present in the intestine of outwardly healtl:^ fish. Home (1928) showed 

 by laboratory experiments that "healthy" carriers were an important 

 soiirce of infection to healthy fish. Carriers harbor the organism mainly 

 in the kidney (Furunculosis Committee 1933)- 



Occurrence 



Arkwright (1912) records that of twenty- three trout which were 

 regarded as healthy controls for experimental work, four yielded cultures 

 of B. salmonicida . All these came from a single hatchery. On examining 

 eighteen live trout from the Kennet River, Home (1928) found that three 

 were harboring the specific organism. Williamson (1929) discovered seven 

 carriers from a trout rearing station. Three were detected from a 

 group of eighty brown trout and four from a group of nineteen rainbow 

 trout. 



In one district where furunculosis had been epizootic for 6 

 years, the Furunculosis Committee (1933) found two carriers. 



The carrier rate in nature no doubt depends on the presence or 

 absence of an epizootic and the trend of an individual epizootic. During 

 the initial peak of an experimental epizootic, it has been shown to be 

 as high as 34 percent (Furunculosis Committee 1935). 



The evidence indicates that carriers are not confined to any age 

 group or to any species of Salmonidae. Blake and Clark (1931) found 

 them among 2- and 3-year-old brown trout (S. trutta ). They have been 

 found among rainbow trout (S. gairdnerii ) and 2-year-old Atlantic salmon 

 (S. salar salar ). Probably'- any species of fish susceptible to the 

 disease is a potential carrier. 



After an experimental epizootic had subsided, the Furunculosis 

 Committee (1935) made a most thorough examination for carriers among the 

 remaining stock. Cultures were made from the heart blood and kidney in 

 all cases and in most cases the contents of the lower intestine, spleen, 

 and liver were cultured. Of 151 brown trout examined only 4 carriers 

 were detected. In each case B. salmonicida was found in the kidney, but 

 only one fish carried the organism in the liver. After this experimental 

 epizootic had subsided, the carrier rate was 2.7 percent. 



As alrea(fy mentioned, it has been shown experimentally that at 

 the initial peak of an epizootic the carrier rate is much higher — 34- 

 percent of the live fish netted for examination. The proportion 

 diminished progressively and by the time cold weather had set in 

 (December), no infected fish could be discovered in the surviving stock 

 (Furunculosis Committee 1935). 



41 



