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Title: Pasteurella haemolytica in sheep : some studies on pathogenesis
Author: Al-Sultan, Imad Ibrahim
ISNI:       0000 0001 3409 1148
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1982
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Pasteurella haemolytica, a common commensal of the nasal passages and tonsils of many healthy sheep, can, under circumstances still imperfectly defined, invade other tissues and cause severe and often fatal disease. Two distinct syndromes are recognized » acute pneumonia due to biotype A serotypes and acute systemic infection of 6 - 12 month old sheep with biotype T serotypes. For neither syndrome is the pathogenesis of disease properly understood and experimental reproduction of disease has been achieved only with pneumonic pasteurellosis. The main objective of the work described in this thesis was to contribute to knowledge of the pathogenesis of ovine pasteurellosis, particularly the systemic disease caused by T serotypes. By repeated tonsillar swabbing of 37 lambs from birth to 12 weeks of age it was established that the tonsils can be colonized within hours of birth, presumably through intimate contact between lambs and their commensally infected dams. Initially isolations were exclusively of A serotypes and untypable strains while T serotypes first appeared at 3 weeks but were the main isolations (>80%) by 9 weeks of age. In lambs under 2 months old a single topical infection of the tonsils with a mixed T serotype culture provoked a definite but transient local inflammatory response. On the basis of the foregoing result further attempts were made to reproduce systemic pasteurellosis in groups of 6 sheep aged between 7 and. 9 months. Superficial abrasion of tonsillar and pharyngeal mucosa caused a moderate, local inflammatory response but did not modify the activity of the pre-existing commensal population of T serotypes. Supplementary infection of abraded tonsils with a mixed T serotype culture did not exacerbate the local inflammatory changes. Conversely, when infection preceded abrasion more severe local lesions developed but there was no dissemination of infection. Experiments in mice demonstrated that the LD₅₀ of T serotypes of P. haemolytica could be reduced significantly (p < 0.0l) by giving the mice ionic iron (ferric ammonium citrate) intravenously immediately before intraperitoneal infection. Similarly, sheep injected with iron developed a severe local reaction to experi¬ mental tonsillar infection with subsequent systemic spread of T serotypes. Verification of these findings was obtained in a single comparative experiment using 7-10 week old lambs. Within the experiment a gradation in severity of response to tonsillar infection was observed between:- a) infection of intact tonsils b) infection of abraded tonsils c) infection of abraded tonsils in iron-treated lambs. Only in the latter group was there evidence of systemic spread of infection. Antisera produced by immunizing chickens with live bacteria of selected individual serotypes were rendered serotype-specific and used for indirect immunofluorescence study of tissues from normal, diseased and experimentally infected sheep. In normal animals T serotypes were distributed only over the tonsillar surface and in relatively small numbers but in diseased and experimentally infected sheep P. haemolytiea had invaded the tonsillar crypts, breached the epithelial barrier and been disseminated to other organs. These findings support the hypothesis that in systemic pasteurellosis the tonsils may act as a primary focus of infection from which bacteria are disseminated to other organs. However, the factors which result in the conversion of commensal P. haemolytica into invasive microorganisms remain undetermined. Though producing neither the full range nor severity of lesions encountered in natural systemic pasteurellosis, infection and abrasion of the tonsils of iron-treated sheep does result in a form of disease similar in onset and general pathology to that encountered in the field. Therefore it is a useful model for further pathogenesis studies.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Veterinary sciences & veterinary medicine