Investigations into bacterial causes of disease in swine with emphasis on airborne pathogens

Abstract

Pig farming is a major agricultural production system globally. Over the years, many aspects of pig farming have been investigated. However, controlling infectious diseases remains an area of ongoing research, with new technologies allowing the development of improved management strategies and vaccines.

The family Pasteurellaceae is mainly composed of commensal species, but includes notable pathogens of livestock. The species of major concern in pigs include Actinobacillus pleuropneumoniae and Glaesserella parasuis. Although publication of the genomes of both species has advanced understanding of their virulence existing vaccines for these pathogens have limited efficacy. Therefore, control of A. pleuropneumoniae and G. parasuis currently relies heavily on farm management.

The studies in this thesis sought to develop novel non-invasive methods for detecting A. pleuropneumoniae on farms. The Coriolis µ air sampler, with a qPCR designed for the detection of the apxIV gene, which is specific for A. pleuropneumoniae, were used for this purpose. Application of the assay on two commercial farms revealed that one farm had circulating levels of A. pleuropneumoniae in houses containing pigs aged between 8 and 21. This method could be used for the detection of A. pleuropneumoniae in clinically healthy populations.

A novel member of the Pasteurellaceae family isolated from pigs with severe respiratory infections was characterised at the pathological, biochemical, genomic and phylogenetic levels. This bacterium was identified as a member of the genus Glaesserella, and found to be most closely related to Glaesserella parasuis. Analysis of the predicted proteins encoded by its genome showed that it had a mosaic structure, with 60% of the coding sequences sharing closest identity with G. parasuis, while 20% had closest identity with other members of Pasteurellaceae and 20% were related to proteins encoded by other bacterial genomes. A gene homologous to A. pleuropneumoniae apxIII was identified in the genome of this new species and is proposed to have been acquired by horizontal gene transfer. The isolation and identification of this novel Glaesserella species highlights the need for large scale epidemiological studies to understand the impact of this pathogen on pig health.

Genome analysis of this organism identified other putative virulence factors, leading to studying its pathogenicity. As many pathogenic species in the Pasteurellaceae family have been explored using mouse models, one was sought for this novel pathogen. The organism was compared to A. pleuropneumoniae in BALB/c mice inoculated either intraperitoneally or intranasally. While no bacteria were recovered from the infected mice, intranasal inoculation resulted in formation of significant lung lesions, and intranasal and intraperitoneal inoculation resulted in significantly increased levels of cytokines in the lungs. These experiments demonstrated that the pathological response to infection with the novel Glaesserella sp. significantly differed from that seen with A. pleuropneumoniae and suggest that the mouse model is a promising method for studying this organism.

Finally, assays were developed to detect changes in the dependence of A. pleuropneumoniae on environmental iron concentrations. Growth in presence or absence of streptonigrin and 2,2′dipyridyl was measured by monitoring the optical density of the cultures over time. There was a clear and consistent difference in growth rate between groups, allowing the differentiation of a clone’s ability to grow under iron restricted conditions. These assays were designed to help assist with the identification of a live attenuated vaccine candidate with an impaired iron uptake capacity.

The studies in this thesis further understanding of respiratory disease caused by members of the family Pasteurellaceae and highlight areas needing further research.