Modelling Chlamydia-koala interactions: coexistence, population dynamics and conservation implications.
A model of koala (Phascolarctos cinereus) and Chlamydia psittaci population dynamics was developed to examine interactions between Chlamydia transmission and pathogenicity, koala mating behaviour and demography, and koala population persistence. Simulations based on sexual and parent-offspring parasite transmission demonstrate that stable Chlamydia-koala coexistence is possible in a small population for a broad range of demographic, behavioural, pathogenicity and transmission parameter estimations. Koala population persistence was most sensitive to reduced annual survivorship of adults (4- to 10-year-old males and 2- to 12-year-old females), highlighting the need for accurate field estimates of adult survivorship in order to assess Chlamydia's impact on specific populations. Extinctions are only predicted if Chlamydia transmission rates also increase (e.g. due to new transmission pathways or increased mating frequency), or other non-disease factors change birth and mortality rates to reduce the koala population's intrinsic rate of increase below 0.1. The most important predicted effect of habitat fragmentation and other forms of human disturbance on this unique host-parasite relationship is the extinction of Chlamydia in populations where koala resistance to disease decreases.