Epidemiology of chlamydial infection and disease in a free-ranging koala (Phascolarctos cinereus) population

Abstract

Chlamydial disease continues to be one of the main factors threatening the long-term survival of the koala (Phascolarctos cinereus). Despite this, large epidemiological studies of chlamydial infection and disease in wild koala populations are lacking. A better understanding of the prevalence, transmission and pathogenesis is needed to improve control measures, such as the development of vaccines. We investigated the prevalence of Chlamydia pecorum infection and disease in 160 koalas in a peri-urban wild population in Queensland, Australia and found that 31% of koalas were Chlamydia PCR positive and 28% had clinically detectable chlamydial disease. Most infections were at the urogenital site (27%; both males and females) with only 14% at the ocular site. Interestingly, we found that 27% (4/15) of koalas considered to be sexually immature (9-13 months) were already infected with C. pecorum, suggesting that a significant percentage of animals are infected directly from their mother. Ocular infection levels were less prevalent with increasing age (8% in koalas older than 4 years), whereas the prevalence of urogenital tract infections remained high into older age (26% in koalas older than 4 years), suggesting that, after mother-to-young transmission, C. pecorum is predominantly a sexually transmitted infection. While 28% of koalas in this population had clinically detectable chlamydial disease (primarily urogenital tract disease), many PCR positive koalas had no detectable disease and importantly, not all diseased animals were PCR positive. We also observed higher chlamydial loads in koalas who were C. pecorum infected without clinical disease than in koalas who were C. pecorum infected with clinical disease. These results shed light on the potential mechanisms of transmission of C. pecorum in koalas and also guide future control measures, such as vaccination.

Fig 1

A: Prevalence of chlamydial infection, as assessed by C. pecorum-specific qPCR, in 160 koalas from a free-ranging population in South East Queensland. The prevalence of C. pecorum infection identified at the ocular, urogenital (UGT) or ocular plus urogenital (UGT) site based on qPCR C. pecorum load. B: Prevalence shown in panel A separated by sex.

Fig 2

A: Prevalence of C. pecorum infection in male versus female koalas between the ages of 9 months and 10 years based on being qPCR positive. B: Prevalence of C. pecorum infection per site of infection in female koalas aged between 9 months and 10 years based on being qPCR positive. C: Prevalence of a C. pecorum infection per site of infection in male koalas aged between 9 months and 10 years based on being qPCR positive.

Fig 3

A: Fit of the general linear model for the probability of disease with increasing age and separated by chlamydial genotype. B: Fit of the general linear model for the probability of being diseased with increasing age.

Fig 4

A: Prevalence of disease amongst male versus female koalas. The prevalence of disease identified at the ocular or urogenital tract site is based on veterinary examination. B: Prevalence of chlamydial disease in koalas between the ages of 9 months and 10 years based on veterinary examination.

Fig 5. Prevalence of chlamydial disease at the ocular and urogenital tract site in koalas residing in the end regions of our study, A versus F.