Chlamydophila and koala retrovirus (KoRV) infection in southern populations of koalas (Phascolarctos cinereus)

© 2012 Jade L. S. PattersonInfection with Chlamydophila has significant impacts on koala populations in Queensland and New South Wales. The factors that determine the prevalence of clinical disease resulting from infection are not well understood. Important variables may include the chlamydial strain involved, co-infection with koala retrovirus (KoRV) and environmental stressors. Knowledge of the epidemiology of Chlamydophila and KoRV infection in Victorian koala populations is limited compared to what is known for northern populations. This project produces a comprehensive examination into Chlamydophila and KoRV infection in 287 free-ranging koalas at three geographically distinct Victorian sites. Detection of Chlamydophila utilised real-time PCR and high-resolution melt curve (qPCR-HRM) technology. Detection of KoRV utilised qPCR. The results from this study showed the presence only of Chlamydophila pecorum (not Chlamydophila pneumoniae) in the Victorian koalas, and only in two of the populations (Raymond Island and Mt Eccles National Park). In these two populations Chlamydophila pecorum was detected only from swabs collected from the urogenital sinus. Chlamydophila was not detected in any conjunctival swabs. The third population (French Island) was determined to be free from Chlamydophila infection confirming previous research findings. Interestingly the prevalence of ‘wet bottom’ (indicating chronic urinary tract disease) was similar between all three populations. For the Chlamydophila-positive populations, significant associations between Chlamydophila infection and both ‘wet bottom’ and abnormal urogenital ultrasound findings were found. Significance for each association, however, depended on whether these populations were grouped or considered separately. There was no association found between abnormal health status, age, sex, presence of young, enlarged lymph nodes, reduced body condition score or reduced gut fill and Chlamydophila infection. The findings of this study suggest that although Chlamydophila infection is associated with wet bottom there is another significant cause (or causes) of chronic urinary tract disease in Victorian koalas. Future studies to investigate these other causes are warranted. The findings also confirm that there is no reliable clinical sign that can be used to detect Chlamydophila infection, as many infected animals appear to be asymptomatic. Results from this study support conclusions from studies in northern koala populations that vertical transmission and horizontal transmission both play an important role in spread of disease, male and females show equal prevalence of infection and infection rates have a variable effect on fecundity rates. KoRV infection status was assessed using quantitative real time PCR (to detect KoRV genomic RNA in plasma samples) and conventional PCR and quantitative PCR (to detect proviral DNA in white blood cells). The quantitative real time PCR was performed using previously published protocols with some modifications that allowed the assay to use SYTO9 double-stranded DNA binding dye, rather than a fluorogenic hydrolysis (TaqMan) probe. This modification brings cost advantages. KoRV infection was found to present in all three Victorian koala populations. The prevalence of KoRV infection was similar between the three populations (33.3 – 38.1%) but was considerably lower than the 100% KoRV infection prevalence detected in studies on northern koalas. In contrast to studies on northern populations, only 16% of animals that were proviral positive for KoRV were viraemic. There were also a small proportion of animals that were viraemic without testing positive for KoRV proviral DNA. The results from this study are consistent with the hypothesis that KoRV may be in the process of endogenising in these southern populations. Significantly, this study did show a positive correlation between KoRV infection and presence of wet bottom in the populations studied. This study did not detect a significant association between KoRV infection and Chlamydophila infection. Nor did it find any significant association between plasma viral load and the presence of proviral DNA or wet-bottom. This study is the largest of its’ kind to date in either northern or southern koalas. It contributes substantial knowledge and understanding of Chlamydophila and KoRV infection in southern koala populations. It is anticipated that the results from this study will not only help to inform future research directions but will also provide important data that will assist sustainable management of southern koala populations into the future. It highlights some noteworthy similarities and differences in both diseases between northern and southern populations of koalas and may provide a valuable insight when considering management of endangered northern koala populations