BACTERIAL DISEASE OF FRESH-WATER FISHES. 267 



also easily rubbed off onto the nets or hands when diseased fish are handled and then can 

 readily be transferred to healthy fish. We have several times infected healthy fish by 

 simply holding them in our hands after having previously handled diseased fish. There 

 is also a possibility that the disease may be transmitted by predacious insects, but no 

 experiments have as yet been carried out along this line. 



Undoubtedly the most important factors in the spread of the disease under ordinary 

 conditions are those which render the fish more susceptible to infection. All the evidence 

 at hand indicates that the bacteria rarely or never attack healthy, uninjured fish under 

 ordinary circumstances. If, however, a fish is injured or its vitality lowered in any way 

 it is liable to contract the disease. Most outbreaks of the disease which have come 

 under the writer's notice have occurred among fish which had been recently handled. 

 As is well known it is almost impossible to handle fish without some slight injury to the 

 fins or body, and even a very slight injury is all that is necessary to enable the bacteria to 

 gain a foothold, and once started they are able to spread rapidly over the body. During 

 warm weather, when the temperature of the water at Fairport averages about 75 to 

 8o° F., an outbreak of the disease is almost certain to occur within 48 to 72 hours after 

 the fish are handled. Well-defined lesions may sometimes appear in 24 hours, but the 

 fish usually do not begin to die until after 48 hours. The greatest mortality usually 

 occurs on the third and fourth days, after which there is a gradual decrease until the disease 

 finally disappears. If the fish are again handled, however, there is almost certain to be 

 another outbreak. 



In a large percentage of cases the disease first makes its appearance on the fins, 

 especially the caudal. This is, of course, readily explained since no other part of the body 

 is so liable to be injured by the struggles of the fish when taken in a net. Of course, any 

 slight abrasion on the surface of the body, such as the removal of a few scales or even a 

 slight injury to the epithelium, is equally liable to infection. It has been found very easy 

 to infect fish artificially by simply scraping away a few scales with a scalpel and applying 

 a small amount of material scraped from a diseased fish. Usually a characteristic lesion 

 will develop around the site of infection in 24 to 48 hours. 



There is also evidence that in some cases lowered vitality unaccompanied by me- 

 chanical injuries may result in the fish becoming infected. This is probably the explana- 

 tion of the epidemic which appeared in one of the ponds at Fairport in which young 

 buffalofish were being held for a feeding experiment. These fish had not been handled 

 for some weeks previous to the outbreak of the epidemic. Just before the disease 

 appeared the water had become noticeably foul and this, no doubt, so lowered the vitality 

 of the fish as to make them susceptible to infection. The writer has also been informed 

 by fishermen that they had seen fish with the same disease in isolated ponds and sloughs 

 along the Mississippi River late in summer. At this time the water in these ponds 

 becomes very warm and stagnant, conditions which would favor the development of the 

 bacteria while tending to lower the vitality of the fish. 



Another important factor influencing the spread of the disease is the temperature of 

 the water. As already pointed out columnar is is very susceptible to temperature changes 

 and grows rapidly only at a comparatively high temperature. The disease is distinctly 

 a warm-weather disease, and so far as our observations go is not ordinarily of great 

 importance when the temperature of the water is low. This was very noticeable at Fair- 

 85781°— 22 ">. 



