648 REPORT— 1902. 



were carried out before or in the earlier days of bacteriology, and this perhaps 

 accounts for the possibility of a primary bacterial infection being overlooked. 



In 1886 G. Murray published a paper in the ' Journal of Botany ' in which 

 he appeared to give conclusive evidence that Saprolegnia was the true cause of 

 the disease, by his experiments on live fish with Saprolegnia taken from diseased 

 salmon and grown on dead flies. By rubbing the fish with Saprolegnia thus grown 

 he was able to produce the growth of Saprolegnia on a number of the fish, which 

 eventually died. At first sight such experiments appear to be pretty strong proof; 

 but when we take into consideration that Murray in his experiments was not 

 working with a pure, but a mixed growth, which might have contained numerous 

 bacteria as well as the fungus, it cannot be said that such experiments proved 

 Saprolegnia to be the cause of the disease. 



AVhat has always struck me as peculiar was that such a fungus as Saprolegnia 

 ferax, which grows so profusely on dead tissue, should grow so readily on live 

 tissue and cause so much damage, and I have always been inclined to think there 

 might be some primary bacterial infection to cause the dead tissue on which the 

 Saprolegnia developed. This view was further supported on three diseased salmon 

 being received for examination at the sanitarj' chambers, for, on cultures in 

 gelatine being made from them almost pure growths of a very rapidly liquefying 

 bacillus were obtained. 



I then resolved to thoroughly investigate the cause of the disease. 



Pure cultures of the liquefying bacillus and Saprolegnia were obtained from 

 diseased salmon. Great difficulty was experienced in obtaining pure cultures of 

 the fungus, as the mycelium was always covered with numerous bacteria, and it 

 was only after numerous cultures had been made that a pure growth was 

 obtained. The fungus was grown in flasks containing liver and water, both 

 sterile. This medium was found most suitable for obtaining spore formation. 



From the results of the numerous experiments carried out with these pure 

 cultures it may be deduced : 



(1) That Saprolegnia ferax is not the cause of salmon disease, as twenty-seven 

 fish of difi'erent kinds (rainbow trout, dace, river trout, sea trout, and gold-fish) 

 inoculated with and kept in direct contact with pure spore-bearing cultures of the 

 fungus in every case remained quite healthy, while there was a profuse growth 

 in each case on the boiled sheep's liver with which the fish were fed, in some 

 cases almost completely filling the jars, showing that the fish had every opportu- 

 nity of becoming infected with the fungus. 



(2) That salmon disease is caused by the liquefying bacillus (Bacillus Salmonis 

 pestis), as the bacillus was found present in almost pure cultures in all the eight 

 diseased salmon examined as well as the infected fish ; while experiments showed 

 the absence of the bacillus from ordinary dead fish. Five fish of different kinds 

 inoculated with pure cultures of the organism all died without showing any 

 growth of the fungus. On the other hand, six fish inoculated with pure cultures 

 of the bacillus + Saprolegnia all died showing growths of the fungus. Again, ten 

 fish inoculated with the fungus + other micro-organisms found in diseased salmon 

 remained quite healthy. 



Further proof was also obtained from the cut sections of the fish examined 

 microscopically, where the bacillus was seen in large numbers invading the 

 healthy tissue, while the fungus, when present, was only observed on the superficial 

 necrosed parts. 



The mode of infection appears to be that the bacilli gain entrance to the 

 softer tissues of the skin by a wound on the healthy fish or by a degenerated 

 mucus gland on the sickly fish, and passing along underneath the skin cuts it ofi" 

 from nutrition ; the skin thus cut off becomes degenerated and forms a suitable 

 nidus for the growth of the fungus. 



(3) Salmon disease is not contracted tvhen the skin of the fish is j"w a healthy 

 state, as two fish not scratched in any way and placed in jars of water containing 

 pure cultures of the organism and Saprolegnia remained alive and healthy ; while 

 on the other hand a fish placed in a jar of water containing the organism + Sapro- 

 legnia, with a layer of finely powdered sand at the bottom, in a very short time 



