Melbourne Veterinary School - Theses

Permanent URI for this collection

Search Results

Now showing 1 - 6 of 6
  • Item
    Thumbnail Image
    Pharmacometrics of the bovine mammary gland
    Woodward, Andrew Paul ( 2017)
    Intra-mammary antibiotics are widely used, and are an important contributor to the control of bovine mastitis. The concentrations of antibiotic drugs in milk after intra-mammary administration is highly important for food protection and treatment efficacy. In this project, novel mathematical and statistical models were developed to describe the pharmacokinetics of antibiotic drugs after intra-mammary administration. This approach combined physiologically-based pharmacokinetic models to capture the unique features of the mammary gland, and large-scale mixed-effects analysis to understand these effects at the level of the treated population. The developed models perform well across various drug types and production conditions, and it is proposed that these models are suitable as a generalized method for the description and analysis of drug concentration in milk. Further, the developed methods are applied to study the statistical performance of the various existing methods used to study milk residues in the regulatory context. A derived model is discussed, which is suitable for application to regulatory conditions for the determination of milk discard times.
  • Item
    Thumbnail Image
    The effect of vaccination against Campylobacter on maiden ewe reproduction in Victoria
    Glanville, Elsa Jane ( 2017)
    Reducing reproductive wastage is important for the Australian sheep industry. Campylobacter fetus fetus and C. jejuni infections in ewes contribute to reproductive wastage through abortions, stillbirths and the birth of small, weak neonates, potentially at greater risk of starvation-mismothering-exposure (SME). A combined vaccine against C. fetus fetus and C. jejuni (Ovilis Campyvax®, MSD Animal Health) is registered in Australia to reduce reproductive wastage due to Campylobacter, but few independent field trials of the vaccine have been conducted in commercial flocks. This study described the effects of Ovilis Campyvax® on maiden ewe reproduction in a randomised controlled field trial on four winter-/spring-lambing Victorian sheep farms. Conception and lamb marking rates were compared amongst nineteen-month-old Merino and Merino-cross ewes randomly allocated to vaccination or control groups at mating on each farm (each n = 211–249/group). Ewes were grazed together from mating until immediately before lambing, when they were set-stocked in treatment groups in matched paddocks. Antibody titres to Campylobacter spp. were measured at mating, mid-gestation pregnancy diagnosis and lamb marking in a subset of ewes. A cross-sectional study of cause of neonatal lamb mortality was also conducted on each farm during lambing. Vaccination had no effect on ewe conception rate (67% to 117% depending on farm). Two of four farms had serological evidence of prior exposure to C. fetus fetus, and variable exposure to this organism occurred during gestation on all farms. Campylobacter jejuni titres were high on all farms at mating, but decreased thereafter. Despite serological evidence of a good response to C. fetus fetus vaccination on all farms, vaccination did not significantly increase lamb marking rates (63% to 100%, depending on farm). The main causes of lamb mortality were dystocia, starvation-mismothering-exposure and predation. There was a suggestion of a difference in the pattern of causes of neonatal lamb mortality between vaccinated and control ewes. The difference was not statistically significant, but corresponded with anecdotal observations made by the flock owners. Additional large scale studies into vaccination and the causes of neonatal lamb mortality are needed to further investigate these observations. Vaccination appeared to prevent Campylobacter-associated neonatal lamb mortality and morbidity on the farm with the greatest exposure to C. fetus fetus. On that farm, 55% of unvaccinated ewes that failed to rear a lamb had ‘high’ (≥ 1:80) C. fetus fetus titres, compared to 0% of ewes that successfully reared a lamb. Additionally, C. fetus fetus was only recovered from necropsied lambs born to unvaccinated ewes. The results demonstrate that ewes can be vaccinated with Ovilis Campyvax® during mating without impacting conception rates. However, the effect of Campylobacter vaccination on reproductive output is complex and multifactorial. Vaccination effects may be obscured by other causes of reproductive loss. Vaccination may reduce the contribution of Campylobacter infections to lamb loss due to SME. However, the dystocia risk in protected ewes may increase depending on ewe nutrition. If this is the case, the nutrition of vaccinated ewes could be managed more economically to obtain the full benefits of vaccination. This is an avenue for future research.
  • Item
    Thumbnail Image
    Exploration of the basis of virulence attenuation and development of recombinant derivatives in a Mycoplasma synoviae live vaccine
    Zhu, Ling ( 2017)
    Mycoplasma synoviae is an important avian pathogen that is responsible for significant economic losses in the poultry industry worldwide. It is able to infect chickens, turkeys and other avian species, causing subclinical respiratory tract infections, synovitis, and egg abnormalities. A live attenuated temperature-sentitive (ts+) vaccine strain, MS-H, is widely used in many countries to prevent infection with M. synoviae. Due to its ts+ phenotype, MS-H does not grow at the core body temperature of birds, but colonises the upper respiratory tract and establishes a solid protection against wild-type M. synoviae. In rare cases, the ts+ MS-H strain may revert to a non-temperature-sensitive (ts−) phenotype, but this is not associated with a significant increase in the pathogenicity in MS-H re-isolates, indicating that factors other than temperature sensitivity may be involved in the attenuation of the vaccine. To better characterise the molecular basis of the vaccine attenuation, MS-H and its parent 86079/7NS were subjected to whole genome sequencing. Comparative analysis of the genome organisations of the MS-H, 86079/7NS and other published M. synoviae strains revealed a number of features unique to MS-H that can be used as novel genetic markers in routine diagnostic tests. In M. synoviae strains MS-H and 86079/7NS, a 50-kb genomic inversion was observed for the first time amongst M. synoviae strains. Two newly identified horizontally transferred genes that encode a deoxyribose-phosphate aldolase and an ATP-dependent DNA helicase were noted for MS-H and 86079/7NS. Genomic comparison of MS-H and its parent strain 86079/7NS revealed 33 single nucleotide polymorphisms (SNPs) (excluding the highly variable vlhA gene), including 11 nonsynonymous SNPs that may havIe functional consequences. A frameshift mutation causing a premature termination of translation was found in oppF-1, a gene that encodes an ATP-binding protein that is a component of an oligopeptide transporter. This frameshift is likely to have inactivated the OppF protein function, suggesting that this mutation may play a significant role in the attenuation of MS-H. A nested PCR combined with high-resolution melting (HRM) curve analysis, based on the frameshift mutation within oppF-1 gene, was developed. This assay could reliably differentiate MS-H from all field strains tested in this study and was efficient for both cultures and swabs from clinical cases. In addition, this study investigated the potential for use of the MS-H vaccine as a delivery vector for foreign antigens using infectious bronchitis virus (IBV) as a model, aiming at the development of recombinant poultry vaccine candidates, especially for those pathogens affecting the upper respiratory system. To achieve this, expression vectors were constructed based on an oriC plasmid (pMAS-LoriC), which is capable of freely replicating in M. synoviae. Gibson assembly was used as a versatile strategy for constructing the vectors. The variable lipoprotein haemagglutinin gene (vlhA) promoter was used to drive expression of partial S1 and N IBV genes in MS-H. The expression of IBV N or S1 mRNA in MS-H recombinants was detected by quantitative real-time PCRs (qPCRs). Western blotting confirmed the expression of IBV partial S1 and N proteins using polyclonal antibodies specific for S1 and IBV respectively. This is the first report of successful expression of a non-native gene in M. synoviae. Thus the studies reported in this thesis have contributed to (1) development of our understanding of putative virulence factors in M. synoviae, (2) development of an assay to discriminate MS-H from field strains, and (3) laid the foundation for the development of novel recombinant vaccines to control other upper respiratory pathogens.
  • Item
    Thumbnail Image
    Antimicrobial stewardship in Australian veterinary practices
    Hardefeldt, Laura Yvonne ( 2017)
    Antimicrobial use by the veterinary profession has been coming under increasing scrutiny by medical, public health and government officials as the threat of antimicrobial resistance becomes increasingly clear. The World Health Organisation has described antimicrobial resistance as one of the major public health challenges of our time. It is clear that at least some drug-resistant pathogens have evolved under selective pressure from antimicrobial use in agriculture and may be contributing significantly to resistance in clinical setting. Antimicrobial stewardship is the selection of the most appropriate antimicrobial for a given disease in a given animal, with the aim of reducing the risk of adverse effects in that animal, and reducing the likelihood of developing resistance on an individual level, on a farm level and on a national level. Currently none of the core elements of antimicrobial stewardship are widely available for veterinarians in Australia, and there is very sparse data available on which to base an antimicrobial stewardship program. This research project aims to address this paucity of data. A range of research methods were used to assess detailed antimicrobial use by veterinarians in Australia and the enablers and barriers to antimicrobial stewardship. These included quantitative methods such as surveys and analysis of pet insurance data, and qualitative methods such as interviews and focus groups. While antimicrobials with low importance rating were predominately used in all species, under-dosing and inappropriate timing of antimicrobial therapy were common particularly in horses and cattle. Few veterinary practices in Australia had antimicrobial stewardship policies in place, or were using antimicrobial use guidelines. The key barriers to implementing antimicrobial stewardship programs were a lack of antimicrobial stewardship governance structures, client expectations and competition between practices, the cost of microbiological testing, and a lack of access to education, training and antimicrobial stewardship resources. The enablers were, firstly, concern for the role of veterinary antimicrobial use in development of antimicrobial resistance in humans, secondly , a sense of pride in the service provided, and thirdly , preparedness to change prescribing practices. This research culminated in the development of a proposed antimicrobial stewardship policy and procedure documents, to enable veterinarians to institute antimicrobial stewardship programs that suit their individual practice requirements. However, it is likely that governance changes will be necessary to compel veterinary practice owners to implement antimicrobial stewardship on a large scale.
  • Item
  • Item
    Thumbnail Image
    An investigation of two significant infectious diseases in populations of Victorian koalas (Phascolarctos cinereus)
    Legione, Alistair Raymond ( 2017)
    The koala (Phascolarctos cinereus), an iconic Australian marsupial, is considered a vulnerable species in parts of Australia due to recent rapid population declines. The role of infectious diseases in population declines in northern koalas (New South Wales (NSW) and Queensland populations) has been highly studied. Chlamydia pecorum and koala retrovirus (KoRV) have both been given considerable attention. C. pecorum in koalas is associated with infertility and blindness through infection of the urogenital tract and conjunctiva, respectively. The prevalence of C. pecorum in northern koalas is as high as 87%. Different genotypes of KoRV have been identified in northern koalas. KoRV-A has been identified in all northern koalas tested to date. KoRV-B appears to be less prevalent but has been implicated as a cause of neoplasia. Molecular testing of C. pecorum and KoRV prevalence in Victoria, a southern population of koalas, has been limited. No genotyping studies have been undertaken on either organism in Victorian koalas. This thesis conducted an extensive survey of Victorian koalas across seven separate regions to establish a prevalence estimate for both C. pecorum and KoRV. A genotyping study for each pathogen was also completed. The estimated prevalence of C. pecorum in Victorian koalas was 15.2% (125/820, 95% confidence interval (CI) 12.9, 17.9%). Molecular evidence of C. pecorum infection in French Island koalas was detected for the first time. Only a single ocular C. pecorum infection was identified in Victorian koalas (1/459). A total of six C. pecorum genotypes were detected, the majority of which were genotype B, which has only been detected in southern koalas. Three of the genotypes were novel, each of which were found in distinct populations. Male koalas were more likely to be infected than females. C. pecorum infection was associated with ‘wet bottom’ (a sign of urinary incontinence and inflammation) in male koalas and reproductive tract disease in female koalas. Not all koalas with ‘wet bottom’ had detectable C. pecorum, suggesting another potential cause. Analysis of the genetic diversity of the bacteria present in urogenital tract samples from ten koalas, of which only five displayed wet bottom, identified 13 operational taxonomic units that occurred at a higher abundance in wet bottom-affected koalas. These bacterial families are of interest for future studies. The genomes of 57 C. pecorum samples from koalas across Australia were sequenced and assembled. The results showed that C. pecorum genomes from southern koalas were distinct from those of northern koalas. KoRV prevalence in Victorian koalas was 24.7% (160/648, 95% CI 21.5, 28.2%). Only KoRV-A was detected. Koalas with ‘wet bottom’ were almost twice as likely to have KoRV detected. There was no association between KoRV and C. pecorum detection. This research highlights that Victorian koalas are experiencing a reduced burden of infection compared to northern koalas, and this may be a factor in southern populations outgrowing the available habitat resources in Victoria, compared to northern populations. Victorian koalas free from C. pecorum and KoRV infection could be sourced from over-abundant populations to assist re-establishment of populations where koalas have become locally extinct. Overall, this research provides valuable information for both future research and koala population management.