Infection dynamics and effective control strategies of tuberculosis in badgers and cattle in Ireland

Abstract

Bovine tuberculosis (bTB) is a chronic inflammatory disease of cattle caused by infection with Mycobacterium bovis. In Ireland, a comprehensive control/eradication programme for M. bovis commenced in 1954, and by 1965, cattle incidence had been reduced by more than 95%. Despite this early and rapid success, the infection has remained relatively stable since. The inability to eradicate the disease by focusing on strategies that aimed at reducing cattle to cattle transmission suggested the presence of one or more reservoirs of the pathogen outside the cattle population. M. bovis was first isolated from a badger (Meles meles) in Ireland in 1974. Since then, a large amount of literature looking at different epidemiological aspects was dedicated to investigating the role of badgers in bTB maintenance. Evidence of the role of badgers as a disease-maintenance-host in areas of known cattle infection was gathered from the results of two large field studies carried in Ireland, the East Offaly and the four area projects. Following the proactive removal of badgers in some of the study areas, these studies demonstrated a significant reduction in cattle incidence in comparison to control areas where no badger removals were carried. In Ireland, badger culling commenced in 2000 as a short/medium term strategy to reduce the incidence of the infection in badgers, with the aim to achieve eradication in cattle. Nonetheless, there were expectations that BCG vaccination of badgers in the field, by helping to reduce M. bovis transmission in badgers and subsequently in the cattle population, would be the long term solution. In this thesis, a field trial enabling quantification of the efficacy of BCG vaccination on M. bovis transmission in badgers under field conditions was designed and analysed. Vaccine efficacy for susceptibility was estimated as VES=59% (95% CI =6.5%-82%) whereas vaccine efficacy for infectivity (VEI) was 0%, implying that there was a complete lack of effect from BCG vaccination on the infectivity of vaccinated and subsequently infected badgers. Also, the quantitative role of each of these two host species in the transmission of M. bovis within the system was assessed. Algorithms to estimate the Next Generation Matrix (NGM) and the reproduction ratio for the cattle-badger system were developed, and all plausible NGMs describing the transmission between the two species were calculated from prevalence data and the number of cattle herds and badgers, collected at the endemic steady state. Taking into account the results obtained for vaccine efficacy, and the reproduction ratios obtained, it was possible to demonstrate that for almost all these plausible transmission scenarios, BCG vaccination of badgers in addition to current controls could successfully achieve eradication of M. bovis infection in the system. Further, the small number of transmission scenarios where eradication of M. bovis from the system (by adding BCG badger vaccination to the current controls) was not feasible, were classified as highly unlikely to represent transmission in the field.