Bovine Tuberculosis in Brushtail Possums (Trichosurus vulpecula): Studies on Vaccination, Experimental Infection, and Disease Transmission

The objectives of the research program were to obtain a better understanding of BCG as a tuberculosis vaccine in possums, and assess its potential as a tool for controlling tuberculosis in wild possum populations. A series of vaccination and challenge experiments were conducted, as well as studies on alternative experimental infection procedures. The program included two field studies, one on the epidemiology of tuberculosis in a population of possums regenerating after localised possum eradication, and the other examined the efficacy of BCG vaccine in a wild population in which tuberculosis was endemic. The first experiments confirmed the earlier published findings that BCG delivered as an intranasal aerosol induced a protective response. The protective response was found to be present 12 months after vaccination and therefore of sufficient longevity to make vaccination a practical control tool. A second study demonstrated that revaccination of possums enhanced protection and a third showed that conjunctival vaccination was as effective as intranasal aerosol. These findings supported the development of a possum activated self-vaccinator that would deliver vaccine as an aerosol. In delivering the spray to both the external nares and the eyes a simple and cheap device could be designed to efficiently vaccinate wild possums. The intratracheal experimental infection procedure used in the vaccination and challenge experiments was not entirely suitable for our purposes. Although it provided an assured level of exposure and repeatable results, all infected possums developed fulminant, rapidly progressive disease, irrespective of the vaccination regime used. Two alternative methods of challenge were examined; the conjunctival route of infection, and natural transmission between experimentally infected possums and susceptible in-contact possums. Conjunctival infection was shown to be a reliable procedure for infecting possums, with the disease that resulted from infection having many of the cardinal features of natural tuberculosis in wild possums. Infection following conjunctival inoculation progressed slowly and may be suitable for studying pseudo-vertical transmission and the efficacy of post-infection vaccination. In studies with captive possums there was little or no transmission of infection between experimentally infected possums and susceptible in-contact possums in the same pen when