Division I.2.2 Histology

Sections of tissue that include gills and digestive gland, shall be fixed for a period of at least 24 hours in Davidson's fixative followed by normal processing for paraffin histology and staining, for example with haematoxylin and eosin. Observations shall be made at increasing magnifications up to × 1 000.

A positive result shall be the presence of parasites as very small cells of 2 to 5 μm wide within the haemocytes or free in the connective tissue or sinuses of gill, gut and mantle epithelium, often associated with an intense inflammatory reaction. To avoid any doubt, the parasite shall be observed inside the haemocyte for a positive diagnosis. The technique is not parasite species specific.