Infection with Bonamia exitiosa

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

Heavy infections may show:

• 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 spp., including B. exitiosa, are protists (intrahaemocytic protozoa) of the phylum Haplosporidia that cause lethal infection of the haemocytes of certain oysters. Some evidence suggests that B.exitiosa in New Zealand may be conspecific with a Bonamia sp. known to infect southern mud oysters in Australia. However, until further research establishes this relationship, Bonamia exitiosa and the Australian Bonamia sp. are currently considered to be different species.

Host range

Molluscs known to be susceptible to the disease:

* naturally susceptible (other species have been shown to be experimentally susceptible)

Presence in Australia

EXOTIC—not present in Australia.

Epidemiology

• Mortalities can occur all year, with highest prevalence in the mid to late summer.
• Bonamia exitiosa infects blood cells, destroying the immune system and interfering with other critical physiological processes.
• The disease dynamics of B. exitiosa in the New Zealand dredge oyster can be affected by exposure to extreme temperatures (below 7^oC or above 26^oC) and salinity (40 per cent), starvation (prolonged holding in filtered sea water), handling (vigorous stirring four times per day), or heavy infection with apicomplexan protists.
• Cohabitation of infected and uninfected oysters in holding tanks appears to promote the spread of infection.
• A seasonal pattern of disease has been observed since 1964, with two separate epizootics in 1985–1991 and 1998–2003 resulting in cumulative mortality of Ostrea chilensis of greater than 90 per cent.

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.

Similar diseases

Infection with Bonamia ostreae

Further images

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

The currently accepted procedures for a conclusive diagnosis of infection with B.exitiosa are summarised at www.oie.int/eng/normes/fmanual/A_00038.htm