Ovine footrot (FR) is an economically important disease that causes lameness and affects sheep flocks worldwide. It is characterized by interdigital skin inflammation (interdigital dermatitis [ID]) with, or without, separation of the hoof horn from the underlying tissue (severe footrot [SFR]). The primary causative agent is the gram-negative anaerobic bacterium Dichelobacter nodosus, which is present in diseased feet and thought to be transmitted via contaminated surfaces. Periods of apparent zero prevalence of FR in a flock can be followed by disease occurrence when the climate becomes favourable for pathogen transmission. This suggests that there are sites where D. nodosus persists in the absence of disease. These sites might include healthy feet, the gingival cavity and faeces of sheep and also the environment. The aim of this thesis was to investigate persistence of D. nodosus, by investigating possible sites of survival of D. nodosus over time. Prospective longitudinal studies were used to investigate persistence. Samples were collected from sheep and from the pasture in three studies (Studies 1 and 2: England, study 3: Spain). Quantitative PCR was used to detect and quantify D. nodosus and to investigate associations between D. nodosus presence in feet, in the gingival cavity and on pasture and a range of predictor variables including climate. A multiple locus variable number tandem repeat analysis (MLVA) suitable for use on mixed DNA and environmental samples was optimized and validated to investigate D. nodosus strains within and between sites. A novel approach to characterize individual strains in a sample was designed. D. nodosus was detected in all sample types in all studies but not on all occasions. The feet of sheep were the only site where D. nodosus was detected in loads exceeding 103 cells per swab. In study 1, D. nodosus was detected in amounts exceeding103 cells in samples collected from the pasture in week 1 only, when detection frequency of D. nodosus on feet was high and the weather was wet. A minimum of 14 strains of D. nodosus were detected on the feet of sheep by MLVA. A decline in detection of D. nodosus in the environment coincided with periods of dry weather, however, dry weather did not coincide with a decline in D. nodosus loads on feet or incidence of disease. D. nodosus was more likely to be detected in the gingival cavity when a sheep had FR. It was detected in 25 % of gingival cavity samples and strain types identified in the gingival cavity were the same as the dominant strain types on the feet of sheep. In study 2, disease prevalence and D. nodosus detection frequencies were lower than in study 1. When sheep from the study group were separated from the main flock in week 1 and moved onto pasture that had been unoccupied for 10 days, D. nodosus was transferred to the study group on healthy feet. One dominant strain of D. nodosus persisted throughout an episode of disease and this strain was present on the healthy feet of sheep until up to 5 weeks before the development of lesions in high bacterial loads. There was a reduction in lesion severity and reduced detection of D. nodosus in soil in a period of dry weather. Only 1 sample from the gingival cavity was positive for D. nodosus. Two faecal sample were positive for D. nodosus, indicating for the first time that faecal shedding is possible. In study 3, there were high loads of D. nodosus on healthy feet of a sheep that was classed as susceptible when there had been no cases of FR for at least 2 month. D. nodosus was still present in the flock during the long non-transmission period in the summer. We conclude that D. nodosus is more likely to persist on the feet of sheep, whereas long-term environmental reservoirs of D. nodosus are unlikely. Future research should focus on the feet of sheep and possibly faeces as possible sites of persistence of D. nodosus in the absence of disease.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:731439 |
| Date | January 2017 |
| Creators | Giebel, Katharina |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/97645/ |
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