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Unlocking archived ticks to investigate a novel putative pathogen

Krige, Anna-Sheree (2017) Unlocking archived ticks to investigate a novel putative pathogen. Honours thesis, Murdoch University.

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Ticks, as vectors of disease-causing bacteria, represent a significant threat to human and animal health. Of particular concern is the genus Borrelia, which contains several recognised tick-borne pathogens of global health importance. In 2015, the microbiomes within modern Australian ticks were profiled using next-generation sequencing (NGS). Complex communities of bacterial species were revealed, including a novel Borrelia sp. that was identified within a tick parasitising an Australian echidna, Tachyglossus aculeatus. This bacterium, named ‘Candidatus Borrelia tachyglossi’, after its association with the echidna tick, Bothriocroton concolor, forms the fourth known Borrelia group. Following this first report of Borrelia in native Australian ticks, additional questions have emerged concerning whether this bacterium resides in other echidna-biting ticks, and if it has a historic presence in Australia.

Around 1,725 tick specimens from 89 registered echidna hosts were collected between 1928 and 2013. Of these, 850 ticks were selected for examination in this study. A total of eight species from three tick genera were morphologically identified. The most common species was B. concolor (89.2%). Interestingly, Amblyomma fimbriatum (22, 2.6%) and Amblyomma triguttatum (19, 2.2%) ticks were recorded for the first time from echidnas. A NGS survey of the bacterial communities within 66 echidna ticks, targeting the V4 region of the 16S rRNA gene, revealed a diverse range of preserved bacteria. Genera of interest included Borrelia (6, 9.1%), Coxiella (19, 28.8%), Ehrlichia (2, 3.0%), Francisella (5, 7.6%), and Rickettsia (23, 34.8%). While NGS of archived echidna tissue biopsies were negative for Borrelia, genus-specific PCR assays amplified the flaB (378 bp) locus in two skin biopsies (2/34; 5.9%) and nine ticks (9/160; 5.6%), with a 99.5-100% similarity to ‘Candidatus B. tachyglossi’ genotypes B and C. Of these positive samples, six ticks and both skin biopsies (6/9, 66.7%; 2/2, 100%) amplified the longer 16S rRNA (1,087 bp) locus, with a 98.2-99.6% similarity to ‘Candidatus B. tachyglossi’ genotype B. Phylogenetic positioning was consistent with previous reports, suggesting ‘Candidatus B. tachyglossi’ formed the fourth Borrelia clade.

This is the first morphological audit of echidna ticks and NGS survey of the archived tick microbiome. This study has confirmed ‘Candidatus B. tachyglossi’ in Australian echidna ticks in the recent past and for the first time, revealed Borrelia in echidnas. However, whether echidnas are a reservoir for this bacterium could not be established. Additional tick transmission analyses and the examination of echidna blood samples are necessary for confirmation.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Oskam, Charlotte
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