PROGRESSION AND RESOLUTION OF MEGALOCYTIC HEPATOPATHY WITH EXOCRINE PANCREATIC METAPLASIA IN A POPULATION OF CULTURED JUVENILE STRIPED BASS MORONE-SAXATILIS

An episode of acute mortality that occurred in a population of pond-reared fingerling striped bass Morone saxatilis in northern California, USA, was investigated. Affected fish were darkened and had bilateral exophthalmos and abdominal distension. Livers were enlarged and pale, and a small amount of a serous fluid was present in the pleuroperitoneal cavity. Microscopic examination revealed panhepatic megalocytosis with individual hepatocyte degeneration and necrosis. Electron microscopic examination of hepatocytes revealed invaginations of the nuclear membrane and subsequent herniation of the perinuclear cytoplasm into the nucleus. Mortality among fish moved from the pond to the laboratory declined and then ceased after 8 wk, providing an opportunity for serial analysis of repair and resolution. At 12 d post-submission, proliferation of bile ductular epithelial cells occurred adjacent to foci of degenerative hepatocytes prior to regeneration of morphologically normal hepatocytes. Multifocal areas of exocrine pancreatic metaplasia were also present at 12 d and persisted until the termination of the study. The etiology ot the disease was not determined but standard procedures ruled out a primary microbial infection. Hepatotoxic agents under consideration include pyrrolizidine alkaloids and blue-green algal phytotoxins. These observations indicate that the striped bass is sensitive to potential toxicants but is capable of repair and regeneration following sublethal injury. A pluripotent stem cell capable of differentiation into 3 epithelial cell types may be essential for this repair and regeneration. Experimental exposure studies are in progress to identify the etiological agent and further study the pathogenesis of the hepatic lesions.