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Genetic variation in Giardia duodenalis and the molecular epidemiology of giardiasis

Meloni, Bruno P. (1992) Genetic variation in Giardia duodenalis and the molecular epidemiology of giardiasis. PhD thesis, Murdoch University.

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In this study the nature and extent of genetic variation in Giardia was investigated to obtain fundamental knowledge on its mode of reproduction, population structure and zoonotic potential, all of which are important in order to fully understand the epidemiology of giardiasis and the taxonomy of the causative agent. Previous studies have contributed little information to these areas due to the comparison of relatively few isolates. Therefore, the major aims of my work were to characterise isolates of Giardia in order to investigate genetic variation and its significance to the epidemiology of giardiasis. Isolates of Giardia. from throughout Australia and overseas, were obtained from humans, cats, cattle, sheep, dogs, goat, beaver and rats and examined using one or more of the following techniques: in vitro cultivation; restriction fragment length polymorphism (RFLP) analysis; enzyme electrophoresis; and amplification of repetitive DNA sequences using the polymerase chain reaction (PCR).

In vitro cultivation of Giardia distinguished intraspecific variants based on observed differences in growth between isolates. Human, cat, goat and sheep isolates of Giardia could be established in vitro while isolates of Giardia from dogs and rats failed to establish. Differences in growth behaviour were also observed between isolates of Giardia from the same host species, namely humans and cats. The observed differences in growth during in vitro cultivation in isolates of Giardia from the same and different host species was usually reflected in genetic differences detected by enzyme electrophoresis.

Enzyme electrophoresis and RFLP analysis produced very similar groupings of isolates of Giardia with identical banding patterns for all zymodemes and schizodemes, including isolates from restricted geographical areas of Western Australia. The significant correlation between the two techniques suggests that the genetic groups identified by enzyme and DNA analyses consist of natural clones of genetically identical organisms and that Giardia has a clonal population structure. This was supported by the finding of widely distributed common genotypes identified by enzyme electrophoresis throughout Australia and overseas.

Electrophoresis banding patterns were of two distinct types. Most isolates produced single-handed enzyme patterns for all the enzymes examined while other isolates produced multiple-banded patterns for a number of enzymes. The observed patterns were interpreted to suggest: (i) that different isolates vary in their ploidy levels with some isolates being functionally haploid and others diploid (or polyploid); and (ii) the possibility of occasional genetic exchange in Giardia. Attempts were made to induce genetic exchange by mixing isolates in vitro and in vivo, but no recombinant genotypes were detected.

Enzyme electrophoretic characterisation of isolates of Giardia from different animals and geographical areas, revealed extensive genetic variation with the identification of 47 different zymodemes. The 47 zymodemes could be divided into three main groups: one comprising isolates from humans and a sheep from Western Australia; a second group containing isolates with a worldwide geographical distribution from humans, cats, dogs, sheep, cattle and beaver; and a third group comprising genetically diverse isolates from humans, rats, cat, dog and goat from Australia and overseas. Isolates of Giardia from Aboriginal communities, from within a restricted geographical area, showed a level of genetic diversity similar to that found for isolates characterised from throughout Western Australia. Humans and dogs from these communities were commonly infected with Giardia: PCR analysis of cysts produced similar DNA banding patterns for a number of human and canine faecal samples, which is suggestive of zoonotic transmission.

The results of this study show that isolates of Giardia duodenalis are genetically diverse. The source of this diversity is consistent with a predominantly clonal population structure for Giardia with the possibility of occasional bouts of genetic exchange. Further, the identification of genetically similar isolates of Giardia from humans and other animals using molecular techniques, and the extensive genetic diversity detected in isolates from humans, provides evidence for the zoonotic transmission of this parasite.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary Studies
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Thompson, Andrew and Lymbery, Alan
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