Infection with Bonamia sp.

Signs of disease

Important: animals with disease may show one or more of the signs below, but disease may still be present in the absence of any signs.

Diseases resulting from any of the Bonamia species are remarkably similar, in that few or no clinical or gross signs are present with light infection. Differentiation of the causative agent requires histological laboratory examination.

Disease signs at the farm level

Infection with species of Bonamia rarely results in clinical signs of disease in oysters. More often, the only visual cue is increased mortality.

Clinical signs of disease in an infected animal

• undergrown oysters, in poor condition
• weakened shell closure, leading to slight gaping
• watery flesh
• high mortalities of oysters in initial outbreak
• algae-covered shell lips after mantle shrinks and no longer reaches edges
• deformities to gill margins

Disease agent

Bonamia are protists (intrahaemocytic protozoa) of the phylum Haplosporidia that cause lethal infection of the haemocytes of certain oysters.

Host range

Molluscs known to be susceptible to the disease:

Presence in Australia

Infection with Bonamia has been officially reported from New South Wales, South Australia, Tasmania, Victoria and Western Australia.

Epidemiology

• Mortalities can occur all year but usually happen in spring and summer.
• Significant mortalities usually occur at water temperatures of 12–20°C.
• Bonamia spp. infect blood cells, destroying the immune system and interfering with other critical physiological processes.
• Animals may be infected without mortalities; the cause of severe outbreaks in some situations and not others is unclear.
• Although the agent may be present, the disease does not necessarily occur every year.

Differential diagnosis

The differential diagnostic table and the list of similar diseases appearing at the bottom of each disease page refer only to the diseases covered by this field guide. Gross signs observed might well be representative of a wider range of diseases not included here. Therefore, these diagnostic aids should not be read as a guide to a definitive diagnosis, but rather as a tool to help identify the listed diseases that most closely account for the gross signs.

There are few or no visual cues to the presence of this disease other than poor condition, shell gaping and increased mortality. It is therefore not possible to differentiate between Bonamia spp. based on gross symptoms alone. Any presumptive diagnosis requires histological laboratory examination.

Light microscopy can further define a diagnosis, but further laboratory examination is required for a definitive diagnosis.

Sample collection

Because of uncertainty in differentiating diseases using only gross signs, and because some aquatic animal disease agents might pose a risk to humans, you should not try to collect samples unless you have been trained. Instead, you should phone your state or territory hotline number and report your observations. If samples have to be collected, the agency taking the call will advise you on what you need to do. Local or district fisheries/veterinary authorities could advise you on sampling.

Emergency disease hotline

For your state or territory emergency disease hotline number, see Whom to contact if you suspect a disease.

Further reading

www.oie.int/fdc/eng/Publicat/Cardsenglish/3.1.1.%20BONAMIOSIS%20September%2000.DOC

The currently accepted procedures for a conclusive diagnosis of infection with Bonamia species are summarised in www.oie.int/eng/normes/fmanual/A_00037.htm

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