Control of Endemic Bovine Tb in New Zealand Possum Populations: Results From a Simple Model

(1) Detailed formulae, and approximate ones based on disease reproductive rates, are derived for the rates of harvesting, sterilization and vaccination required to eliminate Tb from a possum (Trichosurus vulpecula) population, assuming either densitydependent recruitment or density-dependent mortality, asymmetric population growth and pseudo-vertical disease transmission. (2) The threshold density of susceptible possums for disease elimination is about half the uncontrolled population density, achieved by harvesting or sterilizing an annual proportion of the population approximately equal to r (0-2-0.3) or immunizing an approximate annual proportion 3r/2. The required immunization rate for disease elimination is highly sensitive to the nature of host density dependence. (3) Simulation of realistic control policies over time, and including the costs of implementation, suggests that the most cost-effective options currently available are widespread poisoning with 1080 every 6 years, a single intensive poisoning operation followed by less intense ground control, or a poisoning operation followed by subsidized and carefully targeted trapping effort. (4) Controlled areas must be large (>50km2) to prevent breakdown of control through immigration from surrounding diseased populations. Such immigration may preclude the possibility of total disease eradication in many areas but not its reduction to extremely low levels, given indefinitely repeated control. (5) The model suggests the need for research into control of reproduction, preferably using microbiological agents, as the best option for longer-term management both of the disease and of the possum as a pest of New Zealand's native forests. (6) The results of the model form the basis for current control policies, already implemented in response to an urgent need. At the same time specific recommended regimes are being rigorously tested in the field.