Koala retrovirus genotyping analyses reveal a low prevalence of KoRV-A in Victorian koalas and an association with clinical disease
Legione, AR, Patterson, JLS, Whiteley, P, Firestone, SM, Curnick, M, Bodley, K, Lynch, M, Gilkerson, JR, Sansom, FM & Devlin, JM 2017, Journal of Medical Microbiology, vol. 66, pp. 236-244.
The endogenisation of koala retrovirus (KoRV) has resulted in 100% prevalence of KoRV in koalas from New South Wales and Queensland. In contrast, this study found that there was a 24.7% prevalence of KoRV in koalas from Victoria. Additionally, the KoRV provirus was twice as likely to be detected in koalas exhibiting clinical symptoms of KoRV, such as ‘wet bottom’, than in koalas without symptoms.
Quantitative polymerase chain reaction (qPCR) was used to detect and genotype KoRV in samples taken from Victorian koalas. Out of 648 koalas tested, 141 were positive for KoRV-A, while the genotype of KoRV detected in 19 koalas was undetermined. KoRV-B was not detected in any of the koalas in this study. Further univariable analyses revealed that koalas with ‘wet bottom’ were at least twice as likely to be KoRV positive.
The relationship between clinical disease and presence of KoRV has been difficult to study in koalas from regions with 100% KoRV prevalence. In this study, the comparison of clinical disease and KoRV presence identified the possibility that KoRV may contribute to disease symptoms, potentially due to immunosuppression or another unknown mechanism. It is possible that the methodology employed in this study was unable to detect KoRV-B due to variation of the genotype, which could result in nucleotide changes in the primer binding region, or undetectable changes outside of the primer targets. As such, the authors suggest a further study involving deep sequencing of samples from Victorian koalas to validate the absence of KoRV-B observed in this study. KoRV-B is transmitted from mother to young, which could explain the contrast of KoRV-B prevalence in captive and free-ranging koalas. In captivity, the captive breeding and close quarters of koalas may have amplified the prevalence of KoRV-B, resulting in the increased observation of neoplasia. Conversely, there is a low incidence of free-ranging koalas affected by neoplasia, suggesting that KoRV-B is a rare genotype or that the neoplasia phenotype is caused by other pathogens, such as herpesviruses.
In conclusion, as Victorian koalas have lower KoRV prevalence compared to their New South Wales or Queensland counterparts, they may serve as appropriate candidates for the re-establishment of free-ranging and captive koala populations as part of ongoing conservation efforts. However, accurate and quick diagnostic tests for the presence of KoRV and C. pecorum will need to be developed and implemented to avoid lengthy quarantine periods before the koalas can be released into their new environments.
Summarised by Daniel Chew
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