Australian brushtail possum: a highly susceptible host for Mycobacterium bovis

The Australian brushtail possum (Trichosurus vulpecula) is highly susceptible to Mycobacterium bovis infection and serves as the principal wildlife reservoir for M. bovis infection in New Zealand. Following their introduction from Australia in the mid-1800s to establish a fur trade, possums spread throughout New Zealand and are now a noxious pest. It is now well recognized that possums cause extensive damage to crops, forests and native birdlife as well as serving a vector for the spread of bovine tuberculosis (TB) to both cattle and farmed deer. Epidemiological investigations have estimated that the majority of new breakdowns of TB in cattle and farmed deer herds in New Zealand can be attributed to direct or indirect contact with infected possums (Hutchings et al., 2013). The TB–possum problem emerged in the late 1960s (Ekdahl et al., 1970) and lethal control of possum populations undertaken since 1994, currently costing the equivalent of more than US$40 million per year, has been a major contributor to the >95% reduction in the number of M. bovis-infected cattle and farmed deer herds (Hutchings et al., 2013). Infection is fulminating and rapidly lethal in possums, with lesions found predominantly in the lungs and superficial peripheral lymph nodes. The terminal stage of the disease is reached by 2 months following intratracheal delivery of a very low dose of M. bovis (Buddle et al., 1994) and most possums die from natural infections within 6 months (Jackson et al., 1995a; Nugent et al., 2013). Valuable information can be obtained from M. bovis infection in possums as to why this species is so susceptible to a fulminating mycobacterial disease and, second, how the disease can be controlled by vaccination. Other wildlife species such as badgers in the UK and Ireland; white-tailed deer in northern Michigan, USA; and wild boar in Spain also serve as wildlife reservoirs of M. bovis for reinfection of domestic livestock (de Lisle et al., 2001) and the development of an oral bait TB vaccine for possums has application for these other animal species. Comparison of the efficacy of BCG vaccine against experimental and natural infection of M. bovis in possums provides a useful insight into the extrapolation of findings from experimental challenge studies to the efficacy of TB vaccines in the field. A summary of the unique features of M. bovis infection in the possum is shown in Table 18.1.