Histopathology of Diphyllobothrium ditremum plerocercoids in coho salmon Oncorhynchus kisutch

Abstract

Captive and transplanted juvenile coho salmon Oncorhynchus kisutch Walbaum dying from an epizootic of plerocercoid parasitism by Diphyllobothrium ditremum (Creplin) in 2 Alaskan lakes were examined for gross and histological lesions. Clinical signs in moribund fish included severe ascites, abdominal distension with loss of equilibrium, visceral adhesions, hemorrhaging and discoloration of the liver and abundant plerocercoids within major organs and loose within the visceral cavity. The primary microscopic lesion was edema, congestion and hemorrhaging within hepatic sinusoids accompanied by occasional tracks of damaged hepatic tissue caused by migrating plerocercoids. The lesions suggest that fish mortality was due to liver dysfunction, blood loss and osmotic imbalance.