Graduate Thesis Or Dissertation
 

Erythrocytic inclusion body syndrome : a viral disease of salmonid fish

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  • Erythrocytic inclusion body syndrome (EIBS), a viral disease of salmon, was investigated. Reliable artificial infections were established in coho salmon (Oncorhynchus kisutch) via intraperitoneal injection of homogenized kidney, spleen, and blood from naturally infected animals. By examining the exposed fish periodically for viral inclusion bodies and hematocrits, the disease progression for EIBS was determined. Erythrocytic inclusion body syndrome had four stages which were incubation, inclusion body formation, lysis, and recovery. After an incubation period, circulating erythrocytes developed cytoplasmic inclusions and the numbers of infected cells increased to a maximum. The lysis stage followed and was characterized by lysis of erythrocytes with inclusions, lower hematocrits, and an accompanying anemia. When hematocrits were lowest, no cells with inclusions were observed. Following the anemia, the fish began to recover as indicated by an increase in circulating lymphocytes and immature erythrocytes. When the fish had recovered, their hematocrits were normal, serum was protective in passive immunization experiments, the fish showed resistance to reinfection, and tissues from the recovered fish were no longer infectious. The four stages of EIBS were discerned in infected fish held at temperatures of 6, 9, 12, 15, and 18 C but as the temperature increased, the incubation time, time to maximum inclusions, duration of the inclusion stage, and the time to recovery decreased. Numbers of inclusions were highest in fish held at 6 and 9 C. In histological analysis, cells of the liver, anterior kidney, and spleen from infected fish were not necrotic. Hemosiderin, a protein-iron pigment accumulated in the spleen following cell lysis. Utilizing the artificial infection, rainbow (Oncorhynchus mykiss) and cutthroat (Oncorhynchus clarki) trout were determined to be susceptible, but the disease was much less severe in these species than in chinook (Oncorhynchus tshawytscha) and coho salmon, the natural hosts of the virus. Additional results indicated EIBS is transmitted horizontally and that infected fish may be immunosuppressed. Staining of blood smears with acridine orange has indicated the genome of the virus is single stranded nucleic acid.
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