Gastrointestinal parasitism in koalas (roundworms)
Gastrointestinal parasitism in koalas comprises infestations by roundworms (nematodes) living in the stomach or intestines of this animal. These worms are fairly common in koalas, even in healthy animals, but become a problem when they mult...
What is it?
Gastrointestinal parasitism in koalas comprises infestations by roundworms (nematodes) living in the stomach or intestines of this animal. These worms are fairly common in koalas, even in healthy animals, but become a problem when they multiply excessively or when the animal is weakened. It is a recognized cause of digestive disease in wild koalas and those living in sanctuaries or wildlife care centers.
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 are often subtle at first. The koala eats less and loses weight while appearing active.
Diarrhea or loose stools may develop, sometimes associated with abdominal tenderness.
The animal becomes lethargic, its coat loses its shine and becomes dull.
In severe cases, marked dehydration, occasional vomiting, and failure to thrive in young koalas may be seen.
Some heavily infested koalas gradually weaken without obvious signs, making detection difficult without veterinary examination.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Several species of nematodes (roundworms) can colonize the koala's digestive tract, notably worms belonging to genera found specifically in Australian marsupials. These parasites lay eggs that are passed in feces, which then contaminate the environment or eucalyptus foliage consumed by the animal.
Stress, poor diet, concurrent illness (such as Chlamydia infection, which is very common in koalas) or a weakened immune system promote worm multiplication and the development of symptoms.
Recently weaned young koalas, animals kept in captivity in restricted space, and those living in areas with high population density are more exposed to high parasite loads.
Transmission
Infection occurs through ingestion of parasite eggs present on eucalyptus foliage or in the environment soiled by infected feces.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian suspects digestive parasitism when faced with the symptoms described, but definitive diagnosis is based on microscopic examination of faeces (looking for worm eggs) or sometimes autopsy in case of death.
Additional tests (blood work, abdominal ultrasound) may be performed to assess overall health and search for associated diseases.
Treatment
Treatment relies on administering antiparasitic drugs suitable for marsupials, prescribed and dosed only by a wildlife veterinarian, as the koala is very sensitive to certain substances.
Nutritional support and rehydration may accompany treatment in severe cases.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies on good enclosure hygiene in captivity, regular monitoring of parasite load through faecal analysis, and management of stress and diet.
In the wild, there is no direct prevention, but maintaining a healthy habitat with good eucalyptus diversity limits overall weakness in koalas.
Possible complications
Untreated massive infestation can lead to severe dehydration, significant weight loss, anaemia and, in severe cases, death, particularly in young animals or those already weakened by another disease.
When to see a veterinarian
The koala has refused to eat for several days.
Abnormal faeces or persistent diarrhoea are observed.
The animal appears underweight, dull or unusually lethargic.
A young koala shows visible growth or developmental delay.
Vomiting or signs of dehydration appear.
Prognosis
The outlook is generally good if the disease is detected early and treated by a veterinarian with expertise in wildlife medicine.
It becomes guarded if there is heavy parasite burden, concurrent disease (such as chlamydiosis), or delayed treatment, particularly in young koalas or animals already in poor condition.
Frequently asked questions
Are the roundworms found in koalas transmissible to humans or other domestic animals?
No, these nematodes are specific to koalas and other Australian marsupials. They pose no known risk to humans, dogs, or cats.
Can a koala host worms without being sick?
Yes, a low parasitic load is common and generally well tolerated. The problem mainly arises when worms multiply extensively or when the animal is weakened.
How do you know if a koala is infested?
Only a veterinary stool analysis can confirm the presence and quantity of worms. Symptoms alone are insufficient to establish a reliable diagnosis.
Is the treatment risky for the koala?
The koala is an animal highly sensitive to certain medications due to its unique metabolism, which is linked to its eucalyptus diet. Treatment should therefore be reserved for a veterinarian experienced in wildlife care.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
AVMA — American Veterinary Medical Association
Content reviewed by the TunisieVet editorial team · Updated 29 August 2026
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