DISEASE OF SALMON 



45 



tshawytscha, O. kisutch, or Salmo clarhi clarki ex- 

 posed under conditions that caused a high infec- 

 tion rate in O. nerka. We are inclinded to believe 

 that the South American disease is not the same as 

 the one described in this paper. 



Schaperelaus (1941) described a virus disease of 

 rainbow trout, in which the fish became dark in 

 color. The posterior third of the kidney was swol- 

 len, the surface corrugated. The head kidney was 

 arched in shape and mealy, pulp}*, or pasty in con- 

 sistency. The musculature over the kidney was 

 softened. The liver was often light, yellowish, or 

 spotty in appearance. The gall bladder was nor- 

 mal in half the cases investigated, otherwise it was 

 small and filled with a yellowish liquid. The anal 

 region was sometimes swollen and protruding. 

 The disease was produced among healthy fish by 

 injecting filtered material from suspensions of 

 both diseased kidneys and livers. The investigator 

 reported that the course of the disease is slow and 

 that .'50 to (io percent of rainbow trout can be lost 

 during the normal rearing period. He cited one 

 example in which 80,000 rainbow-trout fingerlings, 

 5 to 7 centimeters long, died in 1-t days as a result 

 of this disease. 



The disease among the blueback-salmon and 

 kokanee fingerlings at the Winthrop and Leaven- 

 worth stations seems to ditfer enough from any 

 diseases previously described to warrant its con- 

 sideration as a previously undescribed disease 

 caused by a filtrable agent. 



The development of scoliosis and lordosis among 

 the blueblack-salmon and kokanee fingerlings is of 

 interest. SchrSder (1930) mentioned that spinal- 

 column deformities are more common in fish than 

 in all other vertebrates. He described the ana- 

 tomical changes of a number of curvatures in fish 

 and reviewed the literature. Factors attributed 

 as causes of curvatures are heredity: an unequal 

 development of muscle or its elasticity: parasitic 

 infections in which muscle or cartilage is de- 

 stroyed; nutritional deficiences as avitaminosis or 

 lack of phosphorus or calcium: and oxygen defi- 

 ciency, unfavorable temperatures or improper 

 water balance in the developing egg. 



McCay and Tunison ( L934) found that brook- 

 trout fingerlings raised on a diet containing one- 

 third sheep plucks preserved in (.-percent formalin 

 developed scoliosis after about a year when they 



were approximately 6 inches long. 



SUMMARY 



1. An investigation was made of a disease af 

 fecting blueback-salmon and kokanee fingerlings 

 (Oncorhynehtis nerka) at the Leavenworth and 

 Winthrop stations in L951. The disease did not 

 infect fingerlings of the chinook salmon (O. 

 tshawytscha) , silver salmon ( 0. kisutch), or cut- 

 throat trout ( Salmo clarki clarki ) . 



2. The disease was characterized at the begin- 

 ning of an outbreak by a great increase in its in- 

 fectious nature, and a resulting increase in the 

 mortality of the fish. 



3. The total mortality in a population of 800,- 

 000 to 1,400,000 salmon ranged from 11 to 40 per- 

 cent. A decrease in the epizootic appeared to fol- 

 low sterilization of cleaning equipment. The in- 

 cidence of the disease decreased during summer 

 months, and it was suspected that the higher water 

 temperatures were a favorable factor. 



4. The disease was produced by material that 

 passed through bacteria-proof filters. The infec- 

 tion that was produced in healthy salmon by in- 

 jection of suspensions made from moribund fish 



resembled the disease as it occurred naturally dur- 

 ing the epizootic-. Some injected fish developed 

 scoliosis or lordosis. IJe-nlts of the injection of 

 salmon with heated material were negative. 



.">. The larger fish in a group were attacked first, 

 becoming infected when they were 1 to 1.5 inches 

 in length and about '"> monthsold. 



6. Diseased fish were lethargic and often exhib- 

 ited hemorrhagic areas at the base of the fins and 

 in the isthmus. The stomach was distended with 

 a milky fluid : and the intestine appeared reddened 

 and contained a watery, straw-colored fluid often 

 tinged with blood. The bile appeared normal, but 

 in greater quantity than usual. Ilyperemic areas 

 frequently were present on the air bladder, peri- 

 toneum, or fat. 



7. Spinal deformities developed in later stages 

 among some of the fish that survived the early 

 acute st age of the epizootic. 



8. Experimental infection was accomplished by 

 placing healthy fish in one end of a trough that 

 contained sick fish in the upper compartment. 



