Control of bovine tuberculosis in New Zealand in the face of a wildlife host: a compiled review of 50 years of programme policy, design and research

The control of bovine tuberculosis (TB) infection in cattle is a major animal health issue in many parts of the world. The mainstay of disease control in cattle is provided by testing and removal of reactor animals, which has been remarkably successful for the most part. Unfortunately, the causative agent Mycobacterium bovis is not as host-specific as its name would suggest and in addition to cattle, infection has been confirmed in a wide variety of wild mammal species, where its presence can hamper disease control efforts in livestock. This presents significant challenges for disease managers, particularly where infection is able to circulate amongst a community of wild mammal hosts, with the potential for onward spread to cattle. It therefore seems somewhat paradoxical that a country with no native terrestrial mammals should have generated such a substantial and impressive volume of work on the dynamics of bovine TB infection in wild mammals. But then New Zealand has an unenviable historical track-record in the successful but ultimately unwise establishment and spread of introduced mammals.

The brushtail possum (Trichosurus vulpecula) is arguably the most notorious of New Zealand's introduced mammalian pests. Following their introduction from Australia in 1858 for the fur trade, possums soon established self-sustaining populations in the wild, and spread rapidly throughout much of the North and South Islands of New Zealand. Since then they have had a devastating impact on vegetated ecosystems, particularly indigenous forests, and from the 1950s to the present day they have been the subject of lethal control for the purpose of protecting native biodiversity. The pest status of the possum received a further boost in the early 1970s with its recognition as a source of M. bovis infection in cattle. But although the possum has since then played the leading role in the story of TB management in New Zealand, a supporting cast of three other species of invasive non-native mammals, namely red deer (Cervus elaphus), wild pigs (Sus scrofa) and feral ferrets (Mustela furo), have also focused the minds of researchers.

The papers in this special issue of the New Zealand Veterinary Journal summarise the findings of research to elucidate the role of wild mammals in the epidemiology of bovine TB and to develop and refine practical methods for TB control in both cattle and wildlife. These reviews largely focus on wildlife TB research conducted within the last 20 years, but they also provide a historical context for this work across a programme spanning the last 50 years, and indicate how research has contributed to the diminishment of New Zealand's wildlife TB threat to the point where disease eradication is a realistic outcome. The breadth of work represented in these review papers is impressive, as is their common purpose. Drawing these many strands of research together is a clear focus on asking critical applied questions at the interfaces between ecology, wildlife management and epidemiology. During the period covered by these papers many important questions have been addressed, and although many more remain, this growing body of evidence has been continually integrated into practical management. It is clear that research findings have been a driving force behind the evolution of a long-term strategy which has moved from its initial focus on control of M. bovis infection in cattle and wildlife to the goal of disease eradication. There has been a kind of “virtuous circle” at work, allowing operational experience and research to inform and drive one another; field experience and aspirational disease control targets have directed research questions, and research findings have been directly translated into operational practices, which have in turn been refined by further research. There is no doubt that this close marriage of research and operational experience has been instrumental in the progress made by the National Pest Management Plan for bovine TB in New Zealand.

Unlike many other countries where endemic wildlife poses risks of TB infection for cattle, in New Zealand the spotlight falls on a community of invasive non-native species which are also having a substantial detrimental impact on native ecosystems. For this reason wide scale use of lethal control is made possible, and much research has therefore centred on developing efficient, cost-effective methods. But in order to do so, it has been necessary to tease apart the transmission pathways within these multi-host communities, and to distinguish dead-end hosts from maintenance hosts across a range of ecological conditions. Consequently, a recurrent theme in these papers is the potential for spatial heterogeneity in the risks posed by different wildlife hosts. This highlights the need to consider tailoring control strategies to local conditions, and avoiding a “one size fits all” approach.

Amidst all this talk of managing wildlife reservoirs it is easy to forget the critical role that diagnostic testing and regulation of movements of cattle can play in the control of TB. In New Zealand the management of M. bovis infection in cattle and farmed deer has been supported by initiatives to regionalise testing regimes and movement controls, whilst improving the performance of diagnostic tests. These initiatives have been characterised by recognition of the importance of varying cattle controls to reflect risks from wildlife reservoirs and of the need to change test requirements as the control programme progresses. Once again this emphasises the importance of an adaptive approach to management in both space and time.

Research efforts have more recently supported the strategy shift from control alone towards control and surveillance aimed at achieving disease eradication. Targeted surveillance of TB in wildlife has played a critical role in identifying priority areas for intervention in wildlife populations, in measuring operational success and thereby informing disease control strategies. Research on the role of other species (e.g. ferrets and wild pigs) has confirmed them as hosts of secondary importance and shown that they become infected principally via interaction with tuberculous possums. As a result they serve as effective sentinels for disease surveillance in possum populations, providing valuable information for making decisions on future control efforts. Targeted surveillance has been an important component of TB management in New Zealand as it has become increasingly important to demonstrate freedom from infection. Here, cutting edge mathematical modelling has also been valuable in the development of a method to make best use of surveillance data to estimate the probability that TB has been eradicated from a possum population.

Although the specific techniques described in these papers may not be appropriate to managing TB risks from wildlife reservoirs elsewhere, there is much to learn here in regards to general approaches. Of particular relevance is the development of a long-term plan towards realistic and measurable targets, which is underpinned by existing scientific evidence and subject to adaptation in the light of contemporary research findings.

The final paper in this series provides a synopsis of the history of TB control in New Zealand. It describes a long and sometimes difficult journey, but one that has nevertheless been undertaken with general consensus and with a relatively straight trajectory. As a result, combined efforts to control infection in cattle and wildlife led to a 95% decrease in the number of infected cattle and deer herds between 1994 and 2012 (Livingstone et al. 2015) and the prospect of eradication within the next 20–30 years is now a reality. But make no mistake, as the authors of these papers would be the first to admit, the last few miles may be the hardest so there is no room for complacency or distraction.