Comparative study of two Australian Coxiella species: Coxiella burnetii and nov. sp. Coxiella cheraxi (TO-98)
Powell, Sarah E. (2013) Comparative study of two Australian Coxiella species: Coxiella burnetii and nov. sp. Coxiella cheraxi (TO-98). Masters (Research) thesis, James Cook University.
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Abstract
In current literature, the genus Coxiella is monospecific with Coxiella burnetii as the only described member. Coxiella burnetii is the causative agent of Q-fever, a virulent disease found worldwide spread through inhalation, tick bites and occasionally through the ingestion of contaminated milk products (Center for Disease Control, 2010). Recent research suggests that there are emergent pathogenic members of Coxiella to be described. In 1990, heavy losses in the Australian redclaw crayfish industry led to the discovery of a potential disease pathogen closely related to Coxiella causing up to 80% mortality in crayfish. The previously unrecognized disease was described histologically, revealing an aetiological organism similar to Rickettsia known to cause disease (Owens et al. 1992; Ketterer et al. 1992). An additional genetic study via 16S rRNA analysis revealed that this new pathogen had a 95.6% similarity to Coxiella burnetii, effectively indicating the organisms are of the same genus (Tan and Owens 2000; Cooper et al. 2007). The bacterium was designated Coxiella cheraxi by Tan and Owens (2000) and due to its phylogenetic similarity to Coxiella we are proposing further research to understand it and its infection potential.
This study seeks to compare Coxiella burnetii and Coxiella cheraxi with respect to infectivity, pathogenicity, histopathology and subsequent implications for biosecurity and public health. This was achieved in two parts, the first of which illustrates a novel method and experiment whereby the first known aquatic invertebrate (C. quadricarinatus) was infected with three doses of C. burnetii, a Level 3 pathogen. Twenty crayfish in four replicate dose groups (control, 10⁴, 10⁵, 10⁶ cells/ml) of five animals each were infected and monitored for twenty one days and mortality, histopathology, PCR, qPCR and sequencing assessments were made post-trial. The laboratory design and ensuing experiment were the first of their kind and were successful in illustrating a novel, risk-averse laboratory design for monitoring and maintaining aquatic invertebrates within a Physical Containment Level 3 (PC3/BSL-3) facility. This design is a potential model for future experiments seeking alternatives to vertebrate hosts. The subsequent infection experiment successfully demonstrated the initial host potential of C. quadricarinatus for C. burnetii, with results indicating that crayfish may be less susceptible to C. burnetii exposure and act only as carriers of the pathogen in doses as low as 1 to 2 organisms. These results differed significantly from observed Coxiella cheraxi infections within Australian redclaw which are known to cause high mortality and morbidity.
The second part of this thesis explores the clinical manifestation and origin of a 2012 presumptive C. cheraxi outbreak in Australian redclaw crayfish at the Marine and Aquaculture Research Facilities Unit (MARFU) at James Cook University in Townsville, Australia. Cytoplasmic inclusion bodies, strong Gram negative bacteraemia and heavy mortalities observed in initial screening histopathology were strongly suggestive of previously observed C. cheraxi incidents (Owens et al. 1992, Ketterer et al. 1992). Resultant 16S rRNA sequencing indicated an 86% identity match to C. cheraxi and no other known pathogens thus tentatively suggesting the re-emergence of this pathogen within redclaw stock at MARFU and elsewhere. The work goes on to outline the consequent biosecurity response and diagnostic steps taken, and the successful tracking of the outbreak origin to one of three supplying commercial crayfish farms in northern Queensland. Finally the thesis illustrates the implications of the comparisons between C. cheraxi and C. burnetii as they relate to the fields of aquaculture, biosecurity and public health.
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