Muhling, Jill (2006) Australian porcine circoviruses. PhD thesis, Murdoch University.
Two types of porcine circovirus (PCV) exist, referred to as PCV1 and PCV2. PCV2 has been associated with disease syndromes in pigs, including that designated postweaning multisystemic wasting syndrome (PMWS), which has been identified in all regions of the world bar Australia (Hamel et al., 1998; Allan et al., 1999a; Onuki et al., 1999; Martelli et al., 2000; Kyriakis et al., 2000; Wellenberg et al., 2000; Done et al., 2001; Trujano et al., 2001; Saradell et al., 2004; Castro et al., 2004; Jemersic et al., 2004; Maldonado et al., 2004; Wang et al., 2004; Motovski and Segales, 2004; Garkavenko et al., 2005). PMWS affects young weaner pigs and results in weight loss, tachypnea, dyspnea, enlarged lymph nodes and jaundice (Harding, 1998). PCV2 may also cause or contribute to other swine diseases such as congential tremors (CT) (Stevenson et al., 1999), porcine dermatitis and nephropathy syndrome (PDNS) (Rosell et al., 2000), reproductive failure (Meehan et al., 2001) and several other emerging disease syndromes. PCV1 is currently considered to be non-pathogenic. Although PMWS has not been reported in Australia, information on the distribution, variation and further characterisation of PCV in Australian pigs was necessary as it might provide insights into why there is no PCV-associated disease in this country. The results reported in this thesis involved the detection and further study of porcine circovirus in Australia.
This chapter provides an outline of this thesis and the work undertaken, while Chapter 2 is a review of the relevant literature with particular reference to circoviral diseases. Chapter 3 describes the detection of both PCV1 and PCV2 in the Australian pig herd, using a multiplex PCR designed to differentiate between the two viral types. The association of Australian PCV with two disease outbreaks was also investigated. Following the detection of both viruses, it was important to genetically compare Australian PCV with overseas strains known to cause disease, and this was achieved with a sequencing and phylogenetic study as described in Chapter 4. Possible reasons for the genetic groupings and distribution of different PCV2 strains worldwide are also discussed in this chapter.
As PMWS is as yet unidentified in Australian pigs, the importation of pig meat into Australia from countries with the disease requires careful monitoring. Current protocols for the cooking of imported pig meat were designed to inactivate porcine reproductive and respiratory disease virus (PRRSV), and as such may not be effective against PCV. In this study (Chapter 5), Australian PCV2 was successfully infected into cell culture, and detected using a variety of techniques. Subsequently, thermal stability experiments were performed using a newly-developed immunoperoxidase (IPMA) test. It was anticipated that this study would determine whether current importation protocols require revision, and the results would suggest that this is the case, with PCV2 unaffected by treatment comparable with current cooking protocols.
While no animal experiments were undertaken in this study, it may become necessary to infect pigs with Australian PCV to determine viral pathogenicity. Cell culture inoculums have been used in the past overseas, but problems with contamination and viral titre have been encountered (Fenaux et al., 2001). Viral infectious clones can be used to overcome these problems, so an infectious clone of Australian PCV2 was constructed, as described in Chapter 6. While time constraints prevented the clone from being infected into culture, it is anticipated that the construct would be infectious as it is based on a previously published method (Hattermann et al., 2004). Chapter 7 is a general discussion of the results and conclusions from this study.
The detection and characterisation of Australian PCV as described in this study has provided further information on the status of PCV in the Australian pig herd, and also developed diagnostic tests to assist in future research. These tools will be important when assessing and managing the risk of Australia experiencing PCV-associated diseases.