2-JO BULLETIN OF THE BUREAU OF FISHERIES. 



injuring them, but are capable of increasing enormously whenever proper conditions 

 arise. Such a state of affairs is not unknown as in the case of streptococci and pneu- 

 tnoeocei in man. 



TREATMENT AND CONTROL OF THE DISEASE. 



On account of the great economic importance of the disease special efforts have 

 been made to develop effective methods of control. Early in the investigation it 

 became evident that reliance must be placed chiefly on methods for preventing the 

 spread of the disease rather than to attempt to cure fish already infected. This is due 

 to the fact that the bacteria, although living exclusively on the exterior of the fish, form 

 a thick, matlike growth which protects the bacteria underneath from the effects of 

 chemicals. Furthermore, the bacteria soon make their way underneath the scales 

 which form an additional protection. To kill bacteria in such protected situations 

 requires a comparatively strong solution. 



A number of chemicals have been tried in the effort to find an effective control for 

 the disease, but only potassium permanganate and copper sulphate have been found to 

 be of any value. In all our experiments with chemicals we have first determined the 

 maximum concentration of the chemical which the fish can stand without serious injury. 

 Different species differ greatly in this respect, and unfortunately the fishes which are 

 least resistant to chemicals are often very susceptible to the disease. It was soon found 

 that the buffalofishes are among the most susceptible species, both to chemicals and to 

 the disease, and it is believed that any treatment which will succeed with these fish will 

 be equally successful with most of our common fishes. Having ascertained the strength 

 of the solution which the more susceptible fishes could stand without injury, we then 

 attempted to determine the minimum concentration which will kill the bacteria when 

 fully exposed to the chemical. Since the bacteria can not be grown on culture media it 

 is obviously impossible to determine with certainty when they are killed. It was found, 

 however, that fairly accurate results could be obtained by placing the bacteria on a slide 

 and then treating them with solutions of various strengths. The cessation of all move- 

 ment by the bacteria was taken as evidence that they were seriously injured if not killed. 

 While, of course, there are serious objections to this method it was found in practice to 

 work out remarkably well. 



Of course the time element is important in the use of any chemical. Treatment 

 with a very weak solution may be as effective as a much stronger solution if the time is 

 sufficiently increased. Temperature may also be an important factor, but we have 

 only meager data as to its effects. With few exceptions our experiments have been 

 carried on with river water at a temperature of 75 to 8o° F. 



During the summer of 1918 an extensive series of experiments with potassium 

 permanganate was undertaken. It was found to greatly weaken or kill the bacteria 

 in solutions so dilute that most species of fish could be kept in them for some time with- 

 out serious injury. In order to kill the bacteria it is necessary to treat the fish with 

 a 1 to 50,000 solution for 10 to 15 minutes. Weaker solutions have little effect. Buffalo- 

 fishes can undergo this treatment without serious injury, while the black bass and sunfishes 

 can stand a much stronger solution. A 1 to 20,000 solution was used on black bass 

 for the same length of time with good results. However, very few fishes can stand a 

 1 to 20,000 solution, and weaker solutions do not seem to penetrate far into the bacterial 



