Management of bovine tuberculosis in brushtail possums in New Zealand: predictions from a spatially explicit, individual-based model.
Abstract
Bovine tuberculosis Mycobacterium bovis (Tb) in brushtail possums Trichosurus vulpecula in New Zealand continues to pose a threat to the livestock industry. While significant reductions in cattle reactor rates have been achieved by widespread possum control, there is still a need for significant improvement, particularly in relation to limiting the further spread of the disease. A recently developed oral Tb vaccine based on bacille Calmette-Guérin (BCG) offers a possible alternative to poison baiting for widespread disease control in possums. In addition, there is ongoing development of orally delivered fertility control vaccines (FC) as an alternative method for possum control. However, there is some doubt about whether control strategies based on oral BCG or FC can offer cost-effective solutions for disease control in possums. We developed a spatially explicit, individual-based model (IBM) of Tb in possums to address perceived shortcomings in existing non-spatial models, particularly their failure to model realistically the spatial scale of transmission. The spatial IBM was also used to compare the cost-effectiveness of control strategies using BCG and combined BCG/FC to existing strategies based on lethal control. In particular, we explored the effectiveness of strategies to contain a localized outbreak. Considering the cumulative cost of control to contain a localized Tb outbreak, the most cost-effective strategy combined an initial cull of possums with both BCG and FC applied every 3 years. In addition, the combined culling/BCG/FC strategy required only a 2- to 3- km control buffer around the outbreak site to achieve successful containment. By comparison, control strategies that used BCG or BCG/FC alone were not as cost-effective as they required at least a 5- km buffer around the outbreak site to achieve containment. Synthesis and applications. Current strategies for Tb control in possums rely heavily on lethal control techniques, but there is public antipathy in New Zealand to the widespread use of toxicants for pest control. Strategies for Tb control that use oral vaccination of possums with BCG, or combined BCG/FC, in combination with a single initial cull of the population offer a cost-effective alternative for local eradication and containing disease spread. Such strategies would greatly reduce the need for repeated application of toxicants.