Among humans, chlamydia is the most common sexually transmitted infection in the world, according to the Pan American Health Organization. If left untreated, it can lead to serious complications, such as infertility or ectopic pregnancies — that's when a fertilized egg grows outside of the uterus. But uncomplicated cases of the infection are usually cured within days or weeks by taking antibiotics.
Again: That's in humans. But we are not the only ones susceptible to chlamydia.
Koalas, arguably among Australia's most famous animals, can contract the disease when they are exposed to the feces of sheep or cattle that have chlamydia.
Then the sexually transmitted disease is passed on from mother to child, or during mating. Once the marsupials have chlamydia, they usually fare far worse than humans.
"It's killing koalas because they become so sick they can't climb trees to get food, or escape predators, and females can become infertile," Samuel Phillips, a microbiologist at the University of the Sunshine Coast in Queensland, Australia, told news agency AP.
It's not just the risk of death that's concerning. For a shrinking population — the Australian Koala Foundation estimated in 2022 that there were fewer than 58,000 koalas left in the wild — infertility is just as dramatic a problem.
That's why Australian researchers have started vaccinating wild koalas against chlamydia in the state of New South Wales. The researchers want to see whether the specifically-developed one-shot koala vaccine can protect the animals against the dangerous disease, which causes blindness in addition to infertility and death.
A safe koala vaccine
The research team aims to vaccinate half the koala population in the Northern Rivers region of New South Wales, or about 50 animals. The single-shot vaccine has previously been used to inoculate a few hundred koalas that were brought to wildlife rescue centers.
So, scientists know it is effective and that it doesn't harm the animals. But they don't know how many koalas will need the vaccination for them to develop a group immunity.
"We want to evaluate what percentage of the koalas we need to vaccinate to meaningfully reduce infection and disease," Phillips said.
Capturing koalas to give them the shot
To get there, the researchers first need to capture their study participants. At first glance, that might not seem too difficult. If you've ever seen a koala, you will know they are not the most agile animal. But while they aren't prone to running away at lighting speeds, koalas do spend most of their time high up in eucalyptus trees.
That's where the researchers look for koalas. Once they've spotted one, they set up circular enclosures around the base of the tree, with doors that lead to cages. And then they wait.
After a few hours or days, the koala comes down the tree and wanders into the cage. The animal is taken to a wildlife hospital, where it is anesthetized, vaccinated and then kept for observation for 24 hours after it wakes up to make sure it's okay.
Finally, the vaccinated koalas are marked with pink dye on their backs, to make sure the researchers don't give the chlamydia vaccine to the same animal twice.
Koalas: Endangered since 2022
A vaccination campaign like this among a wild population is unusual. It is not only a hassle to catch the animals, but disturbing them also always comes with the risk of injuring them. In this case, however, the researchers decided the operation was worth it.
The koalas' situation is dire after all. According to some scientists' estimates, around half of all wild koalas in Queensland, another state on Australia's east coast, are already infected with chlamydia. And in February 2022, the Australian government listed the species as endangered.
The conservation status upgrade (from vulnerable) applied to the koalas in Queensland, New South Wales and the Australian Capital Territory.
"The impact of prolonged drought, followed by the black summer bushfires, and the cumulative impacts of disease, urbanization and habitat loss over the past twenty years" have led to the decision to declare koalas an endangered species, said Sussan Ley, who was Australia's environment minister at the time.
Stress increases chlamydia risk
All these threats contribute to making koalas even more vulnerable to chlamydia, which is why vaccinating them is so important, experts say. Wildfires and habitat loss may contribute to increasing the animals' stress levels, weakening their immune systems and making them more susceptible to diseases.
The vaccination campaign in New South Wales is ongoing. The first vaccinated koala was released back into the eucalyptus forest on March 9, and researchers planned to have 50% of the region's koala population vaccinated within three months.
Edited by: Zulfikar Abbany