BACTERIAL DISEASE OF FRESH-WATER FISHES. 265 



of 1919. On account of the limited facilities at our disposal it was only possible to try 

 a few of the simpler media. Standard beef-broth agar and fish agar were tried, and 

 although bacteria appeared in the plates in large numbers no trace of columnaris could 

 be found. Several lots of media with a less acid reaction than the standard were tried, 

 including one with the same reaction as the river water, but the results were all nega- 

 tive. Attempts to grow the bacteria on fish serum were equally unsuccessful. Possibly 

 with facilities for employing a wider range of media it may be possible to isolate the bac- 

 teria. However, the failure to isolate the bacteria did not prove so serious a handicap 

 in the study of the disease as would ordinarily be the case. The bacteria can easily be 

 procured in large numbers from the lesions of infected fish, and their appearance is so 

 characteristic that they can be readily recognized. 



While, of course, the failure to isolate the bacteria in pure culture has rendered it 

 impossible to demonstrate the cause of the disease beyond question, the evidence is so 

 conclusive as to leave little room for doubt. The bacteria can always be found in 

 abundance in the lesions, and the disease can easily be produced in healthy fish by 

 scraping off a few scales and applying a few bacteria from a diseased fish. In advanced 

 stages of the disease there are usually large numbers of bacteria present in addition to 

 columnaris, but it has been found that in small lesions columnaris is invariably enor- 

 mously more abundant than any other organism, and in some cases when the lesions 

 first become visible there is a nearly pure culture of this species present. 



Strong evidence for the causal relationship of columnaris to the disease can be seen 

 in infected bullheads. The reader will recall that the lesions on these fish are charac- 

 terized by an outer reddish zone about 5 mm. wide, while the center of the lesion has 

 a quite different appearance. When very small the entire lesion presents the same 

 appearance as the peripheral zone in later stages. As the lesions increase in size the 

 characteristic blue center appears and enlarges with the growth of the lesion. It is 

 obvious that the red zone is found only where the skin has been recently infected and 

 that the darker color is due to destructive changes produced by the bacteria. If a little 

 material is scraped from the darker central portion of the lesion it will be found that 

 considerable numbers of bacteria are present in addition to columnaris. On the other 

 hand, if material from the red zone is examined it will be found that nearly all the bac- 

 teria present are columnaris. Other species are so few as to be scarcely noticeable. Of 

 course the same thing is true of lesions on other species of fishes, but owing to the absence 

 of scales it is more striking in the case of the bullheads than elsewhere. The great 

 abundance of columnaris around the advancing edge of the lesions is so noticeable that 

 it soon became a matter of routine in our work to always procure the bacteria from 

 this region. The smears from which the photomicrographs (figs. 246 and 247) were 

 taken were made from such material. 



PATHOGENESIS. 



As previously stated the bacteria grow only on the surface of the body or on the 

 gills. No case has been observed where they had penetrated any distance into the tis- 

 sues. When growing on the body they destroy the integument, the muscles sometimes 

 being exposed in late stages of the disease. For obvious reasons the development of 

 the lesions can be more easily studied in the case of scaleless fish, such as the common 



