Abstract
Chlamydial disease threatens many of Australia’s koala populations, and yet our understanding of chlamydial epidemiology and disease dynamics in koalas is limited by a lack of comprehensive, longitudinal population studies. To address this, we utilised longitudinal samples from a large-scale population study of wild koalas in south-east Queensland, to follow chlamydial infections over time and to investigate some of the drivers of disease progression. Our findings show, firstly, that almost two thirds of chlamydial infections progressed to disease, challenging the notion that chlamydial infections in koalas commonly remain chronic and asymptomatic. Secondly, disease progression at the urogenital tract site was associated with infection load, and urogenital tract shedding was significantly higher when koalas acquired a new infection. Thirdly, chronic chlamydial exposure was not necessary for pathogenic sequelae to develop, such as infertility and mortality. Fourthly, ompA-characterised strain sub-types may reflect tissue tropisms and pathogenicity, and the chlamydial status of some chronically infected koalas may be explained by reinfections with novel genotypes. Finally, successful antimicrobial treatment provided only short-term protection against reinfection and disease progression in susceptible koalas. These findings highlight the importance of identifying and preventing chlamydial infections in koalas, informing new population management strategies and research priorities.
Highlights
Australia’s northern koala populations are being pushed towards extinction by ongoing habitat degradation due to urban and agricultural development[1], and as they decline, the threat of chlamydial disease is likely to become more significant
Some northern Australian koala populations are declining towards extinction due to threats that include habitat degradation and chlamydial disease[1,2,3]
Due to the paucity of comprehensive, longitudinal population studies[9,11], largely resulting from historical limitations in the diagnosis of chlamydial infection and disease and the expense associated with such research, this knowledge is lacking
Summary
Australia’s northern koala populations are being pushed towards extinction by ongoing habitat degradation due to urban and agricultural development[1], and as they decline, the threat of chlamydial disease is likely to become more significant. Suggested drivers of chlamydial disease progression in koalas include several pathogen-associated factors, such as the chlamydial strain and infection load. The former study included data from hospitalised koalas, which may have biased the findings These pathogen-associated factors are likely to be important in chlamydial disease progression, our understanding of their role is clearly limited, and chlamydial pathogenesis may be dependent on several host-associated factors. While there was a higher prevalence of chlamydial infections in overabundant Victorian koalas residing in a highly disturbed environment, this was not true for chlamydial disease[9] The interactions between these host and pathogen-associated drivers of chlamydial disease progression are clearly complex, and further research is needed to determine the relative contribution of each factor. To develop an accurate and thorough understanding of chlamydial epidemiology and disease dynamics in koalas, comprehensive, longitudinal population studies are required
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