The protozoan parasite which has been suggested as the causative agent of proliferative kidney disease in rainbow trout has previously been described as an‘amoeba’. However, this ultra‐structural study of the organism (named‘PKX’) has demonstrated many similarities to a group of invertebrate parasites, the genus Marleilia (currently classified in the Haplosporea). The suggested affinity of PKX‘ to Marteilia is based on the presence of internal cleavage,‘haplosporosomes’, and an amorphous cell wall. Other cytoplasmic inclusions also show characteristics of the genus. Since a spore stage has not been recognised in ‘PKX’, a definitive taxonomic statement cannot yet be made.
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