Chlamydia (Yes, the STI) Has Infected Nearly Every Koala Population on Earth

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It might sound bizarre, but a common sexually transmitted infection in humans is currently affecting an adorable tree-dwelling marsupial population. The thought might conjure visions of frisky koalas enjoying some post-coital eucalyptus, but the koala chlamydia epidemic is actually not a laughing matter. There are only 80,000 of these vulnerable animals left in Australia, and nearly all of the world's wild koala populations are infected with this often lethal STI.

How Does a Koala Even Get Chlamydia?

Before your imagination runs wild, let's get one thing straight: This is not the same strain of chlamydia that infects humans. But scientists think adult koalas contract chlamydia just as people do — that is, through sex. Joeys (aka baby koalas) can also get infected by eating pap, a nutritious type of feces that mothers feed their young. The microbes within that pap colonize the joey's gut and help it to break down the otherwise toxic tannins in eucalyptus leaves.

But how did koalas contract it in the first place? Well, a 2016 paper suggested that they were infected by livestock — specifically sheep and cattle. "The mechanism of transmission between livestock and koalas currently eludes us," the authors wrote, but they said it's likely that the koalas got it via contaminated manure, the same way many foodborne illnesses infect humans.

However koalas get chlamydia, it's a huge problem. Recent surveys show that many wild populations are 100 percent infected at this point. That means that nearly all of the world's wild koalas are at risk of blindness, bladder inflammation, infertility, and death. All because of an STI that, in humans, is very treatable.

Marsupial STIs on the Rise

It might seem like a no-brainer: Give chlamydia-infected koalas antibiotics just like we do with chlamydia-infected humans. But it's not that simple. Several studies suggest that antibiotics can upset their gut microbes and make digestion of eucalyptus difficult for them, leading to severe weight loss and even death.

Chlamydia in koalas isn't new. It's likely that at the moment you learned about the STI in health class, koalas were already infected. The illness has been making koalas sick for decades, but until now, scientists weren't quite sure why the animals were so vulnerable. But in 2017, a study published in the Journal of Virology suggested that an HIV-like virus called koala retrovirus type B makes koalas especially prone to contracting chlamydia. Koalas with the virus are also more likely to develop severe symptoms from chlamydia — think urinary and reproductive infections, conjunctivitis, and even cancer.

If all of this is starting to sound a little scary, well, it is. In the past 20 years, scientists believe Australia has lost as much as 80 percent of their koala population. And a good chunk of those deaths come from chlamydia. While car accidents are the leading cause of koala death, chlamydia now comes in second.

A Final Hope for Koalas

There's only one place left on earth where koalas could be safe from chlamydia — or at least where one large population remains uninfected. The options for a safe harbor are extremely limited. As we mentioned before, much of Australia's koala population has been infected by chlamydia. But on Kangaroo Island, as reported in a study published this year in Scientific Reports, researchers swabbed koalas for DNA from chlamydia bacteria and found that all of the island's 170 koalas were free of the infection. Then, just to make sure they had it right, the researchers looked at 22 years of historic data on more than 13,000 Kangaroo Island koalas. Still, they found no chlamydia.

That's good news for koalas, as Kangaroo Island might hold their last hope for rebounding from chlamydia. There, they could act as a breeding population to increase the numbers decimated in other areas. The koalas in this island sanctuary may end up being the saviors of their species.

See how Steve Irwin's family is carrying on the wildlife conservationist's memory 12 years after his death: "Crikey! It's The Irwins" follows Terri, Bindi, and Robert Irwin as they run the extraordinary Australia Zoo in the bushlands of the east coast of Australia. The family has treated thousands of koalas in addition to caring for many other animals in their efforts overseeing a world-class wildlife hospital and conducting high-octane global conservation expeditions. The Irwins' lives are full of adventure, fueled by their love of animals and passion for protecting them. Catch up now and watch every week at 8 pm ET on Animal Planet!

Written by Kelsey Donk October 16, 2019
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