Gill monogeneans in marine fish cultured in Italy: histopathological observations.

Within a two years project (2004-2006) on the pathological effects of parasites on marine fish farmed in Italy, the gill histopathological changes referable to monogenean infections have been studied. The most representative fish species of Italian mariculture were examined: European seabass (Dicentrarchus labrax), gilthead seabream (Sparus aurata) and sharpsnout seabream (Diplodus puntazzo), coming from different farming systems (extensive/intensive inland farms, inshore/offshore sea cages). Gill samples were obtained from parasitized fish after identification of gill monogeneans and evaluation of their infection intensity. Gills were fixed in 10% neutral buffered formalin and subjected to histological examination with Haematoxylin and Eosin (H&E), Giemsa and Crossmon's Trichromic stains. Concerning European sea bass, only Diplectanum aequans was observed, with highest prevalence and infection intensity in inland and inshore sea cage systems. Histological changes were observed mainly at infection intensity >200 parasites/gill arch, with hyperplasia of lamellar epithelium, haemorrhages and phlogosis. Lamellar oedema was also frequently evident; mucous cells were generally hyperplasic and hypertrophic. With reference to gilthead sea bream, Sparicotyle chrysophrii and Furnestinia echeneis were respectively found in sea cages (both inshore and offshore) and in inland and inshore cage systems. No pathological signs were referred to F. echeneis infections, also with 50 specimens/gill arch infection intensity. On the contrary, the polyopisthocotylean S. chrysophrii showed a high pathogenicity at low infection intensity (8 parasites/gill arch) with gross lesions such as gill and systemic anaemia already noticeable at necropsy. In this case histology showed severe hyperplasia of gill epithelium with thickening of lamellae up to fusion, and heavy sloughing off of the epithelial cells. Moreover the gills showed diffused degeneration and necrosis in the residual epithelial tissue. The hematophagous attitude of S. chrysophrii was evident for the presence of several erythrocytes in the parasite gut. Also in sharpsnout sea bream, both monopisthocotylean and polyopisthocotylean species, respectively Lamellodiscus spp. and Polylabris spp., were found during this survey. Sporadic cases of host switching by S. chrysophrii and F. echeneis were also observed in sharpsnout seabream with low prevalence and intensity. Histopathological lesions were observed in gills infected by Lamellodiscus spp. only when its number was >100 specimens/gill arch, with haemorrhages, congestion, oedema and mild epithelial hyperplasia, focal necrosis and cellular sloughing off. Likely S. chrysophrii, Polylabris spp. appeared to cause severe gill anaemia at low infection intensity (8-10 parasites/gill arch). Histopathological examination showed diffuse oedema, mild hyperplasia, necrosis and sloughing off of the epithelial cells with karyorrhexis and pyknosis of nuclei. Although gill monogenean infestations have to be considered always detrimental to respiratory function of the host, mainly polyopisthocotyleans play a high pathogenic role apart from infection intensity while monopisthocotyleans cause histopathological changes strictly correlated to the parasitic burden.