Outcome of experimental porcine circovirus type 1 infections in mid-gestational porcine foetuses

Abstract

Background: Porcine circovirus type 1 (PCV1) has been described as a non-cytopathic contaminant of the PK-15 cell line. Several experimental infections with PCV1 failed to reproduce disease in pigs. Therefore, PCV1 is generally accepted as non-pathogenic to pigs. To our knowledge, nothing is known about the outcome of PCV1 infections in porcine foetuses. This was examined in the present study.

Results: Nine foetuses from three sows were inoculated at 55 days of gestation: three with 10(4.3) TCID(50) of the PCV1 cell culture strain ATCC-CCL33, three with 10(4.3) TCID(50) of the PCV1 field strain 3384 and three with cell culture medium (mock-inoculated). At 21 days post-inoculation, all 6 PCV1-inoculated and all 3 mock-inoculated foetuses had a normal external appearance. Microscopic lesions characterized by severe haemorrhages were observed in the lungs of two foetuses inoculated with CCL33. High PCV1 titres (up to 10(4.7) TCID(50)/g tissue) were found in the lungs of the CCL33-inoculated foetuses. All other organs of the CCL33-inoculated foetuses and all the organs of the 3384-inoculated foetuses were negative (< 10(1.7) TCID(50)/g tissue) by virus titration. PCV1-positive cells (up to 121 cells/10 mm(2) in CCL33-inoculated foetuses and up to 13 cells/10 mm(2) in 3384-inoculated foetuses) were found in the heart, lungs, spleen, liver, thymus and tonsils. PCR and DNA sequencing of Rep recovered CCL33 or 3384 sequences from CCL33- or 3384-inoculated foetuses, respectively.

Conclusions: From this study, it can be concluded that cell culture PCV1 can replicate efficiently and produce pathology in the lungs of porcine foetuses inoculated at 55 days of foetal life.

Figure 1

Different aspects of PCV1-CCL33 replication after inoculation of a 55-day old foetus. a) CCL33-inoculated foetus (S1R1) with a normal external appearance. b) Haematoxylin and eosin staining of the lungs of a CCL33-inoculated foetus (S1R1). Haemorrhages (indicated by a circle) in interlobular regions (magnification 10×). Bar = 200 μm. c) PCV1-positive cells in the lungs of CCL33-inoculated foetus (S1R1). Bar = 100 μm.

Figure 2

Co-localization of PCV1 in the epithelial cells. Co-localization of PCV1 in the epithelial cells of the lungs of a CCL33-inoculated foetus (Bar = 100 μm) and a 3384-inoculated foetus (Bar = 200 μm) collected at 21 days post inoculation (76 days of gestation). Methanol-fixed cryostat sections were incubated (as described in Materials and Methods) with a cytokeratin marker monoclonal antibody AE1/AE3 to stain epithelial cells (green fluorescence). Cells containing PCV1 antigens were localized using biotin-labelled mono-specific porcine anti-PCV1 polyclonal antibodies (red fluorescence).

Figure 3

Co-localization of PCV1 in monocytes (SWC3+ cells). Co-localization of PCV1 in monocytes (SWC3+ cells) of the lungs of a CCL33-inoculated foetus (Bar = 100 μm) and a 3384-inoculated foetus (Bar = 100 μm) collected at 21 days post inoculation (76 days of gestation). Methanol-fixed cryostat sections were incubated (as described in Materials and Methods) with a mAb anti-SWC3 (74.22.15) (green fluorescence). Cells containing PCV1 antigens were localized using biotin-labelled mono-specific porcine anti-PCV1 polyclonal antibodies (red fluorescence).