HISTOLOGICAL COMPARISON OF INFECTIOUS HEMATOPOIETIC NECROSIS VIRUS CHALLENGED JUVENILE RAINBOW-TROUT ONCORHYNCHUS-MYKISS AND COHO SALMON ONCORHYNCUS-KISUTCH GILL, ESOPHAGUS CARDIAC STOMACH REGION, SMALL-INTESTINE AND PYLORIC CECA/

A histological evaluation of selected tissues from juvenile rainbow tr out Oncorhynchus mykiss and coho salmon O. kisutch was conducted. Morp hological differences between the 2 species were detected in gills, es ophagus/cardiac stomach region (ECSR), small intestine and pyloric cae ca at the light microscopy and ultrastructural levels. With respect to gill architecture, only the coho salmon exhibited a dilation of the a fferent filamental artery termed an ampulla or 'bleb'. Gills of both s pecies exhibited differences in the distribution of mucous and chlorid e cells, and the size and orientation of pillar and endothelial cells varied. The esophageal/cardiac stomach region of the 2 species differe d with respect to the epithelial cell architecture of the mucosa and t he appearance and location of mucus-secreting (acinar type) serous car diac glands (MSSG) in the submucosa. The small intestine mucosa of the 2 species also differed, with the coho salmon exhibiting columnar vac uolated absorptive cells, whereas the rainbow trout exhibited columnar nonvacuolated absorptive cells. Juveniles of both species were challe nged in vivo with a virulent isolate of infectious hematopoietic necro sis 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 i ntercellular edema with separation of the mucosal and glandular epithe lia, whereas minimal changes were observed in the coho MSSG. Marked ch anges were also noted at 24 h in the ECSR epithelial cells of coho sal mon. At 24 h post virus exposure, the virus appeared to have had no pa thologic effect on the gills, small intestine or pyloric caeca in eith er species.