Australian scientists have uncovered a strong connection between stress and disease susceptibility in koalas across South East Queensland and the New South Wales North Coast.

The University of Queensland’s Dr Michaela Blyton led the research, which monitored koala retrovirus (KoRV) levels in both wild and captive animals.

“We wanted to see what happened to their KoRV loads over time and how it related to chlamydial infection and levels of the stress hormones cortisol and corticosterone in their faeces,” Dr Blyton said.

According to the findings, koalas with higher KoRV levels showed weakened immune systems, making them more vulnerable to diseases such as Chlamydia—a condition that can cause blindness, infertility, and even death.

Habitat Stress Intensifies Risk

The study also revealed how environmental stress contributes to the problem.
“Poor quality or disappearing habitat may increase stress and the koalas with higher average cortisol levels had higher average KoRV loads,” Dr Blyton explained.

She emphasized that the two most critical threats facing koalas—habitat loss and disease—are closely connected.
“We have confirmed the biggest threats faced by koalas of habitat loss and disease are connected – this is very important to know to improve support for remaining koala populations.”

Stable Virus Load Highlights Root Cause

The research involved 67 koalas, including wild individuals undergoing Chlamydia treatment, participants in a vaccine trial, and a small number of long-term captive animals.

“Our study showed KoRV loads within individual animals were very stable over time,” Dr Blyton said.
“Even when a koala with Chlamydia was successfully treated for that infection, their KoRV load did not decrease.
“That tells us the direction of causation is high KoRV load leading to an increased susceptibility to chlamydial infection and not the other way around.
“KoRV load could be underpinning the Chlamydia epidemic among koalas in northern New South Wales and South East Queensland.”

Conservation Strategies Must Be Holistic

The study was carried out at UQ’s School of Chemistry and Molecular Biosciences, supported by Port Macquarie Koala Hospital, Currumbin Wildlife Hospital, and funded through the NSW Koala Strategy.

“When we’re looking at koala conservation strategies, we need to be taking a holistic approach because disease and environmental factors are linked,” Dr Blyton said.
“The best thing is to preserve good quality habitat and the populations of koalas it supports.”

She also highlighted conservation programs already underway:

• Breeding for resilience: “We are involved in Koala Conservation Australia’s breed to release program, where we are now screening and preferencing animals with low KoRV loads to hopefully give their offspring greater protection against Chlamydia and other infectious diseases.”
• Medical interventions: “We are also aiming to investigate antiretrovirals to reduce KoRV loads in individuals to give them a better chance of fighting off Chlamydia and reducing transmission to the next generation.”
  

The full research has been published in the Journal of General Virology by the Microbiology Society.

Key Takeaways

• Stress and KoRV levels are directly linked to increased disease risk in koalas.
• High KoRV loads make koalas more vulnerable to Chlamydia, which can be deadly.
• Habitat loss elevates stress, indirectly raising KoRV loads.
• KoRV loads remain stable over time, showing the virus drives susceptibility rather than the reverse.
• Conservation efforts must integrate both environmental and medical strategies.
• Breeding programs now screen for lower KoRV loads to build disease resilience.
• Future research will explore antiviral treatments to control KoRV.

Original Publication
Authors: Michaela D. J. Blyton, Tamara Keeley, Lewis McKillop, Astrid Van Aggelen, Shali Fischer, Michael Pyne and Keith J. Chappell.
Journal: Journal of General Virology
DOI: 10.1099/jgv.0.002147
Method of Research: Observational study
Subject of Research: Animals
Article Title: Temporal dynamics of koala retrovirus plasma RNA load in relation to faecal glucocorticoid metabolites and Chlamydia infection
Article Publication Date: 22-Sep-2025
COI Statement: The authors declare that there are no conflicts of interest.