Fish affected with branchial amoebiasis, have marked epithelial fusion and hyperplasia, leading often to loss of normal gill architecture, with obliteration of interlamellar spaces. Several species are involved, some of which invade the lamellar epithelium. Trophozoites can therefore be seen in small groups within the branchial epithelium as well as on the surface. Several diagnostic methods can be used for the detection of amoebae, including whole mounts of fresh gills, histopathology, isolation and culture, in situ hybridization (ISH), electron microscopy and indirect immunofluorescence. Differential diagnoses...