Mikrocytosis, also known as Denman Island Disease, is a disease of oysters that can cause mortalities of up to 40% in affected species.

It usually affects oysters more than 3 years old, however oyster spat also have seen to be infected.

Main sign is apperance of dead and gaping oysters.

Affected oysters show focal lesions in the vesicular connective tissue.

These lesions may develop into pustules, abscesses and ulcerations in the area of the mantle often with corresponding brown scars on the shell.