Graduate Thesis Or Dissertation
 

Bacteriologic, immunologic and pathogenic studies of Vibrio spp. pathologic to salmonids

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  • Fish diseases and various parameters associated with disease caused mortality of fish were monitored at the Oregon State University Marine Science Center and at a private mariculture facility on Yaquina Bay during a period of five years. Nearly all disease problems observed were caused by Vibrio anguillarum and Vibrio spp. Infection by Vibrio spp. resulted in substantial mortality (as high as 50% of a population) among fish which were immunized against two serotypes of V. anguillarum and among non-immunized fish. Naturally occurring levels of V. anguillarum in Yaquina Bay were determined to be ten or less viable cells per ml water. Effluent water from groups of salmonids with naturally acquired vibriosis contained 1.0 to 4.3 x 10³ viable cells of V. anguillarum per ml. The histopathology associated with naturally acquired and experimentally induced infections of vibriosis in chum salmon fingerlings was described for the two serotypes of V. anguillarum which commonly cause epizootic levels of mortality among salmonids reared at mariculture facilities in the Pacific Northwest United States. Results of these studies indicate that different histopathologic changes are produced by the two serotypes of V. anguillarum. One serotype (referred to as V. anguillarum serotype I) produced a bacteremia in early stages of disease with the following organs and tissues being the main targets: blood, loose connective tissue, kidney, spleen, posterior gastrointestinal tract, and gills. The second serotype (referred to as V. anguillarum serotype II) produced a bacteremia in late stages of disease with the following organs and tissues being main targets: skeletal muscle, cardiac muscle, anterior gastrointestinal tract, posterior gastrointestinal tract, and gills. Vibrio anguillarum serotype I cells were evenly dispersed throughout infected fish tissues while V. anguillarum serotype II formed distinct colonies in tissues of fish. Experimentally induced infections of chum, coho, and chinook salmon were studied to compare the histopathologic changes associated with infections of V. anguillarum serotypes I and II and to obtain. quantitative data concerning some specific effects produced in fish infected with these organisms. Differences in histopathology noted above were observed in all three species of fish when infections of the two serotypes of V. anguillarum were compared. Cellular responses were rarely observed during early or late stages of vibriosis. The data suggest that both serotypes of V. anguillarum used in these studies produce a leukocidin in fish because infected fish had 80% to 95% less leukocytes than non-infected control fish. Extremely high levels of V. anguillarum were shown to be present in fish tissues. Pathology observed in the mucosa of the gastrointenstinal tract of infected fish was apparently related to pH. The anterior gastrointestinal tract was strongly acidic and contained no necrosis of the mucosa while the posterior gastrointestinal tract was not acidic and contained massive necrosis and sloughing of epithelial cells in the mucosa. Experimentally induced infections of vibriosis with water born exposure of fish to live bacteria were used to study the progress of disease. Both serotypes of V. anguillarum used in these studies were shown to enter fish by penetrating the descending intestine and rectum. Penetration of the skin is a second means by which V. anguillarum serotype II enters fish. Moribund fish in all studies suffered from hypoxia, possible accumulation of toxins (although not highly potent), loss of fluids in the posterior gastrointestinal tract, and dysfunction of various organs. Death of fish was apparently due to a combination of these ill effects.
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