Infection dynamics and disease prevalence of Chlamydia pecorum in koalas across two geographically separated habitats in Southeast Queensland

The koala, an endangered species in some parts of Australia, faces significant population decline, driven partly by Chlamydia pecorum , a pathogen causing blindness, infertility, and mortality. This study investigated C. pecorum infection dynamics in 285 koalas from Southeast Queensland, from two neighbouring study regions separated by a river into northern (n=147) and southern study regions (n=138) . Ocular and urogenital swabs were screened for C. pecorum presence via quantitative PCR, while age, sex, body condition, and clinical signs, were assessed through veterinary examination. Multivariable logistic regression models were used to identify predictors of infection and disease. C. pecorum was detected in 32% of koalas, with an overall disease prevalence of 24%; notably, 57% of infected individuals exhibited clinical disease. Infection and disease prevalence were significantly higher in the northern study region (46% and 25%) compared with the southern (17% and 11%). Infection prevalence was more frequent in adult males than females (54% vs 37%). Due to the inability to reliably diagnose male reproductive tract disease, sex was excluded from disease models to avoid bias. Age and geographic location were significant predictors of infection, and age, body condition, and infection status predicted clinical disease. Among infected koalas, no single host, pathogen, or environmental factor significantly predicted progression to clinical disease. These findings highlight key epidemiological patterns of C. pecorum in wild koalas, demonstrating that even geographically close subpopulations can exhibit markedly different infection and disease dynamics, and thus require region-specific management strategies to support long-term koala conservation.