Epidemiology of chlamydial infection and disease in a free-ranging koala (Phascolarctos cinereus) population
Nyari, S, Waugh, CA, Dong, J, Quigley, BL, Hanger, J, Loader, J, Polkinghorne, A & Timms, P, 2017, PloS One, vol. 12, no. 12, e0190114.
The aim of this study was to analyse the epidemiology of infection and disease caused by Chlamydia in a koala population from Queensland. It was found that, out of 160 koalas tested, 31% were positive for Chlamydia and 28% exhibited clinical signs of chlamydial disease, with most infections observed at the urogenital and ocular sites.
Polymerase chain reaction (PCR) was used to detect Chlamydia DNA from swabs taken from conjunctiva and urogenital regions of the koalas. Interestingly, 29% of koalas below two years of age were found to be infected by Chlamydia pecorum, with an upwards trend in prevalence of infections with age, as 58% of koalas aged 3 years and above were infected by C. pecorum. In comparison to the ocular site, the urogenital tract (UGT) was found to have a higher prevalence of infection in both sexes, consequently resulting in higher prevalence of UGT disease than ocular disease. Additionally, koalas with clinically observable symptoms of chlamydial disease had lower chlamydial loads than koalas without. Sequencing of the ompA genes found that C. pecorum genotype E’ was the most prevalent, followed by genotype G and genotype F. Further analysis suggested that genotype E’ was in fact more likely to cause disease than the two other genotypes.
This study presents strong evidence supporting the mother-to-young C. pecorum transmission hypothesis as an additional route of transmission to sexual contact, as young sexually immature koala joeys were observed to be infected by C. pecorum. Furthermore, ocular infection appears to be the more common route of infection from mothers to joeys which may occur during birth. Chlamydial load alone does not drive the pathogenesis of chlamydial disease, and is suggested to instead work in tandem with koala retrovirus (KoRV) infection. However, the presence chlamydial disease symptoms in a koala without testing positively for C. pecorum infection may be an indication that the koala has resolved the C. pecorum infection while the physical effects remain. As C. pecorum genotype E’ was identified as having the highest risk of causing clinical disease in the koalas, it could be an important target to consider for development of vaccines against C. pecorum.
This study has produced important data about the prevalence of C. pecorum infection in a wild koala population in Queensland, which are particularly informative for management and vaccination development studies. While this study could be used as model for further studies of population epidemiology, it should be noted that the study was limited by the inability to sample all koalas at a single point in time due to the large population size.
Summarised by Daniel Chew
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