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Avian megabacteriosis (Macrorhabdus ornithogaster)
Avian megabacteriosis is a chronic digestive disease of canaries caused by a fungal microorganism called Macrorhabdus ornithogaster, long called “megabacteria” because of its large size visible under a microscope. This fungus colonizes the...
What is it?
Avian megabacteriosis is a chronic digestive disease of canaries caused by a fungal microorganism called Macrorhabdus ornithogaster, long called “megabacteria” because of its large size visible under a microscope. This fungus colonizes the lower part of the bird's stomach and disrupts digestion, leading to progressive weight loss despite an often preserved appetite. It frequently affects canaries, but also other small passerines and breeding parakeets.
Key point
In canaries, megabacteriosis can progress silently and then cause rapid and severe weight loss; Without treatment, the outcome can be fatal.
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:
The most striking sign is progressive weight loss while the bird continues to eat normally, sometimes even more. The droppings often become soft, larger, or contain poorly digested seeds visible to the naked eye. The bird may appear downcast, less active, with dull, ruffled plumage. Occasional regurgitation and loss of appetite may occur in advanced forms. Some canaries remain carriers without ever showing visible clinical signs.
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 Macrorhabdus ornithogaster, a long rod-shaped fungus that colonizes the junction between the proventriculus (glandular stomach) and the ventricle (gizzard) of birds. It disrupts digestion and damages the wall of this organ, which prevents the bird from properly assimilating its food despite normal or even increased food intake.
Life in a collective aviary or farm with many birds, stress linked to transport or reproduction, young birds with an immature immune system, insufficient hygiene of the cage and equipment, immunosuppression linked to another disease, introduction of new birds without quarantine or prior screening.
Transmission
Transmission occurs mainly by the fecal-oral route, that is to say by ingestion of food, water or equipment soiled by contaminated droppings. Feeding the young by carrier parents also promotes transmission within the same family or aviary.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
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Treatment
Treatment is based on specific antifungals prescribed and adapted by the veterinarian, associated with support measures such as improved diet, acidification of drinking water on veterinary advice and reinforcement of cage hygiene. The duration of treatment is generally prolonged because the fungus is difficult to eliminate completely.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Isolate any new bird before introducing it into an existing aviary and have it screened. Regularly clean and disinfect cages, feeders and drinkers. Avoid overcrowding in aviaries. Quickly remove droppings from the bottom of the cage. Do not share equipment between birds of unknown health status. Monitor the weight of canaries regularly, especially during moulting or breeding periods.
Possible complications
Severe weight loss leading to cachexia, dehydration, ulceration of the wall of the proventriculus which can lead to perforation in the most serious forms, death from exhaustion if the disease is not treated.
When to see a veterinarian
The canary visibly loses weight even though it eats normally.
The droppings become abnormally soft or contain undigested seeds.
The bird is downcast, less active, or has persistently ruffled plumage.
A new bird introduced into the aviary shows one of these signs.
Several birds in the same aviary lose weight at the same time.
Prognosis
The prognosis is variable. Birds treated early, before significant weight loss, often respond well to treatment and regain a correct weight. On the other hand, a late diagnosis with already established cachexia clearly darkens the prognosis, and some canaries cannot be saved despite treatment. Relapses or persistence of the fungus at low levels are possible even after treatment.
Frequently asked questions
Is megabacteriosis contagious between canaries?
Yes, it is easily transmitted from one bird to another by contact with contaminated droppings, particularly in aviaries or when raising young by adult carriers.
Can a carrier canary without symptoms still be dangerous to others?
Yes. Some birds harbor the fungus without losing weight or showing visible signs, while continuing to excrete infectious forms in their droppings, which can contaminate their fellow birds.
Does the treatment completely cure the canary?
The antifungal treatment prescribed by the veterinarian greatly reduces the load of the fungus and often significantly improves the condition of the bird, but total and definitive elimination is not always guaranteed; regular checks remain useful.
Should you isolate a sick canary from the rest of the aviary?
Yes, isolation of the affected bird during treatment, combined with rigorous cleaning of the cage and equipment, limits the spread to other canaries.
References
ESCCAP — European Scientific Counsel Companion Animal Parasites
AVMA — American Veterinary Medical Association
Content reviewed by the TunisieVet editorial team · Updated 11 August 2026
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This information is provided for educational purposes only and is not a substitute for a veterinary consultation. If you have any concerns about your animal's health, consult a veterinarian.
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