312 FISH HAICHERV MANAGEMENT 



KIDNEY DISEASE 



Kidney disease is a chronic insidious infection of salmonid fishes. The 

 disease is slow to develop but, once established, it may be difficult to con- 

 trol and virtually impossible to cure. 



The causative bacterium of kidney disease (Renibacterium salmoninarum) 

 is a small, non-motile, nonacid-fast, gram-positive diplobacillus. 



The course of kidney disease is similar to that of a chronic bacteremia. 

 Once the pathogen enters the fish via infected food, or from contact with 

 other infected fish in the water supply, the bacteria multiply slowly in the 

 blood stream. Foci of infection develop in the kidney and in other organs 

 such as the liver, spleen, and heart (Figure 84). White cellular debris col- 

 lects in blisters and ulcers that develop in these organs are seen easily. Le- 

 sions developing in the posterior kidney are easiest to spot and may reach 

 a centimeter or more in diameter. Some lesions extend into the muscula- 

 ture and result in externally visible blisters under the skin. If the disease 

 has reached the stage in which gross lesions are apparent, therapeutic treat- 

 ment has little effect (Figure 85). At best, drug therapy will only cure light- 

 ly or newly infected fish. This difficulty in the control of kidney disease is 

 the basis for classifying it as a reportable disease. 



Although kidney disease first was reported in the United States in 1935, 

 a similar, and probably identical, condition termed "Dee disease" was re- 

 ported in Scotland in 1933. The disease has been found in 16 species of 

 salmonids in North America. A tendency towards seasonal periodicity has 

 been noted, but the incidence varies at different hatcheries. Chinook, coho, 

 sockeye, and Atlantic salmon and brook trout are highly susceptible, but 

 the disease is not known among nonsalmonids. 



Infected or carrier fish are considered to be sources of infection. Experi- 

 mentally, from 1 to 3 months have elapsed before mortality began. 



Historically, diagnosis of kidney disease epizootics has been based on the 

 demonstration of small, gram- positive diplobacilli in infected tissues. How- 

 ever, the accuracy of such identifications is uncertain and more reliable 

 serological procedures such as fluorescent antibody techniques should be 

 used. 



Until the sources and modes of infection in hatcheries are known, strict 

 quarantine and antiseptic disposal of infected fish are recommended. lodo- 

 phor disinfection of salmonid eggs may be of benefit in preventing 

 transmission of the organism with eggs, but it is not completely effective. 



Under laboratory conditions, erythromycin (not registered by the Food 

 and Drug Administration) given orally at the rate of 4.5 grams per 100 

 pounds of fish per day for three weeks gave the best control but was not 

 completely effective. Treatments under field conditions have given similar 

 results; cures were effected in some lots, but among others the disease 



