Graduate Thesis Or Dissertation
 

Host susceptibility, histopathologic, and transmission studies on Ceratomyxa shasta, a Myxosporidan parasite of salmonid fish

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  • The range of occurrence of Ceratomyxa shasta was extended to include the lower 133 miles of the Willamette River, Oregon. Susceptible salmonids were held in live­boxes within the river system to determine the extent of the river where they would become exposed to ceratomyxosis. No infections were obtained in the exposed fish at or upstream from mile 134. The source of ceratomyxosis for the Willamette appears to be the main river channel and the disease agent does not enter from tributaries. For most years tested, salmonids developed ceratomyxosis when exposed to Willamette River water from early April through the later half of November. Attempts to transmit the disease to uninfected fish in laboratory experiments which used infected tissue, a mud substrate, and flowing pathogen-free water were unsuccessful. Transmission of c. shasta was accomplished by an injection procedure which was used to determine which parasitic cell type was capable of initiating the infection. The suscepti­bility of selected salmonid species to ceratornyxosis by inoculation was also determined. Nine salmonid species were tested for their susceptibi­Lity to ceratomyxosis by natural exposure. A high percent mortality and a short mean time to death characterized sal­monids with high susceptibility to the disease. These in­cluded rainbow, cutthroat and brook trout and churn and one strain of fall chinook salmon. Sockeye and spring chinook salmon had low susceptibility to ceratornyxosis as indicated by low percent mortality and an extended mean time to death. Brown trout, Atlantic and coho salmon were moderately sus­ceptible and had intermediate values for percent mortality and mean time to death. Variation in susceptibility to ceratornyxosis was in­vestigated between different strains of the same species of salmonids. A practical application of this study was to provide data for the selection of a strain of fall chinook salmon for use within the Willamette River drainage. Four strains of fall chinook salmon were chosen. Three strains originated from hatcheries located within the lower Columbia River system, which has been shown to contain C. shasta, while the fourth was from an Oregon coastal river, where the disease does not occur. Fish of these four strains were ex­posed to ceratomyxosis which subsequently was detected in 95 percent of the coastal fall chinook salmon strain and only 9 percent of the Columbia River basin strains. Fish of the strains originating from the Columbia River basin possessed a high degree of resistance to the disease. That resistance can be passed genetically is possible, since none of the in­dividuals tested had been previously exposed to the disease. Two strains of rainbow trout which were cultured for resistance to ceratomyxosis were exposed to the disease. Both strains displayed resistance to ceratomyxosis and il­lustrated that resistant strains could offer a solution to fisheries agencies which manage waters where ceratomyxosis occurs. The histopathology of rainbow trout infected with C. shasta was described. A group of rainbow trout were ex­posed to ceratomyxosis and tissue samples were examined histologically to check the progress of the disease with time. Infection was first detected in the pyloric caeca and descending intestine followed by a massive infection of the digestive tract and its associated organs. Intestinal mucosal epithelium was sloughed off and cells of the lamina propria, stratum compactum, and stratum granulosum pro­liferated to form a fibrous granulomatous layer in places of exposure to the intestinal lumen. Infection of visceral organs other than the intestines occurred from trophozoites which penetrated into the surface of the organs from the body cavity. Death of the host occurred when trophozoites infected the digestive system for most of its length.
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