266 BULLETIN OF THE BUREAU OF FISHERIES. 



bullhead. Figure 250 is a cross section through a lesion which is just beginning to de- 

 velop. The outer layer of the epidermis has begun to disintegrate, but the inner layer 

 is not yet noticeably affected. Figure 251 shows a little later stage. The epidermis 

 is now entirely destroyed in one place at the right in the figure. On either side the 

 epidermis is rapidly disintegrating. A somewhat later stage is shown in figure 252. 

 The outer layer of the corium which has been exposed by the destruction of the epidermis 

 is now beginning to show signs of disintegration. In late stages of the disease the corium 

 may be entirely destroyed in the center of the lesion, as shown in figure 253. 



As previously described , the lesion grows rapidly outward in all directions from the 

 center of infection. Figures 254 to 257 are from photomicrographs of sections through 

 the margins of lesions and show the disintegration of the epidermis caused by the out- 

 ward growth of the bacteria. Usually there is a noticeable hyperemia just underneath 

 the epidermis in this region. This is well shown in figures 255 to 257. The capillaries 

 in the outer portion of the corium become gorged with blood; eventually their walls 

 disintegrate and the blood fills the interstices of the corium and crowds between the 

 corium and the epidermis. Occasionally the blood corpuscles may even penetrate a 

 short distance into the epidermis (fig. 255). This is the cause of the reddish zone which 

 is often quite distinct around the margin. Later as the epidermis is destroyed the 

 corpuscles also disintegrate. It is scarcely necessary to add that the hyperemic zone 

 advances outward as the lesion enlarges. Usually the hyperemia is noticeable only 

 underneath that portion of the epidermis which is undergoing disintegration, but oc- 

 casionally may occur before the overlying epidermis shows any change. No case has 

 been observed where the muscles showed any appreciable pathologic changes. Possibly 

 this may be due to the fact that the fish usually die by the time the lesions have developed 

 to the stage shown in figure 253. 



While the above account is based on a study of the lesions in bullheads the same 

 conditions are found in other fishes with some slight modifications due to the presence 

 of scales. In these fishes the scales become loosened and slough off as the integument 

 disintegrates. 



In all cases there is a thick, matted layer of bacteria covering the lesion. This 

 layer is not shown in the figures since it invariably drops off during the treatment to 

 which the tissues are subjected in preparation for sectioning. Although the bacteria 

 occur in enormous numbers in this superficial layer only a few scattered individuals can 

 be found among the disintegrating cells of the epidermis and corium. 



When growing on the gills the bacteria produce much the same effects as on the 

 integument. The epithelium and blood vessels are entirely destroyed until only the 

 skeletal parts are left. As would be expected gill infections are much more quickly fatal 

 than infections on the surface of the body. 



METHODS OF INFECTION. 



Since the bacteria grow only on the surface of the body and gills it is evident that 

 infection can easily take place by their being carried in the water from one fish to another. 

 As previously pointed out the bacteria are continually being set free from infected fish, 

 especially in late stages of the disease. Where fish are crowded closely together, as in 

 aquaria, the bacteria can thus readily pass from one individual to another. They are 



