Demonstration of porcine circovirus type 2 in the testes and accessory sex glands of a boar. J Swine Health Prod. 14:42-45, 2006.
Rosell C, Segalés J, Plana-Durán J, Balasch M, Rodríguez-Arrioja GM, Kennedy S, Allan GM, McNeilly F, Latimer KS, Domingo M. Pathological, immunohistochemical, and in-situ-hybridization studies of natural cases of postweaning multisystemic wasting syndrome (PMWS) in pigs. J Comp Pathol. 120:59-78, 1999.
Sanchez RE Jr, Meerts P, Nauwynck HJ, Pensaert MB. Change of porcine circovirus 2 target cells in pigs during development from fetal to early postnatal life. Vet Microbiol. 95:15-25, 2003.
Sorden SD. Update on porcine circovirus and postweaning multisystemic wasting syndrome (PMWS). Swine Health Prod. 8:133-136, 2000.