No abstract

Over a 12-month period, 1467 brushtail possums were trapped and then killed or were poisoned along sampling lines on pasture-forest margins and in nearby forest. An additional 334 possums were shot on adjacent farm land. All possums were necropsied, and 141 had macroscopic lesions typical of bovine tuberculosis. Infected possums occurred most commonly in small foci of 2-5 individuals along pasture-forest margins; the furthest from pasture that an infected animal was taken was 4050 m. Lesions occurred in possums of all ages in both sexes, but disease prevalence was highest in immature males and during autumn and winter. Infected possums bred as successfully as non-infected individuals, but were of lighter body weight. Macroscopic lesions occurred in a wide range of visceral and lymph node sites. Lesions were most common in the lungs and axillary nodes, and occasionally formed open sinuses from which infected possums could shed M. bovis readily, and thus were likely to be highly infective. Evidence suggested that disease transmission occurs most frequently on pasture-forest margins, where possum numbers are greatest and possum and cattle mix most freely.

Four control operations, each with paired pre-feed and no-pre-feed treatments, and comprising two baited with carrot and two with cereal, provided strong support for the hypothesis that pre-feeding produces higher possum kills than no pre-feeding. In these replicates, even where high kills were achieved, the modelled two-to three-fold extension of the period of population depression given by pre-feeding was sufficient to warrant the expense of the additional bait sown. The benefits of pre-feeding were clear-cut, and were greatest where poorest kills were achieved, irrespective of bait type. Pre-feeding is thus likely to reduce the probability of failure and should be considered a form of operational insurance.

Mycobacterium bovis was cultured from nine of 25 (36%) tracheal washings but not from any of 38 urine and 38 faecal samples from tuberculous possums cross-sectionally sampled from the wild. One of three tracheal washings, one of three urine samples and one of three faecal samples from terminally ill possums were culture-positive. The respiratory route is implicated as the major route of excretion of Mycobacterium bovis from naturally infected possums in horizontal transmission. Tuberculosis was observed in two young possums and was evidence of probable pseudo-vertical transmission via the respiratory route or ingestion of milk. Discharging fistulae were present in 22 of 71 (31%) cross-sectionally sampled tuberculous possums and were associated with relatively advanced disease. Although the frequent involvement of superficial lymphocentres in early stage disease could not be explained satisfactorily, the respiratory route was implicated as the main route of infection from indirect evidence.

Three years after a population of brush-tailed possums (Trichosurus vulpecula Kerr) in 100 ha of podocarp/mixed hardwood forest in Westland had been trapped to extinction, the study area had been recolonised and the population density was lowest in the alpine forest ( > 600 m) and highest in the forest which bordered improved pasture ( < 400 m). Animals dispersed upwards from this low-altitude forest and laterally into the area along the altitude gradient. Few adults entered the area, and immature (8-18 months old) males constituted the largest dispersive class. Sex-specific and age-specific dispersal rates led to an unstable age structure in the post-kill population. A higher proportion of breeding females and good condition in the post-kill population favoured an accelerated recovery of the population in the area.