Bonamiosis in Oysters
Bonamiosis is a serious parasitic disease affecting oysters, particularly the European flat oyster (Ostrea edulis). It is caused by a tiny single-celled parasite called Bonamia, which invades the oyster's defense cells, known as hemocytes....
What is it?
Bonamiosis is a serious parasitic disease affecting oysters, particularly the European flat oyster (Ostrea edulis). It is caused by a tiny single-celled parasite called Bonamia, which invades the oyster's defense cells, known as hemocytes. Since the 1970s, this disease has caused significant mortality in farmed and wild populations of flat oysters in Europe. The Pacific oyster (Crassostrea gigas), which is more widely used in Tunisia and worldwide, is considerably less susceptible to this parasite. Bonamiosis is a reportable disease to animal health authorities in many countries.
Key point
Bonamiosis is a notifiable disease in many countries owing to its major economic impact on flat oyster farming.
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:
Bonamiosis does not always cause visible signs that can be seen with the naked eye, which makes it difficult to detect without laboratory analysis. When signs do appear, general weakness of the oyster may be observed, along with loss of flesh weight, yellowing coloration, or small lesions on the gills and mantle. Some oysters may show irregular swelling or spots on the inner shell surface. Most often, the disease progresses silently until the animal dies, sometimes within weeks of infection.
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 microscopic parasites of the genus Bonamia, particularly Bonamia ostreae and Bonamia exitiosa, which belong to a group of protozoans. These parasites enter the hemocytes, cells that protect the oyster against infection, where they multiply and progressively weaken the animal and destroy its tissues.
Flat oysters (Ostrea edulis) are much more susceptible than cupped oysters (Crassostrea gigas).
High stocking densities, stress from transport, fluctuations in water temperature, and the presence of the parasite in the natural environment favour disease transmission.
Adult oysters appear to be affected more often than juveniles.
Movement of oyster batches between basins or countries without health controls is a major factor in disease spread.
Transmission
Transmission occurs directly from oyster to oyster in water, most likely through release of the parasite after the death of an infected individual or through close contact between animals. No intermediate host is required. Movement of live shellfish between oyster farming zones is the main pathway for spread over long distances.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is based on specialist laboratory analysis, as clinical signs alone are not sufficient to confirm the disease. Veterinarians or marine biologists examine tissues under the microscope using staining techniques, and use molecular biology tests (PCR) to detect the parasite's DNA. These analyses are performed by official laboratories as part of health surveillance in shellfish production areas.
Treatment
There is no effective curative treatment for bonamiosis. Disease management relies solely on collective health measures: removal of infected batches, restriction of animal movement, and surveillance of production areas.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
There is no curative treatment, so prevention relies on strict health measures.
This includes controlling the movement of oysters between production areas, regular monitoring of batches by approved laboratories, and avoiding the introduction of oysters from infected areas.
Choosing to farm cupped oysters, which are less susceptible, in at-risk areas is also a strategy used by some oyster farmers.
Some selective breeding programmes aim to develop flat oyster strains with improved disease resistance.
Possible complications
Bonamiosis can cause significant mortality within an oyster batch or natural bed of flat oysters, sometimes over several consecutive years. It can increase the vulnerability of oysters to other pathogens or environmental stress factors.
When to see a veterinarian
Unusual or repeated mortality observed in a batch or tank of oysters.
Oysters showing general weakness with no apparent cause.
Before transferring live oysters between tanks or production areas.
Suspicion of disease during routine health monitoring.
Prognosis
The prognosis is guarded for infected flat oysters, as the disease is often fatal in the medium term. It is considerably better for cupped oysters, which are naturally more resistant to the parasite. At the operation level, the economic impact can be substantial in cases of mass mortality event.
Frequently asked questions
Is bonamiosis dangerous for humans?
No, it is not a zoonotic disease. It presents no risk to human health, including through consumption.
Can oyster bonamiosis be treated?
No, there is no curative treatment. The only possible approach is collective health management of the livestock herd and the breeding environment.
Are hollow oysters sold in Tunisia affected?
The Pacific oyster (Crassostrea gigas), which is predominantly farmed and consumed in Tunisia, is significantly less susceptible to this parasite than the European flat oyster.
How to tell if a batch of oysters is infected? Key signs to look for: **Physical appearance:** - Damaged or cracked shells - Discoloration or unusual spots on the shell or meat - Cloudy or milky flesh instead of translucent **Smell:** - Foul or ammonia-like odor (fresh oysters smell of the sea) - Any sour or rotten smell is a warning sign **Texture and consistency:** - Meat that is too soft or mushy - Excessive liquid in the shell - Meat that falls apart easily **Source and storage:** - Check the harvest date and expiration date - Verify they come from certified, safe waters - Ensure they've been stored at the correct temperature (below 4°C/40°F) - Look for the producer's health certification **Best practices:** - Buy from reputable suppliers - Check that shells are closed or close when tapped (for live oysters) - Use within 7-10 days of purchase - When in doubt, don't consume them If you notice any of these warning signs, it's safer to discard the batch.
Only specialized laboratory analyses, such as microscopic examination or PCR testing, can confirm the presence of the parasite, as visible signs are often absent or misleading.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 30 August 2026
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