Author: A. Byrom, G. Nugent, J. McKenzie, T. Porphyre, N. Poutu, J. Shepherd, J. Whitford and I. Yockney
The Northern South Island High Country (NSIHC) is simply too large to impose blanket-coverage vector control. Managers and landowners therefore need reliable information to help them decide when and where control should be applied. This study was undertaken on Molesworth Station in order to identify the principal habitats and vector species requiring control in order to cost-effectively eradicate TB from the NSIHC.
This study demonstrated a sufficiently strong association between trap catch and habitat factors to predict possum abundance on Molesworth Station and for the NSIHC generally. A model (which had a high predictive performance when compared with the observed data) in the form of a GIS-based “risk map” was developed which represents associations between possum abundance and environmental factors. This has the potential to refine vector management in other parts of New Zealand by identifying the specific areas and habitats requiring targeted control of possums.
Despite having the lowest overall prevalence of any of the main hosts sampled, possums are still the main reservoir of TB on Molesworth Station. Possum control alone provided the greatest reduction in TB in sentinel species, compared with the targeting pig or ferret populations individually. Pig-to-pig transmission may occur occasionally on Molesworth Station but not at a level able to independently sustain the disease.
The data on dispersal patterns and seasonal movements of possums suggest that such movements are unlikely to compromise any large-scale control efforts through reinvasion or transport of TB in the landscape. Ferret movements are more likely to provide some low level of risk in spreading the disease from catchment to catchment, but the risk of ferrets transmitting TB to free-ranging cattle on Molesworth Station is likely to be minimal.
On the basis of this study, managers could reliably exclude 11% of the area of the NSIHC from possum control, because possum abundance is too low to present any risk of maintaining TB. For the remainder of the area, there are a range of possible scenarios. At one extreme, control of the 13% of the entire region with the highest possum densities should have the greatest impact on reactor rates for lowest cost, but is unlikely to eliminate ongoing recurrence of infection in cattle. At the other extreme, 87% of the area could be controlled along with pigs and ferrets to rapidly eliminate TB, but the cost is likely to be unaffordable.