FIRST REPORT OF THE MICROSPORIDIAN ENTEROSPORORA NUCLEOPHILA IN ITALIAN FARMED GILTHEAD SEABREAM (SPARUS AURATA)

In order to study the first case of emaciative syndrome due to an enteric microsporidian infection in gilthead sea bream farmed in sea cages in Italy, a parasitological and histopathological study have been carried out. Enterosporora nucleophila is a microsporidian parasite of the family Enterocytozoonidae responsible for an emaciative syndrome in farmed gilthead sea bream (Sparus aurata), recently described in Spain. In December 2013 a disease outbreak occurred in sea caged gilthead seabream in Italy, affecting 0+ year fish with very low mortality rate and showing clinical signs such as lethargy, anorexia and pronounced weight loss up to cachexia. Twenty gilthead sea bream (mean weight 32 g) were sent by the farmer to the DIMEVET for diagnostic purposes. All the fish were necropsied, and subjected to parasitological examination. Samples of intestine were fixed at the sampling site in 10% buffered formalin and processed for routine histology. Four μm sections were stained with H&E and Luna, according to AFIP manual, and evaluated for histopathology. Intestinal tissue was also subjected to molecular analysis; a fragment of the 18S rDNA was amplified by PCR and then sequenced. At necropsy the fish showed emaciation, skin and fin erosive lesions, gill necrosis and keratitis. Hindgut showed a moderate dilatation of the lumen in association with the presence of whitish casts. Results of parasitological examinations allowed to detect, besides gill monogeneans and gall bladder myxozoans at low intensities, the presence of many small clusters of minute spores referable to microsporidians in intestinal scrapings and wall fresh mounts. The sequences obtained from infected intestines showed 99.9% identity with E. nucleophila. In all cases a severe mucosal atrophy with total folds flattening was detected; in all histological sections a limited number of mast cells and lymphocytes was found within lamina propria and submucosa (mild enteritis) accompanied by a mild mucous cells hyperplasia. Multifocally within enterocytes and rodlet cells, the nucleus and/or cytoplasm contained central drop-like and clusters of microsporidian spores tightly packed, more evident with Luna stain. In recent years fish microsporidioses have become increasingly important among the factors threatening aquaculture, both for their economic impact and for lack of effective/authorized treatments. The newly described emaciative syndrome caused by E. nucleophila, microsporidian unique in infecting intestinal epithelium of gilthead seabream and showing, in addition to intracytoplasmic, an intranuclear development in this host, may represent an emerging pathology for the Mediterranean mariculture and should be urgently investigated and understood in terms of biology, epidemiology and pathogenesis. In the cases studied the severe mucosal flattening could be interpreted as an effect of a chronic insult, not only related to the microsporidia infection; other concurrent predisposing factors could be then considered into the development of this disease.