What is it?
Infection with Bonamia ostreae, also known as bonamiosis, is an intrahaemocytic protist belonging to the phylum Haplosporidia.
The Argentinian flat oyster, European flat oyster, New Zealand dredge oyster and Southern mud oyster are all susceptible.
Where and when might it occur?
Bonamia ostreae occurs throughout Europe from along the coast from Spain to Denmark, localised areas in Ireland and Great Britain, and along the west (California and Washington) and east (Maine) coasts of the USA.
The critical host age for development of disease appears to be two years; however, other age classes are susceptible to infection.
Significant mortalities usually occur at water temperatures of 12˚C to 20˚C.
Systemic infection of haemocytes effectively starves the oyster of energy required for survival. As it fights the infection, the animal eventually dies from exhaustion and starvation.
Some studies suggest that prevalence and intensity of infection increase during late winter and autumn, but the disease may occur at all times of the year.
The pre-patent period is up to five months.
Transmission of the parasite can occur directly from host to host and indirectly between oyster beds via the water.
The disease is thought to have spread from California to Europe by human movement of infected oysters.
Diagnosis
Signs of the disease are usually dead or gaping oysters and increased mortality.
Gross pathological signs are:
- generally poor condition
- gills appearing eroded
- yellow discolouration of the gills and mantle.
Infection with Bonamia ostreae rarely results in gross pathological signs of disease in oysters; often the only sign is increased mortality.
Microscopic pathological signs are:
- dense infiltrations of haemocytes, some containing microcell parasites, in the connective tissue of the gill and mantle and in the vascular sinuses around the stomach and intestine
- extensive lesions, including perforated ulcers in the connective tissues of the gills, mantle and digestive gland.
Source: Australian Government, Department of Agriculture, Fisheries and Forestry