Laboratory results 



Hematological studies (Watson et al 1956) 

 of the fish at the Leavenworth hatchery revealed 

 that some of the fingerlings in those troughs re- 

 ceiving the sockeye viscera diet, other than the 

 two troughs showing the typical accelerating 

 mortality pattern, were succumbing to the virus 

 infection though the remaining fish in these 

 troughs failed to show symptoms of the disease. 

 Additional hematological studies of healthy and 

 diseased fish are necessary to establish defin- 

 itely this hitherto unsuspected circumstance. 



All tests of the plankton and insects in 

 the water supply used at the Leavenworth station 

 were negative nor were any "blooms" noted 

 about the time the virus disease was detected. 

 Routine bacterial counts of the water supply re- 

 vealed Gram negative, motile rods as the 

 predominant organism . 



Experiments designed to demonstrate 

 acquired immunity revealed that recovered fing- 

 erlings had complete immunity to the infection . 

 Healthy fish from both stations which had not 

 been previously infected succumbed when inocu- 

 lated intraperitoneally with 0.10 ml. of a 1:10 

 dilution of infectious material obtained from 

 moribund fish while the recovered fingerlings 

 successfully resisted a similar dose of a 1:10 

 dilution. It was noted during the experimental 

 work that among the healthy fish from the Leaven- 

 worth hatchery inoculated as described above, 

 the first mortalities occurred in six days while 

 eleven days were required before the Winthrop 

 reared fish began to die . 



The "carrier" state of recovered fish was 

 also successfully demonstrated. In mid-July the 

 survivors of the infected troughs at the Leaven- 

 worth station were transported to the Seattle 

 laboratory and tested for the presence of the 

 virus by inoculation of a bacteria-free filtrate 

 prepared from these fish into healthy fingerlings . 

 No mortalities occurred among the inoculated 

 fish. Concurrent with this experiment about 50 

 healthy fingerlings were placed in the same 

 trough and just below the recovered fish. The 

 first mortality among the healthy fish occurred 

 one month later but was considered a "normal" 

 mortality. Two weeks later, however, two more 

 mortalities occurred and experimental evidence 



showed that these two mortalities were due to 

 the virus . By mid-September two of the formerly 

 healthy fish developed the characteristic "bent- 

 back" which occurs in recovered populations and 

 by mid-December five more fish had manifested 

 this condition. 



When the adult fish returned to spawn 

 during the fall months, it was again possible to 

 demonstrate the "carrier" state of the adults. 

 Unfortunately, by this time of the year, the 

 healthy fingerlings have reached an age at which 

 they become refractile to the pathogenic agent so 

 the tests are comparatively insensitive. Two of 

 the 24 adults taken at Lake Wenatchee were found 

 to carry the agent and there was some indication 

 that seven of ten spawning adults taken at Cultus 

 Lake in the Fraser River drainage in Canada were 

 carrying the virus though experimental results 

 were inconclusive. Of the 12 sockeye adults that 

 returned to University of Washington School of 

 Fisheries, Seattle, only one fish was infected; 

 ten from the Issaquah, Washington State hatchery, 

 which is higher up in the same watershed, were 

 found to be negative . Of ten kokanees (the land- 

 locked variant of the sockeye salmon) from Lake 

 Whatcom, Washington, three were carriers. 



DISCUSSION AND CONCLUSIONS 



The fact that the virus disease, though not 

 as widespread as in previous years, appeared in 

 three of seven ponds fed viscera at the Winthrop 

 station, while at the Leavenworth hatchery the 

 epizootic affected two of the 23 troughs being fed 

 British Columbia sockeye viscera, is significant 

 in view of the fact that none of the meat-fed con- 

 trols contracted the infection. Because of the 

 high mortality rate the disease may be cyclic in 

 nature, thus accounting for the relatively low total 

 mortality rate throughout 1954. The extensive 

 disinfection of all equipment and the efforts made 

 to prevent the spread of contamination in 1954 

 cannot be overlooked. Using normal hatchery 

 feeding and cleaning procedures allows the spread 

 of the infection before increased mortalities due 

 to the disease occur. Thus, by using sterile tech- 

 niques prior to the onset in the first pond or trough, 

 there is little possibility of spreading the virus 

 before the disease is overtly manifest. 



There can be little doubt that the primary 

 method of entry of the disease into the hatcheries 



