Histopathology of five species of Didymocystis spp. (Digenea: Didymozoidae) in cage-reared Atlantic bluefin tuna (Thunnus thynnus thynnus).

The first parasitological examinations of Atlantic bluefin tuna Thunnus thynnus thynnus reared in the Adriatic Sea revealed high numbers of different digenean trematodes, belonging to family Didymozoidae. Five species were the most frequently isolated: Coeliodidymocystis, abdominalis, Didymocystis wedli, Koellikerioides internogastricus, K. intestinalis and K. apicalis. Parasites were isolated from gills, gill rakers, intestine, serosa of pyloric caeca, and inner layers of the fundic region of stomach, encysted in pairs in connective tissue capsules originating from host tissue. Gross pathology revealed no pathological changes. Histological alterations comprised tissue reaction resulting in lymphocyte infiltration and formation of a collagenous envelope around the parasite cysts; the intensity of observed reaction depended on the size of the cyst and on the tissue infected. The most notable changes were observed in the intestine under provocation by K. intestinalis, while the most potentially pathogenic didymozoid under fish rearing conditions is D. wedli, which if present in high abundance can induce secondary bacterial infections.