Cost-based optimization of the stopping threshold for local disease surveillance during progressive eradication of tuberculosis from New Zealand wildlife

Abstract

Bovine tuberculosis (TB) is managed in New Zealand largely via population reduction of the major wildlife disease reservoir of Mycobacterium bovis, the introduced brushtail possum Trichosurus vulpecula. New Zealand aims to eradicate M. bovis infection from its livestock and wildlife within 40 years, as the culmination of progressive regional eradication programmes. Declarations of regional eradication are decided after extensive possum population control and post-control surveillance; hence, we developed a modelling framework, based on eco-epidemiological simulation data, to provide cost-evaluated options for deciding when to make these declarations. A decision-support framework evaluated potential costs of wildlife surveillance (and recontrol, if required) with respect to the calculated probability of successful eradication of M. bovis from wildlife. This enabled expected costs to be predicted in terms of stopping thresholds, allowing selection of optimal stopping rules based on minimizing costs. We identified factors that could influence optimal stopping values applied during regional eradication. Where vector/disease surveillance was inexpensive (for example, using low-cost detection devices or sentinel wildlife hosts) optimization involved setting a higher rather than lower stopping value, as it would be cheaper to minimize the risk of making a false declaration of eradication than to remedy any such failure. In addition, any cost of recontrol would largely depend on the time to rediscovery of residual M. bovis infection in wildlife, which would in turn be linked to the level of ongoing passive surveillance (with more rapid detection of re-emergent infection among wildlife in farmland situations than in remote forested regions). These two scenarios would favour different optimal stopping rules, as would the consideration of stakeholder confidence and socio-political issues, which are discussed. The framework presented here provides guidance to assess the economics underlying eradication of bovine TB from New Zealand farming; this eliminates reliance upon a pre-determined and uniform stopping rule for ceasing active management.

A concise guide to developing and using quantitative models in conservation management

“…There is a frequent desire for applying thresholds to model outputs, for example, by calculating P ‐values to estimate significance or transforming probabilities of occurrence into binary categories (predicted presence or absence). Thresholds can sometimes be arbitrary and misleading when they are used in the context of conservation management, and it always is important to explain and justify their use (Bestelmeyer, ; Field, Tyre, Jonzén, Rhodes, & Possingham, ; Liu, Berry, Dawson, & Pearson, ; Wasserstein & Lazar, )The development of optimal thresholds to discontinue surveillance or the removal of invasive species, by estimating the costs and benefits associated with deploying different surveying efforts, is a good example of a well‐designed and justifiable threshold in the context of conservation management (Gormley, Anderson, & Nugent, ; Regan, McCarthy, Baxter, Panetta, & Possingham, ; Rout, Kirkwood, Sutherland, Murphy, & McCarthy, ). Ecological tipping points, system thresholds that once exceeded can irreversibly shift the dynamics of the system, are important in conservation management (Groffman et al, ; Guntenspergen, ).…”

“…The development of optimal thresholds to discontinue surveillance or the removal of invasive species, by estimating the costs and benefits associated with deploying different surveying efforts, is a good example of a well‐designed and justifiable threshold in the context of conservation management (Gormley, Anderson, & Nugent, ; Regan, McCarthy, Baxter, Panetta, & Possingham, ; Rout, Kirkwood, Sutherland, Murphy, & McCarthy, ). Ecological tipping points, system thresholds that once exceeded can irreversibly shift the dynamics of the system, are important in conservation management (Groffman et al, ; Guntenspergen, ).…”

Roll-Back Eradication of Bovine Tuberculosis (TB) From Wildlife in New Zealand: Concepts, Evolving Approaches, and Progress

“…The initial stopping rule (posterior P free > 0.95) was chosen subjectively by TB managers and their stakeholders (e.g., farming organizations, governmental funding bodies) to represent what they considered to be an “acceptable” level of risk of disease persistence. Our recent decision-theory modeling ( 15 ) indicates how the choice of stopping rule could be better optimized for each VCZ by explicitly including costs of surveillance and potential re-control costs.…”

“…Our analysis of the total expected costs indicates that where surveillance is relatively expensive compared with re-control, it will usually be more cost-effective to stop earlier than 0.95 at an increased risk of incorrect declaration [Figure 2A ; ( 15 )]. Conversely, where re-control is much more expensive than surveillance, it should be better to carry out more surveillance and choose a stopping threshold that is higher than 0.95 in order to mitigate the risk of incurring expensive re-control (Figure 2B ).…”

“…Use of decision theory to determine the optimal “stopping threshold” probability for declaring a particular local area free of TB ( 15 )…”