Significance and Nature 



Home (1928) was well aware of the importance of carriers when 

 he stated that "carriers are in all probability a chronic source of 

 infection to healthy fish in a river." He felt that a knowledge of 

 carriers would be of great value, since it vrould throw light on such 

 obscure points as the spread and persistence of disease. The difference 

 in the carrier rate in healthy years and epizootic years would be 

 valuable information. It might be possible to gauge the probable course 

 of the disease in a given season and test the effect of such preventive 

 measures as avoidance of stocking and the removal of ill-conditioned 

 fish from a river (Home 1928). 



At first the Furunculosis Committee (1930) was undecided whether 

 carriers of furunculosis were convalescent or paradoxical. In the 

 former the animal has gone through an attack of the disease^ and though 

 recovered, still continued to harbor the specific organism in some part 

 of its body. The parado:d.cal carrier is an animal harboring a pathogenic 

 organism without having suffered any recognizable disease. This type of 

 carrier apparently possesses a certain degree of tolerance to^vards the 

 infection but later nay succumb to an overt attack. 



In support of Arkwright's (1912) original suggestion, Blake and 

 Clark (1931) found that carriers which survived the infection studies 

 proved to be of the incubation type, that is, the infection remained 

 latent for a variable period but eventually the disease developed in 

 fatal form. The work of Blake and Clark (1931) has since been 

 thoroughly supported (Furunculosis Committee 1933? 1935),? and it is now 

 recognized that the true nature of furunculosis carriers is incubatory. 

 In three cases, they noted that a flaring up of infection occurred for 

 no apparent reason, trout dying of furunculosis after remaining alive for 

 at least 4 months after exposure to B. salmonicida . In three other 

 cases, they found that carriers did not recover after kidney puncture 

 but died of general infection by B. salmonicida, indicating that the 

 disease might remain latent until stimulated ty some circumstances 

 injurious to the host. 



In order to demonstrate whether adverse conditions of a less 

 violent nature would also lead to development of active disease in 

 carriers, Blake and Clark (1931) performed a test in v^hich five trout 

 suspected of being carriers were subjected to a gradually increasing 

 temperature (5° to 18° C). This is comparable to a change from mnter 

 to summer temperature but taking place much more rapidly. Four of five 

 trout died in A to 6 days, the survivor proving to be free from infection. 

 The Furunculosis Committee (1933) also demonstrated the death of carriers 

 folloTd.ng a rise in temperature. These facts are of special interest 

 since in nature furunculosis is usually prevalent in summer, and it 

 suggests that trout become in-fected in summer^ harbor a latent infection 

 during winter, and die of active disease when the temperature rises the 

 following summer. 



