A CONTAGIOUS DISEASE OF SALMON, POSSIBLY OF VIRUS ORIGIN 



By R. R. Rucker, 1 Fishery Research Biologist, W. J. Whipple 1 and J. R. Parvin, 1 

 Fish Culturists, and C. A. Evans, 2 Microbiologist 



OUTBREAK OF DISEASE AT THE LEAVENWORTH STATION, 1951 



An outbreak of disease in 1951 among two 

 groups of salmon fingerlings at the U. S. Fish 

 and Wildlife Service station, Leavenworth, Wash., 

 caused extensive losses. One of the affected 

 groups was the blueback, or sockeye. salmon 

 [Oncorhynchus nerka), which spends the first 12 

 to 24 months of its life in lake waters, and most of 

 the next 2 years in salt water. It then returns to 

 fresh water to spawn when about 4 years old. The 

 other group was the landlocked salmon, or kokanee 

 (0. nerka). It is of the same species, but differs 

 from the blueback in that it spends its entire life 

 in fresh water. 



Transmission experiments indicated that a fil- 

 trable agent, presumably a virus, was the cause of 

 this disease. The appearance of the sick fish, the 

 findings at autopsy, and certain other character- 

 istics of this epizootic closely resembled a disease 

 seen previously among kokanee and blueback- 

 salmon fingerlings at the Winthrop, Wash., sta- 

 tion. Diet is not discussed, as the diets employed 

 were used successfully at other stations in rearing 

 similar fish. 



This disease appears to be a significant prob- 

 lem in practical hatchery management. In ad- 

 dition, it is of interest in the field of comparative 

 pathology in view of the probability that the 

 causative agent is a virus. A noteworthy feature 

 seen in a small number of the infected fish, partic- 

 ularly those that survive for relatively long 

 periods, is a peculiar distortion of body shape 

 comparable to scoliosis and lordosis as manifested 

 in certain human diseases. 



Facilities for this study were furnished by the 

 School of Fisheries, University of Washington. 



Technical assistance of J. M. Alexander and S. W. 

 Watson is gratefully acknowledged. 



DISEASE AMONG BLUEBACK-SALMON 

 FINGERLINGS 



A group of about 1,400,000 blueback-salmon 

 fingerlings which started feeding at the Leaven- 

 worth station on January 5, 1951, had insignificant 

 losses — fewer than 300 daily — for 2i._. months. 

 These fish occupied 102 t roughs. < )n about March 

 20, fish in one of the troughs were observed to 

 have a higher mortality than those in the other 

 troughs. Mortality data for the next 10 days are 

 not available, but it is known that by the end of 

 .March about 13,000 fish were left in this trough. 

 They were now approximately "2 inches long and 

 3 months old. Table 1 shows the number of these 

 that died each day, April I to 10. After this date 

 the surviving fish were used for therapeutic 

 studies, and practically all of them eventually 

 died from the disease. 



Table 1. — Daily mortality in 18,000 blueback-salmon 

 fingerlings, Leavenworth station, Ipril 1-10, 1951 



'Fish and Wildlife Service, United States Department of the 

 Interior. 



2 Department of Microbiology. University of Washington Medi- 

 cal School, Seattle. Wash. 



238883° — 53 



A similarly severe mortality occurred in five 

 more troughs. In the six troughs, with a total 

 population in excess of 80,000 fish, the mortality 

 rate during March, April, and the first 3 weeks 

 of May was virtually 100 percent. In contrast, 

 during this same period the mortality rate in the 

 remaining 96 troughs that contained some 1,300,- 

 000 fish was approximately 1 percent a month. 



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