ABSTRACT: A histological evaluation of selected tissues from juvenile rainbow trout Oncorhynchus mykiss and coho salmon O.kisutch was conducted. Morphological differences between the 2 species were detected in gills, esophagus/cardiac stomach region (ECSR), small intestine and pyloric caeca at the light microscopy and ultrastructural levels. With respect to gill architecture, only the coho salmon exhibited a dilation of the afferent filamental artery termed an ampulla or 'bleb'. Gills of both species exhibited differences in the distribution of mucous and chloride cells, and the size and orientation of pillar and endothelial cells varied. The esophageal/cardiac stomach region of the 2 species differed with respect to the epithelial cell architecture of the mucosa and the appearance and location of mucus-secreting (acinar type) serous cardiac glands (MSSG) in the submucosa. The small intestine mucosa of the 2 species also differed, with the coho salmon exhibiting columnar vacuolated absorptive cells, whereas the rainbow trout exhibited columnar nonvacuolated absorptive cells. Juveniles of both species were challenged in vivo with a virulent isolate of infectious hematopoietic necrosis virus or mock-challenged with phosphate buffered saline. The most notable tissue response produced by exposure to the virus was observed in the ECSR and occurred as early as 1 h post viral challenge. At 24 h, MSSG and ECSR epithelial tissue of rainbow trout exhibited severe intercellular edema with separation of the mucosal and glandular epithelia, whereas minimal changes were observed in the coho MSSG. Marked changes were also noted at 24 h in the ECSR epithelial cells of coho salmon. At 24 h post virus exposure, the virus appeared to have had no pathologic effect on the gills, small intestine or pyloric caeca in either species.

KEY WORDS: Rhabdoviruses . Rainbow trout . Coho salmon . Morphology . Histology

Published in DAO Vol. 23, No. 3 (1995) on November 23
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1995