Ten fish from each trough were inocu- 

 lated with the western strain, and ten were in- 

 oculated with the eastern strain . Inoculations 

 were made at weekly intervals for the first 2 

 months and consisted of dual punctures made to 

 a depth of 4 mm. with roughened needles dipped 

 into a heavy suspension of the specific bacter- 

 ium. Stable temperatures were maintained foi 

 17 weeks, but with warm weather the capacity 

 of the refrigeration unit was exceeded, and day- 

 time temperatures rose several degrees during 

 a period of 5 weeks. At that time continuation 

 of refrigeration was deemed impractical, and 

 the replicates were combined and moved to 

 stable 12.5° C. water. 



Results 



There was good agreement in the results 

 among the replicates used in this work. Of the 

 two strains of the bacterium used, the eastern 

 strain again proved to be the more virulent. 

 The pattern of mortality for this strain was 

 similar for both temperatures (fig. 1). At both 

 1° C. and 12 .5° C. the peak mortality occurred 

 about 90 days after the first inoculation. All 

 fish of the 7° C. group were dead at the end of 

 170 days. Mortality continued about 60 days 

 longer at 12.5° C, but only three fish were 

 involved, and in the end all died. 



the two strains of the bacterium. At 12.5° C. 

 eastern strain consistently evoked larger lesions 

 than western strain. Except for a short time 

 early in the experiment when the reverse was 

 true, eastern strain lesions at 7° C. were also 

 the larger. All lesions showed external regres- 

 sion toward the end of the second month -- just 

 before mortality began among fish inoculated 

 with the eastern strain . 



EMscussion 



The fact that some fish which had been 

 repeatedly inoculated by a shallow dermal punc- 

 ture were found to be without bacteria about 10 

 months after inoculations began and 8 months 

 after inoculations ceased can be interpreted as 

 either evidence of probable recovery in an in- 

 stance when initial infection sites were small 

 and rather localized, or of failure of systemic 

 infection to occur. 



Similarly, the clear separation of mor- 

 tality from each of the two strains of the bacter- 

 ium, in spite of the fact that the fish were mixed 

 in common troughs, is significant. This indicates 

 that cross -infection did not occur, and that the 

 disease is relatively difficult to transmit. 



CONCLUSIONS 



At 12.5° C. the fish inoculated with west - 

 ern strain began to die about 60 days after the 

 first mortality among those inoculated with east- 

 ern strain. . As indicated by the slope of the 

 line (fig. 1) the rate of mortality was low, and 

 more than half the fish survived till termination 

 at 302 days . They were killed and examined at 

 that time. Only one fish had kidney disease; it 

 had a surface lesion in which was found the kid- 

 ney disease bacterium, but the organism could 

 not be identified in smears made of the kidney. 

 Mortality at 7° C. among fish inoculated with 

 western strain began about 4 months after the 

 first inoculation and 70 days after mortality be- 

 gan in fish inoculated with eastern strain. It 

 reached a peak soon afterward, then gradually 

 subsided. Four fish survived to the 302d day, 

 and the causative organism was found in the kid- 

 ney of one of them . 



1 . In eastern brook trout under hatchery 

 condition, the oral route is not likely to be the 

 usual portal of entry of the kidney disease 

 bacterium. 



2. It is probable that the dermal route 



is the more likely portal of entry of this organism. 

 Physical factors and/or biological agents which 

 breach the skin barrier are highly suspect in in- 

 itiating kidney disease infections in trout. 



3. The methods of abrasion which were 

 tried are considered incompletely effective and 

 otherwise impractical for producing large num- 

 bers of uniformly infected fish. 



4. Strains of the kidney disease bacter- 

 ium can differ greatly in Oieir ability to produce 

 infection and disease. 



The development of lesions at the site of 

 inoculation reflected the relative virulence of 



5. Kidney disease is a slow-to-develop 

 infection in eastern brook trout. 



