Two studies on the control of wildlife-derived tuberculosis : farmer views and model validation : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies in Epidemiology at Massey University, Palmerston North, New Zealand
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Date
2002
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Massey University
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Abstract
The two studies included in this thesis are part of a larger research programme
evaluating tools to aid in the eradication of bovine tuberculosis (TB) from cattle in New
Zealand. The first study was a survey of farmer’s attitudes towards the National Pest
Management Strategy (NPMS) and tuberculosis control. The second study validated the
population component of PossPOP, a spatial stochastic simulation model of TB in a
wild possum population.
A postal questionnaire survey identified variation of attitudes of 404 farmers in four
regions of varying TB status within New Zealand. Farmers were stratified by region,
enterprise type, area TB classification and herd TB status. Of the farmers contacted by
telephone prior to sending out the questionnaire 91% agreed to participate in the study
and 83% of these farmers returned a completed questionnaire. The questionnaire
contained questions on farmer demographics, TB status, herd TB history, farm
management practices and attitudes towards the control of TB. Farmers with infected
herds were generally positive about the control program and believed that TB could be
eradicated from their herds. A number of farmers were concerned about proposed
changes to the NPMS, such as the implementation of direct payment of TB testing costs
by farmers and removal of compensation for infected cattle. An important finding was
that the majority of farmers were not aware that the Animal Health Board was in charge
of the NPMS.
PossPOP was built using the first 22 months’ data from a longitudinal study of a
possum population run at Castlepoint in the lower North Island of New Zealand. Data
from the remaining 9 years of the study was used set for model validation. PossPOP
was validated by comparing age distribution, sex structure and the proportion of births,
deaths and immigrations in the modelled population against the field population. There
was general agreement between the model and the field population and also published
population patterns. PossPOP produced a stable population over time at different
densities, with similar temporal patterns to the field population. Emergent biological
properties were examined, such as rate of population rebuilding after a major
population cull, the removal of immigration from both populations and age specific
mortality. The field population grew much more rapidly following a cull compared with
the PossPOP population due to home range expansion of possums that were living on
the periphery of the study site, which was not programmed into the model. These
results showed that while PossPOP models a small area, it reflects patterns of control
over large areas making it a useful tool to evaluate large scale possum control
strategies.
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Keywords
Bovine tuberculosis, Cattle diseases, Possum, Trichosurus vulpecula, Brushtail possum