Epizootiology of Chlamydia infections in two free-range koala populations
Jackson, M, White, N, Giffard, P & Timms, P 1999, Veterinary Microbiology, vol. 65, no. 1, pp. 255-264.
This study is the first to use DNA-based assay techniques to compare the prevalence and epizootiology of the chlamydial species infecting koalas in two free-ranging koala populations in Queensland.
Using genus-specific polymerase chain reaction (PCR) and species-specific DNA probe hybridisation methods, researchers quantified and compared the prevalence of Chlamydia pecorum and Chlamydia pneumoniae infections in two separate koala populations south of Brisbane. The populations, Mutdapilly and Coombabah, are 65 kilometres apart. The Mutdapilly population, despite having an 85% infection rate, had a rate of outward clinical disease of only 17%. C. pecorum caused 73% of infections, while only 24% were attributed to C. pneumoniae. All cases of clinical disease were associated with C. pecorum infection and never with C. pneumoniae. It seems, therefore, that C. pecorum is the more pathogenic of the two species. Females in the Mutdapilly population were 2.5 times more likely to have a C. pecorum infection than males. In comparison, only 10% of the Coombabah population had chlamydial infections, and these were equally likely to be of C. pecorum or C. pneumoniae. In both populations, however, C. pneumoniae infections were always low grade whereas C. pecorum infections ranged from low to high grade. Infection rates varied with age, with chlamydial infections detected in 58% of young, sexually immature koalas and 100% of older koalas.
Since it was established that the koala chlamydial strains C. psittaci Type I and II were, in fact, the different species C. pecorum and C. pneumoniae, this is the first study that compares the incidence and epizootiology of the two species in free-ranging koala populations. The authors suggest that the stark difference in infection rates between the two populations studied here relates to environmental or host factors rather than pathogen strain differences. They also suggest some possible infection transmission mechanisms based on the findings of this study. Given that infection rates increase from 58% in young to 100% in old koalas, it is likely that vertical transmission from mother to young is common when the joey is birthed or in the pouch. As koalas enter sexual maturity, infection levels may increase as a result of horizontal transmission via sexual contact. Female koalas may be more prone to infection than males as a dominant male carrier of C. pecorum can transmit the pathogen to many females, whereas a subordinate, non-mating male has fewer opportunities for horizontal transmission.
The findings of this study are valuable for understanding chlamydial infection dynamics in south-east Queensland koala populations and may inform strategies for infection and disease management.
Summarised by Joanna Horsfall
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