Oral vaccination reduces the incidence of tuberculosis in free-living brushtail possums

Abstract

Bovine tuberculosis (Tb) caused by Mycobacterium bovis has proved refractory to eradication from domestic livestock in countries with wildlife disease reservoirs. Vaccination of wild hosts offers a way of controlling Tb in livestock without wildlife culling. This study was conducted in a Tb-endemic region of New Zealand, where the introduced Australian brushtail possum (Trichosurus vulpecula) is the main wildlife reservoir of Tb. Possums were trapped and vaccinated using a prototype oral-delivery system to deliver the Tb vaccine bacille Calmette-Guerin. Vaccinated and control possums were matched according to age, sex and location, re-trapped bimonthly and assessed for Tb status by palpation and lesion aspiration; the site was depopulated after 2 years and post-mortem examinations were conducted to further identify clinical Tb cases and subclinical infection. Significantly fewer culture-confirmed Tb cases were recorded in vaccinated possums (1/51) compared with control animals (12/71); the transition probability from susceptible to infected was significantly reduced in both males and females by vaccination. Vaccine efficacy was estimated at 95 per cent (87-100%) for females and 96 per cent (82-99%) for males. Hence, this trial demonstrates that orally delivered live bacterial vaccines can significantly protect wildlife against natural disease exposure, indicating that wildlife vaccination, along with existing control methods, could be used to eradicate Tb from domestic animals.

Figure 1.

Trapped possums with (black) and without (grey) culture-confirmed Tb lesions in (a) control and (b) vaccinated animals, and number identified as infected for the first time at each trapping session (numbers above columns).

Figure 2.

Model-averaged estimates of the bimonthly probabilities (with 95% confidence intervals) of the bimonthly probabilities of transition from susceptible to Tb-infected states for male and female possums in the control and BCG-vaccinated groups.