DISEASE OF SALMON 



43 



BLUEBACK BROOD OF 1949 



A group of about 140,000 small blueback-salmon 

 fingerlings (Winthrop brood of 1949) showed a 

 tremendous mortality at the Winthrop station 

 during February 1950. These fish had hatched in 

 January 1950, started feeding, and reached a 

 length of about 1 inch in the month. The mortal- 

 ity occurred predominantly among the larger fish 

 in the group. 



The outbreak was characterized by a sudden 

 increase in mortality. Most of the fish in an in- 

 fected trough would die within a period of 2 

 weeks. The disease would appear in one trough, 

 then show up in one or two others, gradually 

 spreading from trough to trough, each suffering 

 severe losses in its turn. Finally, the disease 

 spread through the entile population. A loss of 

 80 percent in this group during the month of 

 February was attributed to this disease. 



Diseased fish were characterized by apathy, and 

 they did not respond normally to moderate stimuli. 

 A few fish developed scoliosis in March, and by 

 May about 5 percent of the remaining stock of 

 2,000 fish showed this affliction. Scoliosis was not 

 found among those [hat died during the earlier, 

 acute stage of the epidemic. 



At autopsy, small hei rhagic areas visible 



through the peritoneum were common; they often 

 extended into the muscle. Hemorrhagic areas at 

 the base of the pectoral or pelvic fins and in the 

 isthmus were frequent. Atrophy of the peduncle 

 was noted in a few cases. 



No significant organism could be found by direct 

 examination nor could any be consistently isolated 

 directly from these fish. A fluorescent pseudomo- 

 nad was frequently recovered from the yearling 

 bluebacks after they had become debilitated fol- 

 lowing injection of material from the small mori- 

 bund fish. This was considered a secondary 

 invader. The disease was infectious, as healthy 

 yearling bluebacks were killed when injected with 

 unfiltered and uncentrifuged saline extracts of the 

 small moribund fish: and their appearance, while 

 ill and at autopsy, resembled that of spontaneously 

 infected fish. Seitz filtrates and heated material 

 were innocuous. 



KOKANEE BROOD OF 1949 



The Winthrop station received 27,<><M). -JU-inch 

 kokanee fingerlings, about ti months old, from the 



Leavenworth station in October 1950. These fish 

 had an insignificant loss at the Leavenworth sta- 

 tion, but the mortality rate increased immediately 

 upon arrival at Winthrop and the losses were 

 hieh. The signs of the disease were similar to 

 those of the bluebacks. All attempts to demon- 

 strate an etiologic agent in tissue smears or by 

 cultural methods failed. Nutritional deficiency 

 was probably not of importance because similar 

 fish at other stations on the same diet were not 

 afflicted. Therapy by sulfonamides, antibiotics, 

 and baths in disinfectants was of no benefit. 



EXPERIMENTAL TRANSMISSION 



Successful experimental transmission of the 

 disease under natural conditions, using diseased 

 fingerling bluebacks, was accomplished at the 

 Winthrop station during the outbreak of March 

 1950. A screen partition was placed in a trough 

 of diseased fish near the lower end of the trough, 

 and about 4,000 diseased fish were held in the 

 upper end. One hundred healthy chinook-salmon 

 fingerlings (O. tshawytscha) , -2. indies long and 

 4 months old. and 100 healthy blueback-salmon 

 fingerlings, Do inches long and :> months old, 

 from disease-free stock at the Leavenworth sta- 

 tion were placed in the lower compartment. Dur- 

 ing the next month 5 percent of the chinooks died 

 from unknown causes. In the same period 9:5 per- 

 cent of the bluebacks died showing evidence of 

 tlic epizootic disease. They did not have the char- 

 acteristic lesions of kidney disease and in our 

 experience this bacterial infection has not been 

 encountered in fish as small as those used in this 

 experiment. Also, we have not transmitted kid- 

 nev disease by contact or feeding experiments. 



RELATION TO EPIZOOTIC AT LEAVENWORTH 



The experiments and observations in transmit- 

 ting the disease at Winthrop make it appear that 

 a Datura! source of infection may lie contami- 

 nated water. However, it is certain that during 

 the 195(1 epizootic at Winthrop, the water supply 

 was not a continuing source of infection. Fish 

 brought to the Winthrop station in March 1950 

 and cared for with equipment kept separate from 

 the diseased fish did not contract the disease. Ap- 

 proximately 12,000 blueback fingerlings were 

 shipped to Winthrop from Leavenworth on 

 March 15. 1950, and an additional 50,000 on 

 June 8. These fish averaged 1 and 1U, inches 



