A new intranuclear microsporidium, Enterospora nucleophila n. sp., causing an emaciative syndrome in a piscine host (Sparus aurata), prompts the redescription of the family Enterocytozoonidae.

The presence of a new microsporidium is believed to be responsible for an emaciative syndrome observed in farmed gilthead sea bream (Sparus aurata) from different facilities along the Spanish coast. Infected fish were approximately half the average weight and significant mortality was attributed to the condition in some facilities. Clinical signs included anorexia, cachexia and pale internal organs. The microsporidium was found mainly in the intestinal mucosa and occasionally in the submucosa. Morphological, histopathological, ultrastructural and molecular phylogenetic studies were conducted to characterise this organism. This microsporidium undergoes intranuclear development in rodlet cells and enterocytes, and cytoplasmic development mainly in enterocytes and macrophages. The nucleus-infecting plasmodium contains several diplokarya and displays polysporous development which occurs without an interfacial envelope. In the host cell cytoplasm, the parasite develops within a membrane-bound matrix. In both infection locations, the polar tube precursors appear as disks, first with lucent centres, then as fully dense disks as they fuse to form the polar filament, all before division of the plasmodium into sporoblasts. Up to 16 intranuclear spores result from the sporogonic development of a single plasmodium, whereas more than 40 spores result from several asynchronous reproductive cycles in the cytoplasmic infection. Fixed spores are ellipsoidal and diplokaryotic, with five to six coils of an isofilar polar filament in a single row. ssrDNA-based molecular phylogenetic inference places this parasite as a sister clade to crustacean-infecting species of the Enterocytozoonidae and closer to Enterocytozoon bieneusi than to other fish-infecting microsporidians presenting intranuclear development, i.e. Nucleospora, Paranucleospora and Desmozoon. Our studies result in the erection of a new species, Enterospora nucleophila, within the family Enterocytozoonidae, and the description of this family is amended accordingly to accommodate the features of known species assigned to it. Severe histopathological damage occurs in intense infections and this microsporidian is considered a serious emerging threat in sea bream production.