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An epidemiological and serological study of Rickettsia in Western Australia

Abdad, Mohammad (2011) An epidemiological and serological study of Rickettsia in Western Australia. PhD thesis, Murdoch University.

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The study was aimed at investigating Western Australian rickettsiae, delving deeper into the epidemiology of a recently described rickettsia, Rickettsia gravesii, and any other rickettsiae lurking in the Western Australian bush. Prior to the discovery of R. gravesii, only Rickettsia typhi was known to be endemic to Western Australia. With the addition of R. gravesii and another novel rickettsia named candidatus “Rickettsia antechini”, understanding and investigation of the potential effects these organisms may have on human and animal health becomes paramount. This research adds to the limited information available in the literature pertaining to Australian rickettsial organisms.

Serotyping of R. gravesii in mice demonstrated distinct specificity differences between R. gravesii, Rickettsia massiliae, Rickettsia australis and Rickettsia honei. R. massiliae was chosen due to R. gravesii’s very close genotypic similarities to the R. massiliae sub-group. R. australis and R. honei were chosen for the possibility of them sharing a similar geographical distribution with R. gravesii.

Collection of ectoparasites, in particular ticks, from humans and larger mammals in south-west Western Australian bush was performed to investigate the prevalence of R. gravesii in tick populations and also to investigate any other rickettsial organisms that may be present. No other rickettsiae were uncovered except for R. gravesii among ixodid ticks collected. Four tick species collected during the course of the study harboured R. gravesii with prevalences of up to 15% in Amblyomma albolimbatum (n=45), 75% in Amblyomma triguttatum (n=187), 51% in Ixodes australiensis (n=63) and 25% in Ixodes fecialis (n=8). No rickettsial DNA was detected in Haemaphysalis
longicornis, though only a single specimen was found. Transovarial transmission of R. gravesii in A. triguttatum ticks from infected females to viable larvae was observed by PCR. Detection of R. gravesii in the salivary glands, ovaries and gut suggests the potential of horizontal, transovarial and transtadial transmission.

Feral pigs (n=148) trapped within the water-catchment areas for Perth were tested for SFG rickettsiae and a prevalence of 49% was observed by microimmunofluorescence at titres of 1:128 and higher. A baseline control group made up of domestic farmed pigs (n=67) was used to determine the cut-off titre (1:128) where no non-specific antigen-antibody detectable by microscopy was observed.

Sero-prevalence in humans was also investigated. Volunteers that are exposed to ticks as a result of occupational and recreational activities were recruited over a 3 year period. Sera were collected and a questionnaire asking about participants’ bush activities was filled out. Two occupational groups were involved in the study, workers from Barrow Island (n=67) and Whiteman Park (n=12). The recreational group was made up of members of the Western Australian Rogaining Association (n=104). The control group used in the study was made up of volunteers among the staff and students of Murdoch University (n=60). Rickettsial prevalence was observed at higher levels in the occupational groups than recreational group participants; 44.8% in Barrow Island workers, 50% in Whiteman Park staff, 23.4% in rogainers and 1.7% in the control group.

Analysis of the questionnaires filled out by participants from the human sero-prevalence study demonstrated a significant association between bush activity and exposure to rickettsial organisms and their vectors. A follow-up of volunteers 10-14 months after the first serum collection from Whiteman Park staff (n=8) and among the rogainers (n=62) showed no significant rates of seroconversion or seroreversion.

Odds ratio analysis (adjusted for age and gender) based on questionnaire results and sero-prevalence of rickettsial antibodies in the different interest groups and control cohort showed that the recreational group had an odds ratio of 13.70 (95% CI=1.73-108.49). Odds ratio for the occupational group is 38.81 (95% CI=5.02-300.34). This shows a significant risk for people performing activities in the bush for extended periods of time, with occupational activities posing the highest risk, potentially due to the longer hours compared to recreational activities in general.

The presence of R. gravesii in Western Australia and the data reported in this study underscores the lack of information available to public health authorities with regards to rickettsial organisms and their hosts. Even though no official reports of debilitating spotted fever infections have been reported in WA in the literature thus far, the potential of it happening cannot be disregarded. Increased awareness of rickettsial organisms and infection need to be instilled among public health officials, doctors and veterinarians. Other animal populations and parts of Western Australia not covered in this study need to be further investigated.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Biomedical Sciences
Supervisor(s): Fenwick, Stan, Berry, Cassandra and Robertson, Ian
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