Koalas “Vax Up” Against Chlamydia

After years of struggling to find an end to the chlamydia epidemic that has had sweeping negative effects on Australia’s koala populations, researchers think they have found a solution.

In 2021, the Australia Zoo Wildlife Hospital in Queensland — also home to the Irwin family — rolled out a new chlamydia vaccine in cooperation with The University of the Sunshine Coast. This vaccine was specifically created to treat koalas with Chlamydia pecorum. The trial included 400 koalas and was the largest test of the vaccine. Researchers have been hoping that this vaccine would help keep koalas off the endangered species list. Although koalas remain endangered, it looks like the scientists’ efforts have borne fruit.

Why Do Koalas Need to “Vax Up”?

Koalas were already a threatened population before the 2020 Australian fire season. Car strikes, dog attacks, land clearing, and other environmental struggles were already negatively impacting koala populations. After the destruction and death caused by the 2020 bushfires, threatened species experts confirmed that koalas were on the road to extinction.

Currently, half of the koala populations in Queensland are believed to be affected by Chlamydia pecorum. This means that as these animals reproduce, they can transmit the disease both between mates and from mother to offspring. Without a vaccine, the disease would continue its rampage through the population.

Long-term effects of this sexually transmitted disease include infertility, blindness, and premature death. In addition, the disease causes immense pain for koalas when urinating. Researchers have been hoping the vaccine would help decrease these effects, improving koalas’ long-term survival and their quality of life.

Previous Tests

Previously tested in smaller groups of 250 koalas, the C. pecorum vaccine has been deemed safe. To date, it has had no negative effects on the animals. Unlike humans, koalas cannot be treated for chlamydia with antibiotics. Not only is it difficult to administer a full course of antibiotics to wild koalas, but the medication also disrupts the gut bacteria they need to digest eucalyptus leaves. The single-dose vaccine addresses both of these challenges. Koalas can receive the vaccine when passing through Australia’s wildlife hospitals. Then they can return to the wild without supervision and continue to eat their primary food source.

Though known to treat current infections, it is still unknown whether this vaccine will prevent the transmission of chlamydia’s dormant bacteria. Researchers hope this trial has provided an answer to that question. This is the first trial in which vaccinated koalas were released back into the wild. In previous trials, the vaccinated koalas were all living in captivity.

Vaccine Trial Results

Researchers involved in the decade-long project recently evaluated the C. pecorum vaccine’s effectiveness. The vaccine was shown to reduce vaccinated koalas’ risk of both developing and dying from chlamydial disease. A survival probability analysis determined there was about a 25% increase in the probability that vaccinated koalas would not develop signs of chlamydial disease when compared to unvaccinated koalas.

Researchers also estimated a vaccine efficacy of 64.7% for prevention of death associated with the signs of the disease. The reduction in deaths from the disease also persisted throughout the lifespans of the vaccinated koalas. Scientists noted that although the current vaccine’s efficacy is lower than desired, the results suggest that incorporating vaccination into a broader koala management plan can help reverse population declines.