Abstract

Examination of the liver from many individuals representing 11 species of dolphin, including free-ranging fisheries by-catch and strandings from California, Texas and other locations, and scattered individuals dying in marine aquaria, as well as review of published articles, reveals common histologic and pathologic features and several problems relating to diagnosis. The purpose of this presentation is describe normal features and to offer suggestions for criteria for diagnosis.

The dolphin liver has several unusual features, probably reflecting adaptation to diving, that should be recognized as they may be a source of confusion in attempting diagnosis. These are unusually thick walls of arteries in the triads, which may be in spasm, other sphincter-like thickening of vascular walls; unusual prominence of centrilobular sinusoids, and cytoplasmic secretion vacuoles. The significance of these vacuoles is not clear, but their ubiquitous distribution strongly suggests that they are physiologic and not pathologic.

Infiltration of lobules and triads by lymphocytes and occasionally plasma cells is common, but does not necessarily indicate 'hepatitis' in the usual sense. Infiltrates may reflect events in the intestine, and not hepatic disease. Criteria supporting a diagnosis of hepatitis include finding of necrotic hepatocytes (acidophilic bodies or Councilman bodies), and/or disruption of the limiting plate (interface necrosis or piecemeal necrosis) associated with inflammatory infiltrates.

Triads normally contain little connective tissue, and any increase indicates fibrosis. Fibrosis may be graded over a four-point scale, from none, through mild, moderate, to severe. 'Mild' indicates portal expansion by fibrous tissue, 'moderate' indicates tracking of fibrosis into lobules with occasional bridging of portal areas, and 'severe' is widespread bridging, with formation of nodules. The diagnosis of cirrhosis requires bridging fibrosis and nodular hepatocellular regeneration.

Parasitic infestation of the biliary tract may result in cholangitis, involving the portal areas, or may produce periductal fibrosis and epithelial proliferation of the larger bile passages. Evaluation of liver histology should include a routine stain, such as H&E, a connective tissue stain, such as Masson's trichrome, a stain for iron, a reticulin stain for evaluation of hepatic plate architecture, and PAS for cytoplasmic inclusions in hepatocytes and Kupffer cells.