Oyster disease Bonamia ostreae – Expert Q&A

Following the discovery of the parasite Bonamia ostreae in Stewart Island oyster farms, the Ministry for Primary Industries has issued a restriction on the area and plans to remove the affected oysters.

Niwa put together a Q&A with fisheries scientist Keith Michael about the parasite and the risk of it spreading further afield.

What is Bonamia ostreae?
Bonamia ostreae is a parasite of flat oysters (Ostrea). It is a small, single cell organism, just two to five thousandths of a millimetre in size.

How does it get into the oyster?
B. ostreae is a waterborne disease and its cells (infective propagules) enter the oyster’s blood through the gills, mantle and gut, affecting the oyster’s blood cells, gill and other tissues.

The B. ostreae cells multiply rapidly in the oyster’s blood cells within these tissues, fuelled by energy from the oyster. This intensification of disease causes blood cell and tissues to break down or rupture, releasing more infective propagules.

What does the disease do?
Mildly infected oysters may appear healthy and in good condition, but heavy infections cause a loss of condition and the development of lesions.  Intense infections kill the oyster. The disease can kill all ages of oysters, but older oysters appear to be more vulnerable. The complete lifecycle of B. ostreae has not been described.

How is the disease related to Bonamia exitiosa?
B. ostreae is closely related to Bonamia exitiosa – the parasitic infection currently well monitored to inform management of the Foveaux Strait oyster fishery – which is thought to be endemic to New Zealand. The two species look very similar. The most reliable way of telling them apart is by using molecular methods targeting species-specific DNA. B. ostreae is a particularly destructive disease compared to B. exitiosa. New incursions of either species must be notified to an international monitoring organisation (the World Organisation for Animal Health, OIE), which the Ministry for Primary Industries (MPI) has done.

Where else is B.ostreae found?
B. ostreae is widespread in the Northern Hemisphere where flat oysters occur. It is mainly spread by the movement of infected oysters. Its known distribution covers the west coast of the US and Canada, the United Kingdom and Ireland, France, Spain, the Netherlands, Denmark, and Morocco.

What damage has it caused overseas?
B. ostreae can cause over 90% mortality in oysters. In Europe, the presence of B. ostreae has primarily been discovered after mass mortalities of oysters, and, by that time, eradication has not been possible.  It has had a catastrophic effect on the European oyster industry, forcing farmers to shift to alternative species such as the Pacific oyster (Crassostrea gigas), which has also been severely affected by another disease, Ostreid herpesvirus.

What could happen here?
Sampling by the Ministry for Primary Industries (MPI) has detected B. ostreae in Big Glory Bay on Stewart Island, however, there is no evidence of B. ostreae in the wild oyster beds in Foveaux Strait.  MPI is taking a number of steps to reduce spread of B. ostreae to other areas, including removing flat oysters from marine farms in Big Glory Bay.

If B. ostreae did spread to the Foveaux Strait oyster population, concurrent infections of B. ostreae and B. exitiosa, and the currently low population size and densities of oysters in the fishery, may be catastrophic.

How easily could it spread?
B. ostreae is usually spread over large distances by the movement of infected oysters, spat and larvae. It is a waterborne disease. The breakdown of oyster tissues releases infective propagules (B. ostreae cells) into the sea. Infection is spread by currents, and in carriers such as other shellfish (mussels), farm equipment and vessels. Infective propagules can persist in seawater for a few days. Research is under way in other countries to assess whether an alternative host or resting phase such as a spore can maintain the infection in an area for long periods of time. In other countries, disease-free oysters introduced back into a previously infected area have become infected and died.

Propagule pressure is the number of B. ostreae cells released by dying oysters. The more oysters to die in a localised area, such as on oyster farms, the higher the propagule pressure. High propagule pressure substantially increases the probability of spread by currents and potential carriers, and increases the likelihood that infection will persist in the environment.

When was B. ostreae discovered in New Zealand?
B. ostreae was first identified in 2015 in the Marlborough Sounds. We don’t know how it got to New Zealand. MPI established Protected and Contained Zones to protect New Zealand’s wild oyster stocks and established a surveillance programme to determine the geographical spread within and from the Contained Zone.

B. ostreae infection was detected in Big Glory Bay, Stewart Island late last month. The circumstances differ markedly from those of other incursions of B. ostreae. MPI surveillance detected infection early. The spread of the disease is caused by dying oysters. There is an unknown latent period between infection and death. Codes of practice for the oyster aquaculture industry recommend that stock and gear movements, and more importantly, animal production data, including abnormal mortalities, are recorded and reported. There have been no reports of heightened oyster mortality on Big Glory Bay farms. The current circumstances provide the best opportunity to contain the further spread of disease.

What happens now?
The latent period between infection and death provides a limited opportunity to prevent the further spread of disease by removing all the oyster hosts on oyster farms before they start dying and releasing infective propagules. MPI is working with flat oyster farmers to urgently remove all flat oyster stocks from marine farms in Big Glory Bay on Stewart Island. They have indicated that further sampling will be carried out to investigate whether B. ostreae has spread to wild oysters, mussels and other potential carriers. This information will inform what more can be done to contain the disease.