Stress and disease create a dangerous mix for koalas

Koalas face two pressures that often strike at once: a virus that lingers in their blood and a bacterial infection that can leave them blind or sterile. A recent study followed 67 koalas in Queensland and New South Wales to examine how stress hormones, virus levels, and infections interact over time.

The results reveal a clear pattern. Koalas with higher average virus loads were more likely to develop disease, and their stress markers tended to rise as well.

Dr. Michaela Blyton of The University of Queensland (UQ) led the work with veterinarians at Currumbin Wildlife Hospital and the Port Macquarie Koala Hospital. The team tracked animals in the wild and in care, measuring hormones in feces and testing for infection.

Dual threats to koala health

Chlamydia pecorum can scar eyes and damage reproductive organs in koalas, and severe cases can be fatal. Documented outcomes include blindness, infertility, painful cystitis, and death if left untreated.

Koala retrovirus (KoRV), a gammaretrovirus, now embeds itself in many northern koala genomes while also circulating as infectious strains. Several KoRV subtypes exist, and the actively spreading forms can drive higher virus levels in blood.

A high viral load can undermine immune defenses. Researchers suspect this is the route by which KoRV increases the odds that Chlamydia will take hold.

The new paper tests that idea over time instead of at a single snapshot. It asks whether virus load rises when disease appears or whether certain koalas carry a consistently higher viral burden that leaves them vulnerable.

Tracking hormones, virus, and disease

The study measured fecal glucocorticoid metabolites (FGMs) as a noninvasive readout of stress and paired those measurements with viral load in plasma and lab tests for Chlamydia. Koalas have validated these hormone methods, which reflect integrated stress over the previous day.

Two hormones were central to the work: cortisol and corticosterone. Both rise with stress and can affect how the immune system performs.

Researchers followed three cohorts: wild koalas under routine tracking, wild koalas admitted with symptomatic infection for treatment, and a small group of nonreleasable koalas kept in care because of prior injuries or illness.

Koala stress rises with infection

“Our study showed KoRV loads within individual animals were very stable over time,” said Dr. Blyton. “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.”

Stress and virus load tended to rise together across the animals, showing a clear link. The researchers reported an R² of 0.27 (a measure that means about 27 percent of the differences in one factor can be explained by the other) between virus load and stress hormones in the healthy sampling windows.

Hormones boost viral activity

There is a plausible biological link between these two signals. The glucocorticoid receptor binds DNA at glucocorticoid response elements and can increase transcription of viral or host genes under some conditions.

The researchers identified a glucocorticoid response element in the KoRV genome. That means elevated cortisol could feasibly boost viral expression in certain cells.

“Poor quality or disappearing habitat may increase stress and the koalas with higher average cortisol levels had higher average KoRV loads,” said Dr. Blyton. Field stressors are part of the picture and must be considered.

Subtypes shape viral burden

KoRV exists in both endogenous and exogenous forms, and their balance differs by region. Prior work shows that exogenous subtypes can dominate circulating viruses.

Animals with a higher proportion of the endogenous KoRV-A sometimes show lower total load, which aligns with their subtype profiles.

These differences could shape risk within and between populations. They also suggest that genetics and maternal transmission early in life help set an individual koala’s long-term viral burden.

The future of koala care

“When we look at koala conservation strategies, we need to take a holistic approach because disease and environmental factors are linked,” said Dr. Blyton. If a high KoRV load raises the odds of disease, then conservation must address both infection and stress.

Preserving high-quality habitat is the most effective way to safeguard koalas, since strong populations depend on the health of the environments that support them. Habitat remains the first line of defense.

Medical tools are expanding as well. Australia recently approved the first national vaccine to protect koalas from Chlamydia, and field results suggest it could reduce mortality among wild koalas.

Targeted screening could further improve outcomes for joeys. The team notes that selecting parents with lower KoRV loads for breed-to-release programs might lower disease risk in offspring.

Next steps for koala health

Observational studies in threatened wildlife must respect ethical and practical constraints. That limits the ability to run controlled experiments on stress and infection, so careful time-series analyses like this one carry extra weight.

Two questions now stand out: Why do some koalas carry a persistently high viral load, and can antiretrovirals lower it safely enough to improve survival and fertility over the long run?

The study is published in the Journal of General Virology.

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