Understanding chlamydia in koalas

There are 2 forms of chlamydia known to infect koalas:

• Chlamydia pecorum
• Chlamydia pneumoniae.

Chlamydia pecorum is the most common cause of chlamydial disease in koalas (Casteriano and Higgins n.d.). The disease can cause infertility, particularly in female koalas. It can also result in blindness, weakness and make koalas more susceptible to other threats such as dog attacks and extreme weather events like severe heat or rainfall.

A national Koala Disease Risk Assessment was published in 2023 by the Koala Health Hub (in collaboration with Wildlife Health Australia and the International Union for Conservation of Nature). 

Chlamydial disease is the top priority for management due to the significant risks it poses to the koala population's viability and resilience, as well as to individual koala health and welfare.

How is chlamydia spread?

Chlamydiosis is predominately found in adult koalas and is spread during mating or close physical contact (Casteriano and Higgins n.d.). However, young koalas can also become infected, possibly during birth or while still in the pouch.

What are the signs of chlamydiosis?

Koalas can be infected with chlamydia bacterium and show no signs of illness (Wildlife Health Australia, 2023). However, in severe cases of the disease, chlamydiosis can result in disease of the eye, urinary tract and reproductive tract.

Visible signs can include keratoconjunctivitis, where both the surface of the eye and the surrounding tissue become swollen and inflamed. This can cause discharge and, in severe cases, lead to blindness (image 1).

Another common sign is a brown-stained, wet, or dirty rump, known as 'wet bottom', which is a consequence of urinary tract infection (image 2).

Reproductive tract infections often show no obvious external signs apart from infertility, especially in female koalas.

How common is chlamydiosis in wild koalas?

The number of koalas infected with chlamydia varies widely between wild populations, from 0% to nearly 90% (Wildlife Health Australia, 2023). Research shows that about two-thirds of koalas that are infected but don't show symptoms will eventually develop signs of chlamydiosis within 4 years (Wildlife Health Australia, 2023).

The koala sentinel program funded by the NSW Koala Strategy is in the process of collecting detailed information on the health of 6 wild koala populations. Initial results show highly variable infection and disease rates across New South Wales.

Chlamydia in south-west Sydney

The New South Wales Government is taking action to support the koalas of south-western Sydney from the threat of chlamydia after 2 rescued koalas tested positive for the disease in the previously chlamydia free area of south-west Sydney in September 2024.

Immediate measures were taken to address the issue, including working with local wildlife rehabilitators and vets to improve biosecurity, as well as organising a survey and testing wild koalas in the vicinity of where the chlamydia positive koalas were rescued.

An intensive drone survey of koalas in the vicinity of Appin will give information on how many wild koalas are infected with chlamydia and how far the disease has spread.

Routine disease testing will also continue for every koala that comes into care.

Chlamydia management strategy

To ensure the ongoing health of koalas in the region and prevent the spread of the disease, the New South Wales Government has commissioned the University of Sydney to develop a chlamydia management strategy for koalas in southern Sydney, due to be completed in late 2025.

The strategy will assess the effectiveness and suitability of the full suite of potential interventions and develop options for the management of chlamydia in southern Sydney.

A key learning from the project will be understanding how to manage chlamydia, as one of many threats impacting koalas in an urban fringe population. Building our knowledge in managing chlamydia effectively may then be scaled to improve koala health more broadly.

This project will also inform future directions and investment to manage chlamydia in other koala populations in New South Wales.

What is being done?

Research and vaccine trials

The New South Wales Government is funding important research and vaccine trials to investigate the best way to support the health of koalas by increasing their resistance to the disease.

A vaccine trial in south-west Sydney and the Southern Highlands, led by the University of Sydney and supported by koala rehabilitators, vaccinated 53 koalas against chlamydia. The koalas were then monitored over 12 months.

The vaccine produced an antibody response, however, 4 of the 53 koalas fully vaccinated contracted chlamydia within 6 months. The trial concluded that vaccination alone was not sufficient to protect wild koalas from infection with chlamydia.

Two vaccine trials are underway in the Northern Rivers of New South Wales.

The University of Sunshine Coast is evaluating the protection provided by its koala chlamydia vaccine at the population level. This project will assess the efficacy of the vaccine for managing chlamydia infections in wild populations. This trial is currently ongoing.

Friends of the Koala Hospital has received a Regional Koala Support grant of $110,000 to deliver a chlamydia vaccination program and establish a koala database. This research project allows Friends of the Koala Hospital to vaccinate 100 healthy wild koalas released per year for 3 years, from July 2023.

Return rates of released koalas will be monitored, and any data collected will be combined with other vaccine trial data to assess vaccine efficacy.

The NSW Koala Research Plan 2019–28 outlines clear priorities to guide research on koalas over a 10-year period. Funded through the NSW Koala Strategy, research projects are selected based on their ability to address critical knowledge gaps essential for koala conservation.

Four funding rounds have been completed (2019, 2022, 2023, 2024), supporting 8 projects focused specifically on koala health and disease.

Monitoring of disease

The New South Wales Government, together with key stakeholders, invests in both regional and statewide initiatives to monitor disease in priority koala populations.

A koala scat sampling project is currently underway across the Southern Tablelands region to explore how widespread chlamydia is in the area, after higher-than-expected infertility was found in the local koala population.

At a statewide scale, the koala sentinel program, is a multidisciplinary program assessing koala populations for disease, genetic diversity and ecology at 6 key populations in New South Wales. Funded by the NSW Koala Strategy, data and samples are collected to investigate the complex factors that influence koala health, movement, and reproduction. 

The program aims to fill critical knowledge gaps, including disease-related questions, about individual koala populations to better understand their ability to survive over time.