An investigation into the status of porcine circovirus in Australia
Raye, Warren (2004) An investigation into the status of porcine circovirus in Australia. PhD thesis, Murdoch University.
This thesis reports for the first time the detection of porcine circovirus virus (PCV) in the Australian pig herd.
PCV DNA was detected in the tissues of pigs from several Australian states using a multiplex polymerase chain reaction (PCR) assay, the primers for which were based on the sequence of PCV1 and PCV2 strains detected in North America and Europe. PCV type 1 or 2 was detected in 80 of 367 (21.7%) pigs tested. In the 80 positives, both PCV1 and PCV2 were detected in 14 samples. Virus was detected in pigs from all states from which samples were obtained: Western Australia, South Australia, New South Wales and Queensland.
The complete genomes of 13 strains of Australian PCV were sequenced. Analysis of the data indicated there was extremely high homology between the Australian strains of PCV1 and PCV2 and previously published sequences of PCV1 and PCV2 strains from North America and Europe.There were no consistent differences between the genome of the Australian strains and strains in North America and Europe.
The widespread occurrence of PCV in the tissues of pigs was confirmed by a small scale serological study of the Western Australian pig herd using an immunofluorescence assay, which did not discriminate antibody to PCV1 and PCV2. This assay detected PCV antibody in 11 of 14 pig herds in Western Australia, with a prevalence rate in positive herds varying from 25 to 47%, but it was unable to differentiate antibody to PCV1 and PCV2.
A PCV2-specific recombinant viral capsid protein was produced in insect cells with a baculovirus expression system and this was used to develop a PCV2-specific ELISA and a Western immunoblotting assay. These assays were applied to samples from a national pig serum bank and detected PCV2 antibody in 33% of 3933 serum samples. The highest seroprevalence to the recombinant PCV2 capsid antigen was detected in the samples from Victoria where there was a 51.3% seroprevalence rate, and the lowest in Western Australia where there was an 11.4% seroprevalence rate.
An in situ hybridisation (ISH) technique was developed for the detection of PCV in tissues of infected pigs and infected cell cultures. A monoclonal antibody specific for the capsid protein of PCV2 was also produced and has application for the development of immunocytochemical procedures for the detection of PCV2 in tissues and cell cultures.
The high prevalence of PCV in the Australian pig herd and the absence of reports of PMWS suggested that the Australian strains of PMWS detected may have been of low virulence. To examine the pathogenicity of Australian strains, two animal experiments were conducted where the type species of PCV1 present in persistently-infected PK15 pig kidney cells and an Australian PCV2 strain were cultured in vitro in cell cultures and inoculated into weaner pigs. As expected, the PCV1 replicated well in pigs but did not result in the induction of clinical signs or lesions in the inoculated pigs. The inoculation into weaner pigs of cell culture replicated PCV2 with an apparent virus titre of 103 virus particles/mL resulted in infection of only some of the inoculated pigs and it was concluded that the PCV2 inoculum contained insufficient virus to infect all pigs into which it was inoculated. The PCV2 did not induce any disease syndrome and could not be visualised in tissue sections of infected pigs using immunohistochemical techniques.
In conclusion, techniques were developed for the detection of PCV in the Australian pig herd. PCV of both genetic types were detected at prevalence rates similar to those reported in other countries where PMWS has occurred, and the widespread occurrence of PCV was confirmed by serological assays. The PCV strains present were genetically indistinguishable from those present in North America and Europe. The reason for the absence of PMWS in Australia is most likely not due to differences in the characteristics of the PCV strains present.