Koala Chlamydiosis
Chlamydiosis is a bacterial disease that is very common in koalas, caused mainly by the bacterium Chlamydia pecorum, and sometimes by Chlamydia pneumoniae. It affects primarily the eyes and the urogenital system (urinary tract and reproduct...
What is it?
Chlamydiosis is a bacterial disease that is very common in koalas, caused mainly by the bacterium Chlamydia pecorum, and sometimes by Chlamydia pneumoniae. It affects primarily the eyes and the urogenital system (urinary tract and reproductive organs). This disease is considered one of the major threats to the survival of wild koala populations, particularly in Australia, their natural habitat. The infection can remain silent for a long time before producing visible signs, sometimes years after the initial infection.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Signs depend on the area affected.\n\nIn the eyes, conjunctivitis is often seen: redness, eyelid swelling, purulent discharge, and sometimes cloudiness of the eye that can progress to blindness if left untreated.\n\nIn the urinary and genital area, the koala may have difficulty urinating, urine tinged with blood, matted and wet fur around the rear end (hence the term "wet bottom" in English), and inflammation of the reproductive tract that can lead to infertility in females.\n\nThe affected animal is often tired, thin, and less active than normal.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by infection with the bacterium Chlamydia pecorum, an intracellular microorganism that colonizes the eye, urinary, and genital mucous membranes. Chlamydia pneumoniae can also be involved, particularly in respiratory or disseminated infections.
Stress is a major aggravating factor: habitat loss, drought, wildfires, high population density, or any situation weakening the immune system can trigger progression from silent infection to active disease. Young koalas infected by their mother and animals already weakened by other illnesses are particularly vulnerable.
Transmission
Transmission occurs mainly through direct contact between koalas, especially during mating, between mother and young (the young koala feeds on maternal droppings called "pap," a natural stage in its development), or through contact with contaminated secretions (eye, urinary, genital).
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian performs a complete clinical examination, with careful observation of the eyes and urogenital area. Definitive diagnosis relies on samples (eye, urine, or genital swabs) analysed in the laboratory using molecular detection techniques (PCR) or bacterial culture. Ultrasound may be useful to assess involvement of the urinary or reproductive tract.
Treatment
Treatment is based on appropriate antibiotic therapy, prescribed and monitored by a specialist veterinarian, combined with supportive care (hydration, pain management, eye cleaning). Management is often prolonged and requires close monitoring in a specialised wildlife care facility.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention focuses on limiting stress and preserving koalas' natural habitat. A vaccine exists and is used as part of conservation programs, notably in Australia, to reduce disease incidence in wild and captive populations. Regular screening of animals in care facilities or wildlife parks allows early management.
Vaccination
A vaccine against Chlamydia pecorum has been developed and is used primarily in Australia within conservation programs and certain wildlife parks, with encouraging results in reducing disease severity. It is not in routine use outside this context.
Possible complications
Without treatment, the disease can progress to complete blindness, kidney failure, permanent infertility, or death. In females, infertility resulting from reproductive tract damage poses a significant threat to the renewal of wild populations.
When to see a veterinarian
Discharge or redness around the eyes
Difficulty urinating or abnormal urine (blood, strong odour)
Fur soiled and damp around the rear end
Weight loss or unusually decreased activity
Any koala found weakened or lethargic should be reported to a specialized wildlife care centre
Transmission to humans
The risk of transmission to humans is considered very low and is not significantly documented for Chlamydia pecorum. As a precaution, handlers of sick koalas (carers, veterinarians) follow standard hygiene measures.
Prognosis
Prognosis depends heavily on how early treatment is started. When detected early, the disease often responds well to treatment. However, an advanced infection can cause irreversible complications (blindness, infertility) and threaten the animal's life. This is a disease that significantly impacts the demographics of wild koala populations.
Frequently asked questions
Is chlamydiosis in koalas dangerous for humans?
The risk is considered very low. It is not recognized as a major public health problem, unlike other forms of animal chlamydiosis.
Why does this disease affect so many wild koalas?
The stress caused by habitat loss, wildfires, and drought weakens koalas and facilitates the progression from latent infection to active disease, which explains its high prevalence in the wild.
Is an infected koala always sick?
No. Many koalas carry the bacterium without developing clinical signs for a long time. The disease can remain silent before manifesting, sometimes years later.
Is there a vaccine?
Yes, a vaccine has been developed and is being used in certain conservation programs in Australia, with promising results in limiting disease severity.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 29 August 2026
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