Abstract
In New Zealand, managing the threat of bovine tuberculosis (TB) to livestock includes population reduction of potentially infectious wildlife, primarily the brushtail possum (Trichosurus vulpecula). Population control is often targeted on forested buffer zones adjacent to farmland, in order to limit movements of possums across the buffer and reduce the risk of disease transmission to livestock. To assess the effectiveness of buffers in protecting livestock we analysed GPS telemetry data from possums located in untreated forest adjacent to buffers, and used these data to characterise patterns of movement that could lead to possums reaching farmland during the season when most dispersal occurs. Analyses of movement data showed that the direction of dispersal by sub-adult and adult possums and the extent of long exploratory movements were not biased toward forest buffers, even though these provided vacant habitat as suitable for possums as untreated forest. Instead, dispersal and exploratory movements were uncommon even for sub-adult possums and such events typically lasted <10 days. Dispersing possums settled predominantly in river valleys. A simulation model was developed for the 3-6-month dispersal season; it demonstrated a probability of <0.001 that an infected possum, originating from a low-density population with low disease prevalence in untreated forest, would move across 3 km of recently controlled forest buffer to reach farmland. Our results indicate short-term reduction in the risk of TB transmission from possums to livestock in New Zealand by the use of depopulated buffer zones, while acknowledging that the threat of disease spread from untreated forest is likely to increase over time as possum population density and, potentially, TB prevalence among those possums, increase in the buffer zone.
Highlights
The agricultural industry in New Zealand, in common with that in the British Isles, has experienced outbreaks of bovine tuberculosis (TB) attributable, at least in part, to transmission of Mycobacterium bovis infection from a ubiquitous wildlife reservoir host [1,2]
Unlike badgers (Meles meles), the principal wildlife TB host in the British Isles, possums in New Zealand are not subject to species protection legislation, and this recurrent source of infection has been controlled with the aid of population reduction [3]
We developed a simulation model written in the Python programming language [35] to explore how the probability of incursion onto farmland of at least one M. bovis-infected possum (a ‘TB incursion event’) would be influenced by the width of the buffer and by the population density and disease prevalence in adjacent uncontrolled forest (Fig 1)
Summary
The agricultural industry in New Zealand, in common with that in the British Isles, has experienced outbreaks of bovine tuberculosis (TB) attributable, at least in part, to transmission of Mycobacterium bovis infection from a ubiquitous wildlife reservoir host [1,2]. Unlike badgers (Meles meles), the principal wildlife TB host in the British Isles, possums in New Zealand are not subject to species protection legislation, and this recurrent source of infection has been controlled with the aid of population reduction [3]. This approach, in combination with conventional farming practices for TB management, has seen New Zealand’s annual livestock herd reactor rate decline by >95% over the last 20 years [4]. Depopulated buffer zones have been used in New Zealand for many years [12] while vaccine-protected buffer zones for wildlife remain theoretical [13]
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