Outcome of Porcine Circovirus Type 1 (PCV1) Infections in Mid-Gestational Porcine Foetuses

Summary of paper presented by D. Saha of Ghent University in Belgium at the 6th International Symposium on Emerging and Re-Emerging Pig Diseases in Barcelona, Spain in June 2011.

1 January 2012

5 minute read

Introduction

It is accepted that PCV1 is non-pathogenic to pigs (1,2). However, PCV1 has been isolated from cases of congenital tremors and stillborn piglets (2,3). Until now, nothing is known about the outcome of PCV1 infections in porcine foetuses. In the present study, virus replication and pathology were examined in porcine foetuses, inoculated with two PCV1 strains at 55 days of gestation.

Materials and Methods

Three conventional sows were submitted to laparotomy at 55 days of gestation. Three foetuses of each sow were inoculated: one foetus with PCV1 cell culture strain ATCCCCL33; one with PCV1 field isolate 3384 (2) and one foetus with medium. Inoculations were performed by transuterine injection with 10^4.3TCID₅₀ of PCV1 into the foetal peritoneal and amniotic cavities of the foetuses (4). At twenty-one days post-inoculation (dpi), the sows were euthanised and all foetuses were collected. All foetuses were examined for gross lesions and tissue samples from various organs were taken for histopathology, virus isolation and titration on PK-15 cells and for localization of PCV1 antigens by indirect immunofluorescence staining. DNA was extracted from organs of PCV1-inoculated and adjacent foetuses and PCV1 PCR was performed. In order to verify that the foetuses had been inoculated with the specific PCV1 strain, the full-length genome of PCV1 was amplified using heart and/or lung tissue from infected foetuses followed by sequencing. Abdominal fluids were tested for the presence of PCV1-specific antibodies by IPMA.

Results

All six PCV1-inoculated foetuses had a normal external appearance (Figure 1a). Microscopic lesions include severe haemorrhages in the interlobular regions were observed in the lung tissues of two foetuses inoculated with PCV1 strain, CCL33 (Figure 1b). Microscopic lesions were not present in the other four PCV1-inoculated foetuses. Gross and microscopic lesions were not observed in mock-inoculated and noninoculated foetuses.

Figure 1. Different aspects of PCV1 replication after inoculation of a 55-day old foetus. a) ATCC-CCL33 inoculated foetus without showing any gross pathology. b) Haematoxylin and Eosin staining of lungs of ATCC-CCL33 inoculated foetuses. Haemorrhages (indicated by arrow marks) in interlobular regions. Bar=200μm. c) PCV1-positive cells in the lungs. Bar=100μm.

High PCV1 titres (10^2.9, 10^4.6 and 10^4.7 TCID₅₀/g tissue) were found in the lungs of CCL33-inoculated foetuses. All other organs were negative (<10^1.7 TCID₅₀/g tissue) by virus isolation. All collected organs from 3384-inoculated foetuses were negative by virus isolation. PCV1-positive cells (28 to 121 for CCL33-inoculated and 1 to 13 for 3384-inoculated foetuses per 10mm² tissue) were observed in the lungs of all PCV1-inoculated foetuses (Figure 1c).

In general, heart, lungs, spleen, liver, kidney, thymus, tonsils and ileum of all PCV1-inoculated foetuses were positive by PCR. PCR and DNA sequencing recovered pure CCL33 and pure 3384 sequences from CCL33- and 3384-inoculated foetuses, respectively. No evidence for mixed samples (e.g. containing more than one PCV1 strain) was seen. All adjacent foetuses of PCV1-inoculated foetuses were negative by PCR. All mock-inoculated foetuses and their adjacent foetuses were also negative by PCR. All PCV1- inoculated foetuses had a low anti-PCV1 Ab titre of 10 to 40, except one foetus inoculated with CCL33, which had a titre of 160.

Discussion

In this study, PCV1 strain CCL33 was found to be pathogenic to porcine foetuses inoculated at 55-days of foetal life. Severe haemorrhages were present in the lungs of CCL33 inoculated foetuses (Figure 1b). These lesions were correlated with high titres (up to 10^4.7 TCID₅₀ per gramme of tissue) in the lungs.

This study also shows that the lung tissue is the main target organ of CCL33. On the other hand, the field strain 3384 remained non-pathogenic to porcine foetuses. A high PCV1 load could be essential to induce pathology.

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

Acknowledgements

The authors acknowledge G.M. Allan for providing the strain 3384. The authors also acknowledge C. Boone and I. Vanherpe for their technical assistance.

References

1. Tischer et al. 1986. Arch Virol. 91:271-276.
2. Allan et al. 1995. Vet Microbiol. 44:49-64.
3. Choi et al. 2002. Can J Vet Res. 66:217-224.
4. Saha et al. 2010. Vet Microbiol. 145:62-68.