The culprit is chlamydia, a sexually transmitted bacteria that infects more than 100 million people worldwide annually and can cause infertility in humans if left untreated.
For koalas, uncontrolled chlamydia can cause blindness and painful cysts in a animal’s reproductive tract that may lead to infertility or even death.
Worse still, antibiotics used to treat the disease can destroy the delicate gut flora koalas need to consume their staple diet of eucalyptus leaves, leading some to starve to death even after being cured.
The disease can also spread quickly.
In 2008, there was a “very, very low chlamydial prevalence” – about 10% – in the koala population in Gunnedah, a rural town in northeast New South Wales, according to Mark Krockenberger, a professor of veterinary pathology at the University of Sydney.
By 2015, that figure had risen to as high as 60%. Now, about 85% of that koala population is infected with the disease, Krockenberger said.
“If you think about it, that’s not a viable population anymore because of infertility. Pretty much every female that’s infected with chlamydia becomes infertile within a year, maybe two years maximum … Even if they survive, they’re not breeding,” he said.
Experts say situations like that in Gunnedah are playing out among koala populations across Australia, threatening populations already vulnerable to worsening bushfires and habitat loss due to deforestation.
Koalas may appear, at first glance, to live lives of leisure. They sleep upwards of 20 hours a day, and spend many of their waking hours sitting in the treetops munching eucalyptus leaves.
But in reality, it’s not easy being a koala. Since 2022, they have been listed as endangered in eastern Australia. And an Australian government inquiry last year found that without urgent action to protect them, they could become extinct in New South Wales by 2050.
They face habitat loss from development and wildfires. They’re often hit by cars or attacked by dogs.
But one of their greatest threats is chlamydia, which can lead to blindness, widespread infertility, and fatal kidney failure.
In New South Wales alone, about 80 per cent of koalas are infected with the disease, according to Mathew Crowther, a conservation biologist at the University of Sydney who monitors the condition in koala populations.
The origins of chlamydia in koalas aren’t confirmed, but many scientists suspect they initially caught it from exposure to the feces of infected sheep and cattle. Now it spreads sexually, or passes from mother to offspring.
But their unique biology makes them resistant to treatment.
“Koalas are exceptionally good at detoxifying agents, including antibiotics. So we have to give up to four times the dose of what we would for a normal animal, and prolonged doses,” Phillips said.
So instead of just trying to cure chlamydia in sick koalas, scientists are now trying to prevent it in healthy ones. And they’ve already had some luck.
Vets at wildlife rescue centres in Queensland have already successfully tested the single-shot vaccine on a few hundred koalas who were brought in by other afflictions, Phillips said.
But that approach has limits. Because the critters were often treated for multiple conditions, Phillips says it’s hard to properly monitor the vaccine’s effectiveness.
“Just vaccinating koalas [brought into] wildlife hospitals only results in about two per cent of the koalas being vaccinated,” he said. “And we estimate that we need to get at least 20 to 40 per cent of the population of koalas to be effective at lowering disease.”
So this time, they’re aiming to dose half the koalas in Northern Rivers region of New South Wales. That’s about 50 animals.
Researchers use binoculars to spot koalas in eucalyptus trees, then construct circular enclosures around the tree bases with doors leading into cages. After a few hours or days, the koalas will eventually climb down from one tree to seek tasty leaves on another, and wander into the harmless traps.
After a check-up to make sure the animals are in good condition, researchers administer anesthesia, vaccinate them, then keep them under observation for 24 hours after they wake up, to confirm there are no unexpected side effects.
Each vaccinated koala is marked with pink dye to ensure they’re not trapped more than once, as well as as microchips and ear tags, to track their progress long term.
“We’ve done a little bit of modelling to show that hopefully, if we vaccinate 50 per cent of the population, we can achieve a 70 per cent reduction in the chlamydia infection rate within these koalas,” Phillips said.
