Characterisation of community-derived Hymenolepis infections in Australia

Marion G. Macnish

Hymenolepis nana is a ubiquitous parasite, found throughout many developing and developed countries. Globally, the prevalence of H. nana is alarmingly high, with estimates of up to 75 million people infected. In Australia, the rates of infection have increased substantially in the last decade, from less than 20% in the early 1990's to 55 - 60% in these same communities today. Our knowledge of the epidemiology of infection of H. nana is hampered by the confusion surrounding the host specificity and taxonomy of this parasite. The suggestion of the existence of two separate species, Hymenolepis nana von Siebold 1852 and Hymenolepis fraterna Stiles 1906, was first proposed at the beginning of the 20th century. Despite ongoing discussions in the subsequent years it remained unclear, some 90 years later, whether there were two distinct species, that are highly host specific, or whether they were simply the same species present in both rodent and human hosts. The ongoing controversy surrounding the taxonomy of H. nana has not yet been resolved and remains a point of difference between the taxonomic and medical literature. The epidemiology of infection with H. nana in Australian communities is not well understood as the species present in these communities has never been identified with certainty. It is not clear which form of transmission commonly occurs in Australia, whether the H. nana 'strain/species' present in the north-west of Western Australia is present in human and rodent hosts, or whether humans harbour their own 'strain/sub-species' of Hymenolepis. Furthermore, it is not known whether mice are a potential zoonotic source for transmission of Hymenolepis to human hosts. In this study, 51 human isolates of H. nana were inoculated into highly susceptible laboratory rodent species. However, these failed to develop into adult worms in all instances, including when rodent species were chemically and genetically immunosuppressed. In addition, 24 of these human isolates were also cross-tested in the flour beetle intermediate host, Tribolium confusum. Of these, only one isolate developed to the cysticercoid stage in beetles, yet when inoculated into laboratory rodents, the cysticercoids also failed to develop into adult stage. Since isolates of H. nana infecting humans and rodents are morphologically indistinguishable, the only way they can be reliably identified is by comparing the parasite in each host using molecular criteria. In the current study, three regions of ribosomal DNA, the small subunit (18S), the first internal transcribed spacer (ITS1) and the intergenic spacer (IGS) were chosen for genetic characterisation of Hymenolepis spp. from rodent and human hosts from a broad geographic range. In addition, a mitochondrial gene, the cytochrome c oxidase subunit 1 (C01) gene and a non-ribosomal nuclear gene, paramyosin, were characterised in a number of Hymenolepis isolates from different hosts. A small PCR fragment of 369 bp, plus a larger fragment of 1223 bp, were sequenced from the 18S gene of reference isolates of H. nana and the rat tapeworm H. diminuta. Minimal sequence variation was found in the two regions of the 18S between these two morphologically distinct, phylogenetically recognised species, H. nana and H. diminuta, and this indicated that the 18S gene was too conserved for further genetic characterisation of isolates of H. nana from different hosts. A large number of human isolates of H. nana (104) were characterised at the ITS1 using PCRrestriction length fragment polymorphism (PCR-RFLP). The profiles obtained were highly variable and often exceeded the original size of the uncut fragment. This was highly suggestive of the existence of ribosomal spacers that, whilst identical in length, were highly variable in sequence. To overcome the problems of the variable PCR-RFLP profiles, further characterisation of the ITS1, by cloning and sequencing 23 isolates of H. nana, was conducted and this confirmed the existence of spacers which, although similar in length (approximately 646 bp), differed in their primary sequences. The sequence differences led to the separation of the isolates into two clusters when analysed phylogenetically. This sequence variation was not, however, related to the host of origin of the isolate, thus was not a marker of genetic distinction between H. nana from rodents and humans. Indeed, the levels of variability were often higher within an individual isolate than between isolates, regardless of whether they were collected from human or mice hosts, which was problematic for phylogenetic analysis. In addition, mixed parasite infections of H. nana and the rodent tapeworm H. microstoma were identified in four humans in this study, which was unexpected and surprising, as there have been no previous reports in the literature documenting humans as definitive hosts for this parasite. Further studies are required, however, to determine if the detection of H. microstoma in humans reflects a genuine, patent infection or an atypical, accidental occurrence. Sequencing of the mitochondrial cytochrome c oxidase 1 gene (C01) in a number of isolates of Hymenolepis nana from rodents and humans identified a phylogenetically supported genetic divergence of approximately 5% between some mouse isolates compared to isolates of H. nana from humans. This provided evidence that the mitochondrial C01 gene was useful for identifying genetic divergences in H. nana that were not resolvable using nuclear loci. Despite a morphological identity between isolates of H. nana from rodent and human hosts, the genetic divergence observed between isolates at the mitochondrial locus was highly suggestive that H. nana is a species complex, or 'cryptic' species (= morphologically identical yet genetically distinct). In addition, whilst not supported by high bootstrap values, a clustering of the Australian human isolates into one uniform genetic group that was phylogenetically separated from all the mouse isolates was well supported by biological data obtained in this study. To confirm the phylogeny of the C01 tree a small segment of the nuclear gene, paramyosin, was sequenced in a number of isolates from humans and rodents. However, this gene did not provide the level of heterogeneity required to distinguish between isolates from rodent and human hosts. The high sequence conservation of the paramyosin gene characterised in this study did not refute the finding that H. nana may be a cryptic species that is becoming host adapted. It simply did not provide additional data to that already obtained. A DNA fingerprinting tool, PCR-RFLP, of the ribosomal intergenic spacer (IGS), was developed in this study in order to evaluate its usefulness in tracing particular genotypes within a community, thus determining transmission patterns of H. nana between rodent and human hosts. Analysis of the IGS of numerous H. nana isolates by PCR-RFLP identified the presence of copies of the IGS that, whilst similar in length, differed in their sequence. Similar to that observed in the ITS1, the existence of different IGS copies was found in both rodent and human isolates of H. nana, thus the variability was not evidence of the existence of a rodent- or humanspecific genotype. Evaluation of the intergenic spacer (IGS) as a fingerprinting tool suggests that this region of DNA is too variable within individuals and thus, cannot be effectively used for the study of transmission patterns of the tapeworm H. nana between different hosts. In summary, it appears that the life cycle of H. nana that exists in remote communities in the north-west of Western Australia is likely to involve mainly 'human to human' transmission. This is supported by both the biological and genetic data obtained for the mitochondrial locus in this study. The role of the intermediate hosts, such as Tribolium spp., in the Hymenolepis life cycle is still unclear, however it would appear that it may be greatly reduced in the transmission of this parasite in remote Australian communities. In the future, it is recommended that further genetic characterisation of faster evolving mitochondrial genes, and/or suitable nuclear genes be characterised in a larger number of isolates of H. nana. The use of techniques which can combine the characterisation of genotype and phenotype, such as proteomics, may also be highly valuable for studies on H. nana from different hosts.

Characterisation of community-derived Hymenolepis infections in Australia

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