the daily mortality rate just before moving was 1 percent a day. Five 

 other groups of infected fish, each with a population of 200,000 were re- 

 tained in five rearing ponds as controls. 



Results 



Characteristics of the virus 



Demonstration of causative agent. — Numerous attempts were made to 

 culture this agent on various bacteriological media; however, neither 

 bacteria nor any other microorganisms were consistently demonstrated 

 to be present in diseased fish. All attempts made to demonstrate the 

 infectious agent in diseased tissues were negative — either by examin- 

 ing wet mounts with the phase-contrast microscope or stained smears with 

 the light microscope. 



Infectious nature. --Although it was not possible to demonstrate 

 the causative agent of this disease, either by direct observation or by 

 cultural techniques, the infectious nature of the etiological agent was 

 easily proved. Sockeye-salmon fingerlings were readily infected orally, 

 intraperitoneally, and by contact. The relative effectiveness of each 

 route of infection is shown in figure 6. In one experiment, one group 

 was inoculated by intraperitoneal inoculation of a 10~* suspension. In 

 another experiment, groups of healthy fish were infected by one of the 

 following methods: Intraperitoneal inoculation of a 10 "2 suspension, 

 holding the healthy fingerlings in a trough with sick fingerlings, feeding 

 the fish a 1:1 mixture of ground, infected fish, or by holding dead fish 

 in the water supply of a healthy group of fingerlings . Although the intra- 

 peritoneal inoculation was the most effective method of infection, the 

 fish were easily infected by oral inoculation and by two methods of external 

 inoculation. It was likewise demonstrated that this infectious agent was 

 serially transmissible. Serial transfers were made of the infectious 

 material by inoculating a group of healthy fingerlings with a 10~ c suspen- 

 sion of the infectious material; when the fish showed symptoms of the 

 disease they were sacrificed and a filtrate was prepared and inoculated 

 into an additional group of healthy fingerlings. After 12 such serial 

 transfers, with a final dilution factor of lO - ^, n0 decrease in virulence 

 of the infectious agent was found. 



Filter ability . — In addition to showing that the agent was infectious, 

 the experiments also demonstrated that the agent was filterable. The 

 filtrate of a 10 suspension of diseased tissue retained its infectious 

 nature after it had passed through any of the common bacterial filters. 



Effect of storage. — For the laboratory study of .any virus, it is 

 desirable to have a homogeneous standard stock suspension of i-nown virulence, 

 For most virus studies, stock suspensions are stored in glycerine or frozen 



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