262 BULLETIN OF THE BUREAU OF FISHERIES. 



DESCRIPTION OF THE DISEASE. 



The disease is easily recognizable, being distinguished by well-defined character- 

 istics. Ordinarily the first indication of the disease is the appearance of one or more 

 characteristic dirty-white or yellowish areas on some part of the body. The infected 

 areas are usually quite conspicuous and increase rapidly in size. In some cases a large 

 proportion of the body may eventually become infected. With the increase in the size 

 of the lesions the fish become greatly weakened and usually die from 24 to 72 hours after 

 the lesions first become visible. 



The lesions may occur on any part of the body, but in the majority of cases first 

 appear on the fins, especially the caudals, and from there spread to adjoining portions of 

 the body (figs. 231-237). In late stages of the disease the lesions may cover from one-half 

 to two-thirds of the body, while the fins become badly frayed, the caudal sometimes be- 

 coming worn to a mere stub (figs. 233 and 234). There is considerable variation in the 

 site of the initial infection. As will be shown later the disease is usually the result of 

 injuries and first makes its appearance in the injured region. There is also considerable 

 variation dependent on the species of fish infected. In the crappies, for instance, the 

 disease is usually confined to the fins and gills (figs. 231, 232, and 243) and only rarely 

 does the infection spread to the body. Possibly this may be due to the fact that these 

 fishes are especially susceptible to the disease and die before there is time for the bacteria 

 to become widely spread over the surface of the body. 



Lesions on the gills first appear as small, white patches (figs. 243 and 244). These 

 spread rapidly, and the fish usually dies within a few hours (fig. 245). 



On bullheads (Ameiurus mclas and A . nebulosus) the lesions have a somewhat different 

 appearance from those on scaled fishes. They usually first appear as numerous small, 

 circular areas with sharp, distinct outlines (fig. 238). 



The centers of the lesions are dark blue overlaid by a whitish veil or cloudiness. 

 Surrounding this is a well-defined zone about 5 mm. wide characterized by a distinctly 

 reddish tinge due to hyperemia. This region, like the central portion of the lesion, is 

 overlaid by a slight cloudiness. Later the lesions often become confluent and cover the 

 greater portion of the body (figs. 239-241). For some reason, in fingerling bullheads 

 the disease more commonly starts on the caudal fin, as in other fishes, and the infected 

 area gradually advances toward the anterior end, the entire posterior end of the body be- 

 coming a dirty white (fig. 242). 



In many respects the lesions resemble those produced by Saprolegnia and, in fact, 

 have usually been confused with them. A careful examination will, however, readily 

 enable one to distinguish between the two, since the bacterial lesions do not present the 

 fuzzy appearance so characteristic of those infected with fungus. Of course they can 

 easily be distinguished by a microscopical examination, but this is usually not necessary. 



In some instances a bacterial infection may be followed by an infection with Saproleg- 

 nia, but in such cases the fungus is of secondary importance. During the summer very 

 little fungus has been observed at Fairport and infection with Saprolegnia appears to be 

 dependent on the temperature of the water. When the temperature is high (above 75 ° F.) 

 the bacteria develop rapidly and the fish die so quickly that the fungus does not have 

 time to develop to any appreciable extent. However, the bacteria are much more sus- 

 ceptible to any decrease in temperature than is the Saprolegnia, the result being that 



