troughs infected in the latter part of the epizootic than in troughs 

 infected at the first part of the epizootic. 



Within 2 weeks after the onset of the disease at the Leavenworth 

 hatchery in 1953, and thereafter for the next 3 months, infected fish 

 were taken at random from infected troughs and titrated into healthy 

 sockeye f ingerlings . Fifteen days after the onset of the disease on 

 June 30, the titer of the infectious material was 10"°; by July 3, how- 

 ever, the titer had risen to 10"10 t ^ titer of 10"^ was found through 

 July, but by August 1, when daily mortalities started to decline, the 

 titer had decreased to 10~6 and by August 13, it was 10'^. In another 

 week, the titer decreased to 10"'- . By October 1, the daily mortality 

 rate had returned to nearly normal and no filterable infectious agent 

 could be demonstrated in fish dying in the ponds or troughs (table U). 



It is believe that this change in titer of the infectious material 

 was indicative of a change in the virulence of the causative agent rather 

 than a change in the resistance of the fish to the disease. During the 

 period when the titer was 10~2 the virus was serially transmitted twice 

 and the titer returned to 10~°. 



Table h. — Variation of titer during course of the 



epizootic" at Leavenw orth hatchery in 1953 



June 30 10" 6 



July 2 10~ 8 



July 3 10~ 10 



July 16 10~ 10 



July 21 10-10 



July 23 10" 10 



August 1 10 -6 



August 13 10"k 



August 21 10 -2 



September 26 infectious agent could not be 



demonstrated 



In late August, a healthy trough became infected by contact from an 

 adjacent trough. In an attempt to gain additional information on this 

 apparent change in virulence, material from infected fish was titered 

 periodically during the course of the disease. The periods at which the 

 titrations were done is indicated by the arrows on figure 7. In contrast 

 to the previous experiments, the titer did not rise, but remained at 10~° 

 through the first "22 days of the eoizootic. The titer then dropped and re- 

 mained at lCT^ through the 36th day of the epizootic. The titer on. the 

 U2nd day was 10"^ (each titration based on that of at least 50 dead or 

 moribund fish) . 



25 



