Control of bovine tuberculosis in British livestock: there is no ‘silver bullet’

Abstract

Bovine tuberculosis (bTB; Mycobacterium bovis) is a bacterial infection of cattle that also affects certain wildlife species. Culling badgers (Meles meles), the principal wildlife host, results in perturbation of the badger population and an increased level of disease in cattle. Therefore, the priority for future management must be to minimize the risk of disease transmission by finding new ways to reduce the contact rate among the host community. At the farm level, targeting those individuals that represent an elevated risk of transmission might prove to be effective. At the landscape level, risk mapping can provide the basis for targeted surveillance of the host community. Here, we review the current evidence for bTB persistence in Britain and make recommendations for future management and research. Wildlife hosts for bovine tuberculosisMany infections of humans and their livestock are able to cross-infect multiple host species [1]. The presence of infections within a multiple-host community can complicate attempts to reduce disease [2,3], not only because of varying resistance to infection between the different host species but also because of ecological and behavioural differences.Bovine tuberculosis (bTB; Mycobacterium bovis) persists in various countries in the developed world. In some of these countries (including Britain, Ireland and New Zealand), the presence of additional wildlife hosts for the infection -principally Eurasian badgers (Meles meles) in the UK and Ireland, and brush-tailed possums (Trichosurus vulpecula) and ferrets (Mustela putorius) in New Zealand -is thought to have contributed to the persistence of the disease in cattle, despite considerable investment in culling programmes to reduce the populations of these wildlife hosts. Badgers are particularly well suited as a host for bTB because they are a group-living animal. They exhibit a range of social structures, with social group size varying between two individuals in Poland and 25 individuals in Britain, and group territories ranging from 0.14 km 2 in Britain to 14 km 2 in Poland and Finland [4].In Britain, the total economic costs associated with bTB between 2004 and 2011 are expected to reach £1 billion [5]. A test-and-slaughter strategy to detect and remove bTB-infected cattle from British herds had all but eradicated bovine bTB in a large part of Britain by the 1970s, and persistent infection was limited to distinct areas in southwest Britain. Since that time, badgers have been implicated increasingly in the persistence and reemergence of bTB as the disease has spread once more into new areas, both within southwest Britain and beyond.A large-scale field trial, the Randomized Badger Culling Trial (RBCT), was initiated in 1999 to determine the extent of badger involvement in bTB in cattle and the effectiveness of culling badgers in reducing bTB infection in cattle [6,7]. The field trial showed that badger culling over 100-km 2 areas could result in a decrease in cattle bTB inside these areas, with effectiveness increasing with distan...

Risk of tuberculosis cattle herd breakdowns in Ireland: effects of badger culling effort, density and historic large-scale interventions

“…This is most likely due to Co. Donegal being the most exposed county, with poor soils and mountainous areas [19]. There have been previous attempts in Britain to use badger abundance metrics to produce risk maps for bTB to cattle herds at national levels [28,42]. Bessell et al [29] used one of these risk maps, the probability of main-sett presence, to model its relationship with cattle herd breakdown risk at a national scale.…”

“…We added a variable which represented a metric of badger social group density (derived from the maps within [19]), as a means of controlling for underlying heterogeneity of badger density. This variable represents the suitability of landscapes for social group density based on main sett abundance [28,29]. This metric was scaled to values between zero and one; higher values denote higher densities (see [19] for details).…”

Contact Networks in a Wildlife-Livestock Host Community: Identifying High-Risk Individuals in the Transmission of Bovine TB among Badgers and Cattle

“…Failure to account for the complexity of social networks through which diseases may be transmitted may in turn be responsible for the failure of disease control strategies based simply on one-off population reduction [6]. For example, a study of possums Trichosurus vulpecula Kerr in New Zealand showed a non-linear relationship between contact rates and population density [7], clearly in contrast to the traditional assumption of density dependence.…”

Risk factors for bovine Tuberculosis at the national level in Great Britain

“…While the RBCT showed that badger culling operations and cattle herd breakdowns rates are strongly and significantly associated with each other [8-10], relatively little is understood about the role of badgers in herd breakdowns across GB, though a recent paper has analysed risk factors for badger presence [11]. These analyses have been extrapolated to produce a prediction of badger distribution in GB, features that have previously been suggested could be used to predict risk of bTB to cattle [12]. …”