224 TWELFTH ANNUAL REPORT OF THE 



which the previously smooth overlying scales set up. Pressure with the 

 fingers on the roughened scales will force out the contents of the scaly 

 pouch in a jet. 



At the same time scale defects appear, and at different points ecchy- 

 moses are found which are especially bad on the fins. 



In the early stages of the disease the fish seem to suffer little from it. 

 Later on their movements are less rapid, their respiration is quickened, 

 they become constanly more feeble until the caudal end often seems entirely 

 paralyzed and becomes immovable and the fish, lying on its back struggling 

 for breath, dies after one da}-. 



In section the skin of the belly is seen to be greatly reddened — occa- 

 sional hemorrhagic spots appear in the kidney and liver. The belly cavity 

 usually contains a bloody humor. 



Cause of the Disease. 



Dr. Marianne Plehn, of the Biological Experimental Station at Munich, 

 attributes the disease to the crab pest bacillus {Bacillus pestis Astaci) 

 which is found in the blood and in all the organs of the sick fish. A com- 

 mon infection is produced under which the fish succumb before the charac- 

 teristic symptoms appear on the skin and especially the roughened scales. 

 Under what special conditions this general infection with the forming of an 

 exudation in the scale pockets develops is not yet clearly understood. The 

 bacillus agrees fully in all morphological and physiological attributes with 

 the crab pest bacterium. If crabs are injected with pure bacteria culture 

 from fish, the characteristic symptoms of the crab pest will be produced. 

 The only difference between the crab pest bacteria obtained from fish, and 

 those which are usually found in crabs is that they first show a smaller 

 degree of virulence which increases in degree, however, so that one 

 repeatedly conveys the bacteria through the crab's body. 



As to the origin of the scale disease, it has long been known that the 

 infection proceeds from the skin outward. This is corroborated by the 

 fact that the roughening of the scales usually begins on the places on which 

 scale defects were already present — therefore the impossibility of infecting 

 uninjured fish if placed in crab-bacteria infected water, while the infection 



