The effect of manipulating population density on the probability of den-sharing among common brushtail possums, and the implications for transmission of bovine tuberculosis

Common brushtail possums (Trichosurus vulpecula) act as a reservoir of bovine tuberculosis (Mycobacterium bovis) in New Zealand and the simultaneous sharing of dens may result in the transmission of Tb between possums. The effect of manipulating population density on the per capita probability of simultaneous den-sharing among possums was investigated at a site near Dunedin, New Zealand. Den characteristics that could affect the probability of simultaneous sharing were also investigated, though none were found to be significant. The daily probability of a possum sharing a den was estimated to be 0.07 for possums denning within uncontrolled areas of the study area. Den-sharing was most common between female pairs, though sharing was also recorded between male–female and male–male pairs. The highest number of possums recorded sharing a single den was four. Reducing population density significantly lowered the probability of possums simultaneously sharing dens within the study area, with greater than 60% reductions estimated to eliminate simultaneous den-sharing altogether. The relationship between the contact rate arising from den-sharing and population density was convex-down, rather than convex-up, as often hypothesised for animal–animal disease contact processes. The implications of simultaneous den-sharing for the transmission and control of bovine tuberculosis in brushtail possum populations are discussed.