Host-Parasite Relation 



Incubation period. — Experimentally, the incubation period of this 

 disease varied with the amount of infectious material inoculated, 

 virulence of the virus (titer), source of inoculum, route of inoculation, 

 and the water temperatures at which the inoculated fish were held. 



The maximum incubation period was not demonstrated in this investiga- 

 tion since it was impossible to determine whether the fish which died in 

 the latter part of an experiment died from the original inoculum, or from 

 a reinfection contracted by contact with fish which had become diseased 

 during the initial phases of the experiment. However, deaths have occurred 

 in groups of sockeye fingerlings up to h$ days after the original inoculation, 

 The minimum incubation period was found to be 1 day. 



In general, the higher the titer of the infectious material, the 

 shorter the incubation period, and conversely, a corresponding increase 

 in the incubation period as the dilution increased. "When the titer of the 

 infectious material was 10~° at the start of the epizootic at Leavenworth 

 in 19!? 3 , the incubation period was 3 to 5 days, but when the titer rose 

 during the middle of the epizootic to 10"-*-, there was a drop in the minimum 

 incubation period to 1 to 2 days. Likewise, in the latter part of the 

 epizootic at this hatchery, the titer dropped to 10"^ and the minimum in- 

 cubation period increased to 7 to 10 days. When hundredfold dilutions of 

 diseased tissue were made and inoculated into healthy fingerlings, the 

 minimum incubation period in the fish inoculated with the 10~2 suspension 

 was from 1 to U days shorter than in those inoculated with the highest 

 dilution which was still infectious. 



The incubation period was also contingent upon the route of inocula- 

 tion. The difference in the incubation period with different routes of 

 inoculation is seen in the comparison of groups of sockeye fingerlings. 

 each infected by a different route of inoculation (Figure 6). The disease 

 in those fish infected with material having a titer of 10"^ by intra- 

 peritoneal inoculation, or by being held in a suspension of infected 

 tissue, had a shorter incubation period than when the fish were fed in- 

 fected food or held in contact with dead fish. 



In other experiments, using material having a lower titer, similar 

 differences in incubation periods were found; however, with a decrease 

 in titer there was an increase in the incubation period. 



Change in virulence during an epizootic . —Two phenomena were found 

 during the epizootic at the Leavenworth hatchery in 19^3 indicating a 

 change in virulence of the virus during the course of an epizootic. 

 First, it was found that the virulence of the infectious agent increased 

 shortly after the onset of an epizootic in any given trough, but decreased 

 at the end of the epizootic. Second, it was found that fewer fish died in 



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