New Syndrome of Mixed Bacterial and Viral Etiology in Cultured TurbotScophthalmus maximus

Abstract

A new disease occurred from August to October 1987 in cultured turbot in northwestern Spain. During the outbreak, there was continuous but low mortality, with a total cumulative loss of 4% of the affected population. Diseased turbot did not display unusual swimming behavior; the external signs of the disease were abdominal distension and hemorrhagic foci in the anus and base of the fins. Internal examination of affected fish revealed a swollen stomach and intestine, which were filled with a mucous liquid. The internal wall of the peritoneal cavity was hemorrhagic, and there was an accumulation of reddish fluid in the cavity. Bacteriological analysis revealed a sucrose-negative Vibrio in the kidney, liver, and spleen. This bacterium's biochemical and serological characteristics resembled those of V. splendidus. Samples of internal organs were processed for virological analysis and inoculated on chinook salmon embryo (CHSE-214) cells, which showed a cytopathic effect after 14 d at 15°C. The viral agent appeared to be a reovirus because of its cytopathology (formation of syncytia in cell cultures), its morphology (70-rim icosahedral particles with a double capsid), and its resistance to lipid solvents (i.e., it has no envelope). Acridine orange stain and DNA inhibitor assays indicated that the virions contained doublestranded RNA. The virus caused a cytopathic effect in CHSE-214 and bluegill fry (BF-2) cells, but not in rainbow trout gonad (RTG-2), brown bullhead (BB), or fathead minnow (FHM) cells. Although the Vibrio bacterium was pathogenic in rainbow trout Oncorhynchus mykiss (formerly Salmo gairdneri), treatment with oxolinic acid was not totally effective for arresting the mortality, which indicates the turbot reovirus may play a role in this pathologic condition.