Pests and Pestilence by Phil Lester

Author:Phil Lester
Language: eng
Format: epub
Publisher: Victoria University Press
Published: 2022-07-15T00:00:00+00:00

Fig 4.6: The number of cattle herds in New Zealand infected by bTB has been declining, indicating that the controls and eradication programme may be working. There are, however, still infections occurring, even in ‘Vector Free Areas’ where possum control is undertaken.

To understand the high infection rates on the West Coast, a small experiment was performed. A herd of just 29 calves was grazed in a paddock known to be in an area with possums that had high rates of bTB infections. After six months, 16 of the 29 calves were infected and had ‘gross lesions’. Possums were implicated as disease spreaders. That small experiment has gone on to influence bTB management around the world.

A following experiment culled possums, resulting in a drop of bTB infections in the local cattle population from 16% to 4% after three years.39 The possums interact with cattle by foraging, urinating and defecating in the same fields, or by inquisitive cattle displaying an interest in dead possum carcasses.

bTB is now under an eradication plan in New Zealand, which is enforced under the Biosecurity Act 1993, called the National Bovine Tuberculosis Pest Management Plan.40 The plan costs approximately NZ$80 million per year and is funded by both the government and groups including dairy farmers. Key management practices involve the testing and management of diseased cattle and deer, movement restrictions of infected herds and from areas with high bTB infections, and ground and aerial operations to reduce pest numbers (primarily possums) which carry and spread TB to farmed animals. Rates of infection have dropped dramatically under this eradication plan. So far, bTB appears to have been eradicated from nearly 1.6 million hectares, and the number of infected herds has reduced to 43. The goals are now to eradicate bTB from cattle and deer herds by 2026, from possums by 2040, and from New Zealand by 2055.41

This case highlights the role that alternative hosts, and their management, can play in disease dynamics and eradication. Statistical analysis and modelling have predicted that possum control will eradicate TB from possum–deer–pig host communities in forest habitats. But, in grassland ecosystems, TB is predicted to persist in the ferret–pig host complex, even in the absence of possums, potentially jeopardising the effectiveness of possum-only control programmes.42

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