A conserved retrovirus, koala retrovirus (KoRV)-A, is present within the genome of koalas in most Australian populations. Additional divergent sequences and subtypes are thought to be exogenously transmissible and more pathogenic. We present a comprehensive analysis of KoRV genetic diversity within two captive koala populations and statistically significant evidence of exogenous transmission occurring primarily through maternal transmission. These findings suggest strategies which may help limit the spread of pathogenic KoRV subtypes. We raise the possibility that captive breeding programs could target use of antiretroviral drugs to dams during the breeding season. While substantial research is needed to demonstrate safety and effectiveness, this presents a potentially important conservation strategy for koala populations severely affected by disease and currently in rapid decline.
Koala populations are currently in rapid decline across Australia, with infectious diseases being a contributing cause. The koala retrovirus (KoRV) is a gammaretrovirus present in both captive and wild koala colonies that presents an additional challenge for koala conservation in addition to habitat loss, climate change, and other factors. Currently, nine different subtypes (A to I) have been identified; however, KoRV genetic diversity analyses have been limited. KoRV is thought to be exogenously transmitted between individuals, with KoRV-A also being endogenous and transmitted through the germline. The mechanisms of exogenous KoRV transmission are yet to be extensively investigated. Here, deep sequencing was employed on 109 captive koalas of known pedigree, housed in two institutions from Southeast Queensland, to provide a detailed analysis of KoRV transmission dynamics and genetic diversity. The final dataset included 421 unique KoRV sequences, along with the finding of an additional subtype (KoRV-K). Our analysis suggests that exogenous transmission of KoRV occurs primarily between dam and joey, with evidence provided for multiple subtypes, including nonendogenized KoRV-A. No evidence of sexual transmission was observed, with mating partners found to share a similar number of sequences as unrelated koala pairs. Importantly, both distinct captive colonies showed similar trends. These findings indicate that breeding strategies or antiretroviral treatment of females could be employed as effective management approaches in combating KoRV transmission.
Author contributions: M.D.J.B. and K.J.C. designed research; B.A.J. and M.D.J.B. performed research; B.A.J., M.D.J.B., and K.J.C. analyzed data; and B.A.J., M.D.J.B., S.D.J., P.R.Y., and K.J.C. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission. M.V.E. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.2024021118/-/DCSupplemental.
All sequences reported in this paper have been deposited in GenBank and assigned accession numbers MW283966–MW284386.
- Received July 8, 2021.
- Accepted May 18, 2021.
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