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Title: Life-history correlates of Myobacterium bovis infection in individual Eurasion badgers (Meles meles)
Author: Tomlinson, Alexandra Jane
ISNI:       0000 0004 2734 4452
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2011
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Bovine tuberculosis caused by Mycobacterium bovis is a disease of global importance. In the UK, it has serious economic and welfare implications for cattle farming enterprises, and its control confers substantial costs on UK taxpayers. Disease control strategies, in particular those pertaining to its main wildlife reservoir, the Eurasian badger (Meles meles), are highly problematic and continue to divide opinion. The aim of the present study was to investigate life-history correlates of infection with M. bovis in individual badgers, using data from the long-term badger trapping and sampling research programme at Woodchester Park in south-west England. An understanding of disease manifestations at the individual level is essential to elucidate transmission dynamics at the social group and population levels, and is therefore also important in the development and optimisation of disease control strategies. Epidemiological analyses centre on the correct interpretation of diagnostic test results. In the case of bovine tuberculosis, this is hindered by the complexity and variability of the immune response. Recent data from the Woodchester Park population presented a rare opportunity to observe the temporal progression of the cell-mediated response as measured by the gamma-interferon assay in a population of free-living naturally infected badgers. Analysis demonstrated fluctuation and decline in the interferon response over time following initial detection. In addition, the magnitude of the initial response was positively correlated with the likelihood of disease progression. These data provide a useful framework on which to further our understanding of the pathogenesis of naturally acquired M. bovis infection in badgers. Using retrospective data collected over a 24 year period, condition loss was shown to be a feature of disease in badgers, but only when mycobacterial excretion was detected. Furthermore, adult female badgers appeared to show more resilience to the physiological impact of disease than male badgers, as they survived for longer, gained weight as per the normal seasonal cycle, and continued to reproduce successfully despite intermittently excreting M. bovis. Shorter survival times were also reported for badgers in which the onset of excretion was characterised by positive culture from a bite wound or lymph node abscess. A more intensive study of six badger social groups in the study area over three years from 2007 to 2010 revealed no significant association between the magnitude of the IFN response and either the presence or intensity of helminth or coccidial burdens in individual badgers, providing no evidence to support a simple relationship between parasite burdens and the immune response to M. bovis infection. The ability of infectious adult females to continue to reproduce and rear cubs successfully resulted in significantly higher risks of both the acquisition and progression of infection in cubs captured in the same social group. In addition, the highest probability of pre-emergent infection was observed in cubs from these high-risk groups. There was a decreasing risk gradient observed from the infectious breeding female to seropositive breeding females to other adults of excretor then seropositive status, and there was no evidence to support a protective effect of maternally derived antibody in cubs. However, groups with infectious breeding females were in the minority during the study period from 1982 to 2010, and the majority of emergent cubs in the population were not detected as infected during their first year of life. These findings highlight the importance of social structure and the role of infectious females in disease dynamics. The value of potential control strategies such as the targeted selective culling of seropositive adult females, and annual vaccine delivery to as many cubs as possible prior to infection, either solely or in combination, are discussed in the light of these findings.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral