科研之友
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摘要
1. A mixed deterministic/stochastic model is described for the epidemic spread of bovine Th in New Zealand possum populations. The model is a modified SI one with birth and dispersal pulses, equations replicated over a spatial grid and with disease transmission between as well as within grid squares.
2. An explanation is proposed for the assumption in this and previous models, based on observations, that the disease contact rate (potentially infectious contacts per infectious animal per year) is independent of the equilibrium density (K) but is dependent on manipulations of density below K. The explanation is the dependence of contact rate on the extent of den site sharing and use. Since K largely reflects that abundance of den sites in the habitat, den site use per possum, hence contact rate, is independent of K.
3. The predicted rate of spread of the disease was sensitive to the assumptions about endemic disease behaviour. It varied from 0.6 km year-1 for a chronic disease with long infectious period to 3 km year-1 for an acute disease with short latent and infectious periods.
4. The model suggests that a 3 km 'buffer zone' around an endemic area, in which possums are effectively eliminated, reduces the number of diseased possums leaving the zone by about 70%. However, in practice and with the less intensive control currently recommended, a buffer zone alone has little ultimate effect on disease spread unless its width approximates the maximum likely juvenile dispersal distance (12 km). This is because some infected juveniles cross the buffer, and because the remaining possums within the buffer allow the presence of a stable level of Th in the face of immigration of diseased animals.
5. Control regimes were based on an earlier model for endemic disease, involving a 75% kill followed by a 50% kill when possum densities reached 40% of their pre-control level. Combining a buffer zone, routinely controlled in this way, with localized control operations, in which possums are controlled for 5 years beyond the buffer zone where cattle test positive for Tb, successfully prevents the spread of disease if the probability of possum-to-cattle transmission is sufficiently high. The most cost-effective option involved the widest buffer (12 km), giving an average of about 3 km2 controlled per year per km of the Th front. Localized control alone, without a buffer, was the most expensive option, requiring an average of 9-2 km2 of annual control. Increasing the follow-up kill from 50% to 75% and decreasing the threshold density from 40% to 25% of pre-control levels both increased the cost-effectiveness of control.
6. The level of control required to prevent Tb spread was sensitive to the possum carrying capacity (disease-free equilibrium density). At 3 ha-1 rather than 10 ha-1 the annual area controlled was reduced by 50%.
7. Possum control operations within an endemic zone must be carried out over a wide area (>50 km2) to prevent immigration of diseased animals from seriously reducing control effectiveness. Immigration per unit area is a non-linearly decreasing function of the total area controlled.
8. The model identifies a need for more data on possum/cattle transmission of disease and on short-range dispersal behaviour of adult possums.
- 出版日期1993
