Veterinary Science Collected Works - Theses
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ItemCoxiellosis on Commercial Dairy Goat Farms in Australia: Prevalence, Risks Factors, Risk Assessments, and Surveillance( 2022-12)Coxiella burnetii is the causative agent of coxiellosis in animals and Q fever in humans. Multiple animal species can acquire coxiellosis. Being highly infectious and resilient, C. burnetii is a threat to both animals and humans. Clinical signs of coxiellosis rarely occur in animals except for reproductive disorders such as abortion, stillbirths, weak offspring, reduced milk yields and mastitis. Infected domestic small ruminants can excrete C. burnetii from their milk, urine, faeces and birthing products, therefore being a crucial source of human infections. Once excreted outside the host animal, C. burnetii takes its small cell variant (SCV) form, which can withstand high temperatures and disinfectants, and travel long distances as airborne particles. In 2012, the largest Australian farm-related Q fever outbreak was reported in an intensive dairy goat farm in Victoria. This thesis aims to improve the understanding of C. burnetii status among commercial dairy goat farms in Australia and attempt to establish a framework of a program to minimise the possibility of C. burnetii infection among commercial dairy goat farms. This aim was achieved by a series of studies on the prevalence of C. burnetii among commercial dairy goat farms, risk perceptions among commercial dairy goat farmers and an evaluation of different surveillance methods. A cross-sectional study (Chapter 2) was conducted to quantify the prevalence of C. burnetii infections among commercial dairy goat farmers in Australia and identify risk factors associated with farm positivity. The apparent herd prevalence was 10% (95% CI: 4, 22) and the true herd prevalence estimated to be 3% (95% CI: 0, 18). Samples from herds with >900 milking goats were 6.75 (95% CI: 1.65, 27.7) times more likely to return a C. burnetii positive result compared with farms with no less than 900 milking does. Farms with an increased dairy goat density had higher odds of BTM sample positivity, increasing by a factor of 2.53 (95% CI: 1.51, 4.22) for each order of magnitude increase in the number of goats per acre. In the following chapter (Chapter 3), a study was conducted to identify the pattern of C. burnetii Com1 PCR results in bulk tank milk (BTM) samples as well as production factors that may affect testing results. This longitudinal BTM test study found that Com1 PCR tests fluctuate in positivity. C. burnetii DNA concentration in BTM was associated with the season, farm and fat concentration of the BTM sample. These findings are important for informed decisions when making BTM surveillance plans for C. burnetii infection in dairy goat herds. Based on the findings from Chapter 3, risk perceptions of farmers from test-negative farms for C. burnetii introduction into their herds were investigated and comprehensive risk assessments were undertaken (Chapter 4). Participants perceived Q fever as an important risk but their self-efficacy level was ambiguous. Medium overall risk of C. burnetii introduction was reported by four out of seven participating farms. The introduction of infected goats was perceived to be the most important introduction route, followed by transmission on fomites, introduction from neighbouring domestic animals and spillovers from wildlife. An evaluation of different surveillance methods for detecting C. burnetii infections for herds with different starting probabilities of freedom was conducted in Chapter 5. Seven surveillance strategies were constructed from three candidate surveillance system components, and their performance was evaluated quantitatively. The results show that the most efficient combination of surveillance system components depends on a good understanding of initial herd C. burnetii status and the probability of introduction of infection. Collectively, the findings of this thesis identify, at the time of writing, a relatively low C. burnetii prevalence among commercial dairy goat farms in Australia. However, the risk factors for detecting C. burnetii infection on a farm were related to farm size/intensity and the industry is undergoing change in this regard. Overall, this thesis presents many elements of a framework for developing a market assurance program to achieve confidence in C. burnetii freedom and maintain such status
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ItemCoxiella burnetii environmental contamination from a large multi-site goat farm and its spatial risk profile( 2022)Coxiella burnetii is a zoonotic bacterial pathogen that can infect multiple animal species. Animals rarely develop clinical disease from infection with C. burnetii. When infection occurs in animals, C. burnetii can be found in relatively high concentrations in the reproductive tract and are released into the environment during parturition. Intensively managed small ruminant farms can play an important role in the epidemiology of C. burnetii due to the potential for abundant release of the bacteria into the environment when large numbers of animals give birth during synchronized kidding/lambing events. Outside the host, C. burnetii can attach itself to dust particles and travel by the wind to places distant from the site of disposition. Human infection, Q fever, manifests itself as clinical disease in about 40% of cases and has the potential to be fatal if not treated. Q fever is endemic in Australia, with 2 cases per 100,000 population notified annually and Q fever seroprevalence in Australia is (at the time of writing) the second highest in the world. However, the knowledge of its spatial transmission from infected sources and validated methods to detect C. burnetii in the environment are limited. This thesis assesses the level of C. burnetii environmental contamination in and around a known infected intensively managed multi-site dairy goat farm in Victoria, Australia. The overarching aim of this work was to improve understanding of the environmental epidemiology of C. burnetii using as a case example the geographic distribution of contamination around a known C. burnetii-positive source. As a first step in addressing this aim, a systematic review (Chapter 3) was conducted to identify the main environmental substrate types, sampling, and testing methods available. Critical appraisal of the available evidence showed that a variety of factors play a role in the ability to detect the organism during field sampling and laboratory testing. Chapter 3 concludes with a framework that can be used by future researchers as a guide for environmental field sampling to detect C. burnetii. Given that the primary mode of transmission of C. burnetii is inhalation, determining the level of bacteria circulating in air is important when considering environmental contamination. In Chapter 4, three air sampling devices were compared and validated in a laboratory-based experiment to determine their ability to detect known concentrations of C. burnetii. This chapter showed that the air samplers performed similar at detecting aerosolized C. burnetii and provided detection and quantitation limits for each sampler with the PCR protocol validated in the study. Chapters 5 and 6 were field sampling studies centred around the dairy goat farm in which coxiellosis was endemic. In Chapter 5, an understanding of the level of environmental contamination in and around the kidding sheds was obtained while standardizing laboratory testing methods for different environmental substrates. Chapter 5 served as an assessment of the feasibility and assisted in the design of the larger-scale geospatial field study presented in Chapter 6. The field study found that C. burnetii soil positivity was higher closer to rivers and creeks. The detected association could be due to either contamination of the environment arising from wildlife preferentially aggregating around waterways or runoff of deposited material on topsoil accumulating in and around waterways. Considering the findings of this thesis and previous work in this field, it is evident that C. burnetii environmental contamination is context specific, depending on many factors including but not limited to the source of bacterial release, surrounding terrain and weather conditions. Overall, the work presented in this dissertation serves as a guiding model for research on C. burnetii geospatial contamination elsewhere.
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ItemQ fever in intensively managed dairy goats: Transmission dynamics, production losses and control interventions( 2020)Q fever is a zoonotic disease with a significant impact on public health worldwide. It is caused by the highly infectious bacterium Coxiella burnetii. In humans, Q fever typically manifests as a self-limiting febrile syndrome. C. burnetii infection can also lead to chronic disease presentations and be life-threatening for individuals with underlying risk factors. Australia ranks among the countries with the highest Q fever report rates. Livestock species are the main source of infection for humans and dairy goats have been linked to some of the largest Q fever outbreaks recorded in recent times. In Victoria, Australia, a Q fever outbreak linked to a large dairy goat enterprise occurred in 2012 – 2014. This was the largest farm-associated Q fever outbreak recorded in the country. In the context of a steadily growing dairy goat sector, an improved understating of Q fever dynamics and the efficacy of available control strategies in dairy goats is required. This thesis documents a series of studies designed to better understand factors contributing to the spread of Q fever in this herd and the potential efficacy of different control measures. As a first step, we developed a software prototype for reporting herd performance in intensively managed dairy goat herds. This allowed productivity, reproductive performance, health and mortality to be documented, and enabled the performance of Q fever positive and Q fever negative does to be compared. With this system in place, a panel study was conducted to document the prevalence of C. burnetii shedding at the time of kidding. A heterogeneous C. burnetii shedding pattern was found, characterised by the presence of small numbers of ‘super shedder’ does. Further, does identified as ‘super shedders’ had reduced total lactation milk yields compared with C. burnetii negative does. Demographic data gathered using the herd health software prototype and data on Q fever prevalence from our panel study were used to develop a within-herd transmission model of Q fever. The model was used to assess the expected time to eradication by means of vaccination. In addition, the efficacy of segregation of pregnant does and culling of C. burnetii shedders at the time of parturition or abortion was assessed. Vaccination consistently led to disease eradication, although it required this intervention to be sustained for at least 6.5 (95% CrI: 4.1 to 11.3) years. A combination of vaccination with segregation of pregnant does or culling of C. burnetii shedders at parturition or abortion shortened the median time to eradication. The model was adapted to account for heterogenous shedding patterns and used to test the potential efficacy of a test and remove intervention. We found the removal of super shedders does alone would lead to Q fever eradication. However, tests with a high sensitivity for early detection of super shedders are required for this strategy to be effective.