Koala Retrovirus Load Stays Stable And Signals Higher Risk Of Chlamydia Infection

By Pooja Toshniwal PahariaReviewed by Lauren HardakerOct 1 2025

In a recent study published in the Journal of Generic Virology, researchers traced koala retrovirus (KoRV) load alongside Chlamydia pecorum infection and fecal glucocorticoid metabolites to clarify their roles in disease susceptibility. They found that high KoRV loads remain stable over time, and koalas with consistently high viral loads are more likely to develop chlamydial infection.

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Elevated viral loads are associated with stress hormones, although this correlation was observed at the population (between-individual) level rather than within individuals, and stress hormones were not directly associated with chlamydial infection. These findings provide evidence that KoRV load is a key predisposing factor, but not yet definitive proof of causality.

Koala populations in New South Wales (NSW) and Queensland (QLD) are declining rapidly, primarily due to chlamydia infections and KoRV. While studies link high KoRV loads to chlamydial disease and poor condition, the causal relationship remains unclear. Since ethical and logistical constraints limit experimental work, longitudinal studies provide an essential way to evaluate changes in KoRV loads following chlamydial infection and stressful events, helping clarify temporal dynamics and supporting future conservation strategies for vulnerable koala populations.

About The Study

In the present temporal study, researchers investigated whether KoRV predisposes koalas to chlamydial infection, whether viral load increases in response to disease, or whether physiological stress influences these conditions.

The team sampled koalas across three cohorts. They monitored seven non-releasable exhibit koalas at Port Macquarie Hospital during routine health checks from May 2023 to February 2024. They tracked 38 wild-type koalas from southeast Queensland using VHF collars, sampling them every three to six months between December 2022 and April 2024 as a routine investigation into a chlamydia vaccination trial. In addition, they sampled 22 rehabilitated koalas (7 from Currumbin Wildlife and 15 from Port Macquarie) on admission, before antibiotic treatment, and again one to three times following recovery.

The researchers diagnosed chlamydial infection by loop-mediated isothermal amplification (LAMP) or quantitative polymerase chain reaction (qPCR) from ocular and urogenital swabs. They classified infections that did not present with clinical pathologies as asymptomatic. Symptomatic infections demonstrated pathology consistent with chlamydiosis (ocular, urogenital, renal, or reproductive), identified by loop‐mediated isothermal amplification (LAMP) and ultrasound.

The team analyzed blood samples for KoRV plasma ribonucleic acid (RNA) load via quantitative polymerase chain reaction (qPCR), and the env gene was sequenced to determine subtype expression. Fecal samples were collected concurrently to assess glucocorticoid function. Fecal cortisol and corticosterone metabolites were quantified using enzyme immunoassays, providing an integrated measure of stress hormone activity less affected by acute capture stress.

Statistical analyses included linear mixed-effects modeling to test temporal associations between KoRV load, chlamydial infection status, and glucocorticoid metabolites within and between individuals. Additionally, the researchers compared KoRV load before and after antibiotic treatment (doxycycline for two weeks or chloramphenicol for up to six weeks) to evaluate whether disease resolution influenced viral dynamics.

Results

KoRV loads remained largely stable among individuals across time in all cohorts. In wild koalas, KoRV load showed no consistent increase upon C. pecorum shedding by animals or decrease following treatment or clearance. In exhibit koalas, the average log₁₀ changes minimally, 0.3 over 5.3 months in healthy wild koalas and 0.47 over four months.

In the vaccination trial, wild koalas testing C. pecorum-positive had higher KoRV loads (mean, 6.3) than those who remained healthy throughout (mean = 6.01). Koalas undergoing treatment for chlamydiosis maintained elevated viral loads, suggesting that high KoRV predisposes individuals to infection rather than responding to the disease.

Initial loads were highest in koalas admitted for treatment (mean = 6.79), with successfully released individuals showing lower loads (mean = 4.51) than their counterparts who were deceased or euthanized (mean = 6.4). The authors also observed that KoRV load increased in treated koalas that survived rehabilitation but decreased in those that later died, highlighting complex patterns that require further study.

KoRV load varied seasonally, peaking during the breeding season, and regionally, with NSW exhibiting koalas showing lower loads than healthy wild QLD koalas. A higher expression of KoRV-A was associated with reduced KoRV RNA levels.

Higher average KoRV loads correlated with elevated fecal glucocorticoid metabolites (FGMs), indicating a potential interaction between viral replication and physiological stress. However, FGM levels were not consistently associated with chlamydial infection, and temporal changes in stress hormones within individuals did not explain intra-koala variations in KoRV load.

Fecal corticosterone decreased following antibiotic therapy for symptomatic chlamydiosis, while cortisol levels were variable. This suggests corticosterone may better reflect chronic stress reduction during treatment, while acute stressors more influence cortisol.

Conclusions

The study confirms that koala retrovirus load is a stable, individual-specific trait and a strong predictor of susceptibility to Chlamydia pecorum, rather than a viral response to infection.

Elevated KoRV loads were associated with consistent patterns of immunosuppression and elevated stress hormone levels, highlighting complex viral-host interactions that increase disease risk. However, the relationship between KoRV, immune suppression, and stress physiology remains to be mechanistically demonstrated.

Future research should focus on the mechanistic pathways connecting KoRV load, immune function, viral subtypes, and glucocorticoid physiology. These studies could identify mechanisms linking viral load, immunosuppression, and stress to improve wildlife conservation efforts aimed at maintaining ecological balance and biodiversity.

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Journal Reference

Michaela D. J. Blyton et al. (2025). Temporal dynamics of koala retrovirus plasma RNA load in relation to fecal glucocorticoid metabolites and Chlamydia infection, Journal of General Virology, 106:002147. DOI 10.1099/jgv.0.002147. https://www.microbiologyresearch.org/content/journal/jgv/10.1099/jgv.0.002147