Caitlin Wood PhD thesis review seminar, "Detection and epidemiology of Coxiella burnetii infection in beef cattle in northern Australia and the potential risk to public health"

Caitlin's advisors are Prof Nigel Perkins, Dr Sarah Tozer, Prof Michael McGowan, Dr Justine Gibson, Dr John Alawneh

Abstract:

Coxiella burnetii is a zoonotic bacterium able to infect multiple species, including but not limited to livestock and humans. It is the causative agent of Q fever in humans and coxiellosis in animals. Knowledge of the true prevalence and distribution of coxiellosis in cattle populations across northern Australia is limited due to minimal surveillance and no standardisation of diagnostic test methods. The overall aim of this PhD project was to improve the understanding of the epidemiology of coxiellosis in beef cattle in northern Australia and gain insights into the potential risk to public health. To explore this aim, four research chapters were executed.

Initially, a focused molecular survey was conducted to detect C. burnetii at an abattoir in South-east Queensland that slaughters cattle from many regions across Queensland and northern New South Wales. We hypothesised that active disease surveillance through an abattoir was likely to be an effective method of 1) estimating the prevalence of C. burnetii infection in a population of beef cattle going to slaughter, and 2) would enable identification of specific genotypes of C. burnetii infecting cattle in this population. Cattle originating from several Queensland regions and northern New South Wales had reproductive tissue and liver tissue collected. These samples were analysed using polymerase chain reaction (PCR) molecular methods. The frequency detected within the positive regions was very low, ranging from 0.5% (95% Confidence Interval (CI) 0.1 – 2.6%) to 1.6% (95% CI 0.4 – 5.6%) and the overall animal prevalence was 0.8% (95% CI 0.3 – 1.6%). C. burnetii DNA was detected, although at a lower than expected frequency, from the liver of cattle originating from Darling Downs South West, Central Queensland and North Queensland regions. There was however, no evidence of C. burnetii infection in any placental tissue or amniotic fluid samples collected from pregnant cattle post-mortem. Samples were not suitable for additional molecular genotyping methods. Results from this thesis chapter indicate that an abattoir surveillance system could be suitable to estimate and monitor the overall level of infection in cattle within infected regions, however it may not be an ideal method for quick and effective identification of C. burnetii strains infecting cattle in Australia.

The second research chapter aimed to develop and validate an indirect immunofluorescence assay (IFA) for the detection of phase specific IgG antibodies against C. burnetii in cattle serum for the purpose of estimating prevalence of infection to facilitate epidemiological investigations. The test optimisation process and determination of cut-off values were described, and the repeatability and reliability of the assay assessed. Direct comparison of the developed IFA with a commercial enzyme-linked immunosorbent assay (ELISA) was accomplished by testing serum samples from distinct cattle populations across the east coast of Australia and New Zealand. Bayesian latent class analysis was used to estimate the diagnostic test accuracy (sensitivity and specificity) of the serological tests in the absence of a gold standard reference test. It was found that the estimated diagnostic sensitivity (DSe) of the IFA (at the 1/160 cut-off) was 73.7% (95% Credible Interval (CrI) 64.0 - 82.0). This was lower than the ELISA (87.9%; 95% CrI, 73.6 - 96.5) in detecting IgG phase I and/or phase II C. burnetii in bovine serum; however, diagnostic specificity (DSp) was slightly higher for the IFA (98.2%; 95% CrI 0.951 - 0.997) compared to the ELISA (97.8%; 95% CrI 93.3 - 99.7). This study identified that although the IFA was less sensitive than the ELISA, the assay had very good specificity and the ability to identify phase specific serological patterns. This assay may be appropriate for large seroprevalence studies, where the ELISA could be prohibitively expensive, to facilitate epidemiological studies and assess dynamics of infection.

The third research chapter was designed with two main objectives: firstly to investigate the prevalence and distribution of coxiellosis in a sample of extensive beef breeding cattle from northern Australia, and secondly to analyse the spatial distribution of retrospective human Q fever case notifications from Queensland. To fulfil the first objective, a total of 2012 sera samples and 1570 vaginal swab samples, previously collected from cattle on 60 properties, were testing using serological and molecular methods. Molecular detection of C. burnetii DNA from vaginal swabs was only identified in 1 of the 60 properties. Overall 53.3% of the 60 properties showed at least one seropositive sample for C. burnetii IgG. The animal level sero-positivity varied significantly between cattle from the Northern Territory and Queensland. A Bayesian hierarchical latent class model (LCM) was then developed to estimate the true prevalence of exposure to C. burnetii using the IFA serological test results. The LCM incorporated the hierarchical data structure by included random effects for property and region. The DSe and DSp of the IFA test were also accounted for in the model. Within-herd true prevalence (TPwithin) and regional level true prevalence (TPregion) estimates were estimated for 5 regions of northern Australia. Results identified that cattle in this study originating from Northern, Central and Southern Queensland have very similar TPregion ( 7.43% (95% Credible Interval (CrI) 3.75, 13.92); 7.78% (95% CrI 3.91, 15.23); 7.20 (95% CrI 2.57, 16.65) however cattle from the Northern Territory and South-east Queensland regions showed much lower TPregion ( 0.36% 95% CrI 0.00, 2.17; 0.63% 95% CrI 0.00, 4.53). Cattle in this study from the Northern Territory have statistically significantly lower TPregion of C. burnetii exposure than cattle from 3/4 regions of Queensland with the exception of South-east Queensland. The South-east region of Queensland has the lowest regional estimate for the state, however the CrIs still overlap with TPregion of Northern, Central and Southern regions. Basic GIS mapping techniques were used to visualise the distribution of results at the property and regional levels.

The final research chapter of this thesis has utilised a large dataset investigating causes of reduced reproductive performance in beef cattle across northern Australia to investigate if C. burnetii exposure has a relationship to cattle reproductive indices. Two analysis were performed, firstly the outcome variable defined as “pregnant within four months of previous calving” (P4M), then secondly with the outcome variable defined as “annual pregnancy rate”.

Overall, this thesis project has explored several aspects of coxiellosis in beef cattle in Australia in order to improve our ability to detect exposure and infection and increase understanding of the impact from an animal health perspective and with relevant potential public health aspects.

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