41st Annual Meeting of the Society for Invertebrate Pathology
and 9th International Conference on Bacillus thuringiensis

To date, only one genera (Nucleospora) within one family (Enterocytozoonidae) of microsporidians contain species that are parasitic within the nuclei of their host cells. All described intranuclear species of the genus Nucleospora exist in fish. We have recently discovered the first intranuclear microsporidian parasite within an invertebrate, the European edible crab (Cancer pagurus). Whilst infected crabs displayed no obvious external symptoms, histologically, infected hepatopancreatic tubules were characterised by the presence of varying numbers of hypertrophic and eosinophilic nuclei within epithelial cells. Parasite stages appeared as eosinophilic granular accumulations causing margination of host chromatin. In advanced cases, degeneration of the tubule epithelia occurred, with parasites and sloughed epithelial cells appearing in tubule lumens. Ultrastructurally, all life stages of the parasite were observed within host nuclei. Uninucleate meronts were not detected though bi-nucleate stages were observed. Multinucleate plasmodia (sporogonal plasmodia) contained up to 22 nuclei in section. Significantly, late stage plasmodia contained multiple copies of apparatus resembling the polar filament and anchoring disk that appeared to associate with individual plasmodial nuclei. As such, aggregation and early assembly of sporoblast components took place within the intact sporogonial plasmodium, a unique feature of the family Enterocytozoonidae. Liberation of sporoblasts from plasmodia or the presence of liberated sporoblasts was not observed in this study. However, maturing and mature spores were observed in direct contact with the host nucleoplasm. In many cases, large numbers of spores were observed within a given section of host nucleus. Spores measured 1.3 ± 0.02 x 0.7 ± 0.01 mm. By considering the shared features of this parasite with microsporidians of the family Enterocytozoonidae and the unique presence of this parasite within the nucleoplasm of hepatopancreatocytes from a decapod crustacean, it was proposed that this parasite is the type species of a new genus of microsporidian (genus Enterospora). Further work is now required to provide molecular taxonomic data for comparison of Enterospora to the two other genera, Enterocytozoon and Nucleospora that reside within the Enterocytozoonidae and with other hepatopancreatic microsporidians of crustacean hosts.