Division I.2.1 Cytology (imprint cytology)

After drying gills or heart tissues on absorbent paper, several imprints shall be made on a glass slide. Slides shall be air-dried, fixed in methanol or in absolute ethanol and stained using a commercially available blood-staining kit (namely such as Diff- Quik®/Hemacolor®.), in accordance with the manufacturer's instructions. After rinsing in tap water and drying, the slides shall be mounted with a cover-slip using an appropriate synthetic resin. Slides shall be observed first at × 200 magnification and then under oil immersion at × 1 000 magnification.

A positive result shall be the presence of small spherical or ovoid organisms (2 to 5 μm wide) within haemocytes. However, the parasite might also occur extracellularly. These organisms show a basophilic cytoplasm and an eosinophilic nucleus (colours may vary with the stain used) and, because they spread on the slide, they can appear wider on imprints than on histological examination. Multinucleated cells may be observed. The technique is not parasite species specific.