Bonamia spp., including B. ostreae are protists intrahaemocytic protozoa of the phylum Haplosporidia that cause lethal infection of the haemocytes of certain oysters, and in the UK this is the European flat oyster, Ostrea edulis. Infection in oysters rarely results in clinical signs of disease, and the only visual cue is increased mortality. The disease can occur at any time of the year, although the prevalence and intensity of infection increase during the warmer months. In the northern hemisphere, prevalence is highest in September with significant mortality usually occurring at water temperatures of 12°20°C. 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.

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