Australia begins vaccinating wild koalas against chlamydia

  • In March, Australia captured and vaccinated its first wild koala against chlamydia in a new trial aimed at protecting marsupials from infertility, blindness and death.
  • About 80% of the koala population in northern New South Wales is currently infected with chlamydia, compared to only 10% of the animals were infected in 2008.
  • Australian scientists plan to vaccinate half of the koala population in the Northern Rivers region of New South Wales, which is estimated to number 50 animals.

Australian scientists have begun vaccinating wild koalas against chlamydia in an ambitious field trial in New South Wales.

The goal is to experiment with a method to protect the beloved marsupials from a widespread disease that causes blindness, infertility and death.

“It’s killing koalas because they get so sick that they can’t climb trees for food or escape predators, and female koalas can become sterile,” said Samuel Phillips, a microbiologist at the University of the Sunshine Coast who contributed to develop the vaccine.


The scientists’ initial goal is to capture, vaccinate and monitor about half of the koala population in the Northern Rivers region of New South Wales, which means vaccinating around 50 animals.

The safety and efficacy of the single-dose vaccine, which was designed specifically for koalas, was previously tested by vaccinating a few hundred koalas brought into wildlife rescue centers for other afflictions.

Now scientists want to understand the impact of vaccinating a population of wild koalas. “We want to evaluate what percentage of koalas we need to vaccinate to significantly reduce infection and disease,” Phillips said.

The first koalas were captured and vaccinated in March and the effort is expected to last about three months.

Researchers use binoculars to spot koalas in eucalyptus trees, then build circular enclosures around the bases of the trees with doors leading to the cages. After a few hours or days, koalas will eventually descend from one tree to seek out tasty leaves on another and wander into harmless traps.

“It’s hard to confuse a koala with any other animal—they’re pretty easy to spot,” said Jodie Wakeman, veterinary care and clinical director at Friends of the Koalas, a nonprofit that runs a wildlife hospital where they are taken koalas vaccinating.


A koala sits in a tree at a koala park in Sydney, Australia May 5, 2023. Australian scientists have begun vaccinating wild koalas against chlamydia in a pioneering field trial in New South Wales. (AP Photo/Mark Baker)

After a check to make sure the animals are in good condition, the researchers administer anesthesia and vaccine injections, then observe them for 24 hours after they wake up to confirm there are no unexpected side effects, Wakeman said.

The goal is to vaccinate healthy koalas to prevent them from becoming infected with chlamydia.

Before release, the researchers mark the koalas with a small amount of pink dye on their backs, to ensure the same animals aren’t caught twice.

When the first vaccinated koala returned to its habitat on March 9, the scientists placed its cage at the base of a tree and opened the door. He quickly emerged and clung to the tree trunk.

Koalas are iconic Australian marsupials, like wombats and kangaroos. They spend most of their time eating and sleeping in eucalyptus trees, and their paws have two opposing thumbs to help them grip and climb trunks.

Populations of wild Australian koalas have declined dramatically over the past two decades.

Last February, the Australian federal government declared koalas “endangered” in eastern New South Wales, Queensland and the Australian Capital Territory.


Facing threats compounded by disease, habitat loss and road collisions, koalas could be extinct by 2050, according to a 2020 assessment by the New South Wales government.

Scientists estimate that around half of wild koalas in Queensland are already infected with chlamydia.

In deciding to vaccinate, scientists are balancing the risk of disturbing the animals with the danger of allowing the disease to spread. The process has been approved by several government bodies, including the Australian Department of Agriculture and the New South Wales Department of Planning and the Environment.

The origins of chlamydia in koalas are unconfirmed, but scientists believe it is likely that the marsupials first contracted the disease from exposure to feces from infected sheep and cattle. Then it spreads sexually, or is passed from mother to offspring.

While humans and livestock infected with the bacteria that cause chlamydia can be treated with antibiotics, it’s not so simple for koalas.

The “complex” microbes inside koalas’ stomachs are designed to neutralize toxins in the eucalyptus leaves that are their main food source, said Mathew Crowther, a conservation biologist at the University of Sydney. But their digestive systems can also neutralize some medications, so “that means they don’t respond well to antibiotic treatment,” he said.

Crowther has been monitoring a koala population in northern New South Wales for more than a decade. In 2008, 10% of the animals tested there were infected with chlamydia. Today that rate is 80%.


“It’s been devastating — there’s very, very low fertility,” she said. “You hardly see any children.”

The other threats koalas face, including habitat destruction from logging and climate-driven fires, can increase their stress levels, weakening their immune systems and making them more susceptible to diseases including chlamydia, said Crowther.

Rebecca Johnson, now chief scientist at the Smithsonian National Museum of Natural History in Washington, DC, previously led the Koala Genome Consortium in Australia. She said seeing the effects of the disease up close was heartbreaking.

An autopsy of a koala with advanced chlamydia that was suppressed revealed “ovaries completely encased in cysts” and “intestines filled with hard lumps of food, evidence that it couldn’t digest food properly,” Johnson recalled. “She was obviously barren and in pain.”

There are only a handful of other examples around the world of scientists attempting to capture and inoculate endangered wildlife for conservation. In 2016, scientists began vaccinating Hawaiian monk seals against a deadly strain of morbillivirus. Two and a half years ago, Brazilian biologists began vaccinating golden lion tamarins against yellow fever.

“Vaccination for wildlife is not yet routine,” said Jacob Negrey, a biologist at Wake Forest University School of Medicine. “But whether it should be used more often is a fundamental question that conservation biologists are really arguing with right now.”

Johnson of the Smithsonian said the benefits are likely to outweigh the risks for the koalas. “Vaccination is an incredibly resource-intensive thing. Koalas live high up in trees,” he said.

“But because the effects of chlamydia are so debilitating, I think it’s worth it.”


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