Differential Diagnosis of Gas Emboli in the Gills of Steelhead and other Salmonid Fishes

Abstract

Gas bubble disease (GBD), resulting from gas supersaturation of water, can cause extensive mortality and morbidity in fish. Recent interest in using controlled spilling of water from dams in the Snake and Columbia rivers to promote safer outmigration of salmonid smolts has resulted in renewed concerns regarding GBD. Consequently, outmigrant salmonid smolts are being examined at monitoring facilities for clinical signs of GBD. Monitoring during the 1994–1996 outmigration periods revealed that it could be difficult to identify and distinguish true gas emboli from other microscopic tissue structures found during the examination of wet mounts of gill tissues. Laboratory exposures of steelhead Oncorhynchus mykiss and Chinook salmon O. tshawytscha to gas-supersaturated water and field examination of 477 outmigrant steelhead smolts were conducted to characterize the appearance of true gas emboli in gills and to differentiate these from other similarly appearing structures. Gas emboli completely dissipated from excised gills within approximately 2–15 min under typical observation conditions and within about 30 min in intact gills of fish removed from water. Gas emboli were glistening, elongated, rapidly diffusing intravascular bodies when examined in wet mounts of gill tissue. Other glistening ameboid globules found at the distal aspect of primary lamellae were nondiffusing. Histochemical analysis demonstrated that these structures were osiniophilic, indicating their lipid character. The source or function of these extravascular osmiophilic structures is not known but, in some cases, they may have pathologic consequences. Diffusibility, shape, location in the gill filament, and reflectance are the key characters to distinguishing lipid bodies from true gas emboli.