Mikrocytosis here refers only to the disease in oysters caused by Mikrocytos mackini on the west coast of Canada. If detected outside the known range of M. mackini , electron microscopy or molecular probes, if available, must be used to identify and distinguish the detected organism from other microcell species ( e.g. Mikrocytos roughleyi, Bonamia ostreae, B. exitiosus) . The presence of these pathogens in any bivalve should be regarded as potentially serious and the OIE Reference Laboratory should be consulted.
Mikrocytos mackini has uncertain taxonomic affinities, but it is not closely related to other microcells ( 8, 10, 11 ) . Mikrocytosis is also known as Denman Island disease ( 2, 13 ) and microcell disease of oysters . Mikrocytos mackini infects Crassostrea gigas, Ostrea edulis and O. conchaphila (= O. lurida); and infects experimentally Crassostrea virginica( 1, 3, 4, 9 ) . Crassostrea gigas seems to be more resistant to the disease than the other species.
Mikrocytos mackini occurs on the south-west coast of Canada. It is probably ubiquitous throughout the Strait of Georgia, including Henry Bay, Denman Island, and is confined to other localities around Vancouver Island ( British Columbia ) .
Mikrocytos mackini produces a focal intracellular infection of muscle and vesicular connective tissue cells, which results in haemocyte infiltration and tissue necrosis ( 5 ) . The parasite may induce yellow-greenish pustules, abscesses and ulcers, mainly on the mantle, with corresponding brown scars on the shell. Macroscopic lesions are not always present. Abscesses are composed of granular haemocytes and hyalinocytes, and may contain small cells of 1-3 µm in diameter. Severe infections appear to be restricted to oysters over 2 years of age.
The mortality rate has been recorded at around 40% for older oysters at low tide levels ( 13 ) . The disease occurs most often in April and May, after a 3-4-month period when temperatures are less than 10°C. Harvesting or moving large oysters to locations high in the intertidal zone prior to March and not planting oysters at lower tide levels before June have been successfully implemented to control the disease ( 1 ) .
For diagnosis, the recommended guidelines for sampling are those stated in Chapter 1.1.4 and Chapter I.2. of this Aquatic Manual.