of September, fish which had died of furunculosis were found regularly. 

 Both before and after these periods the effects of the disease were not 

 apparent. In another case the Committee observed that after a rise in 

 temperature and a fall in water level an outbreak of furunculosis 

 occurred. Thus during the months -svhen water may easily reach a critical 

 temperature^ the sequence of events appears to be as follows; After a 

 flood, fish ascend from the sea or from lower doTm in a riverj they 

 congregate in pools j the water level falls | they acquire the infection 

 and rapidly succumb. This need not be invariably the case, since in 

 other districts in Great Britain fish die of the disease, though not in 

 such great numbers, where there is no fall in water level nor any great 

 aggregation. 



If given a suitable temperature in an area where an infection 

 exists, and provided there is any aggregation of salmon, an outbreak is 

 almost certain to take place, unless the water level is such that the 

 fish continue their movement upstream,, giving them the best possible 

 conditions for maintaining their health (Furunculosis Committee 1933)« 

 Areas of greatest mortality are those where the fish are checked by an 

 obstruction in the river, and v^here of necessity they must collect in 

 considerable numbers relative to the size of the body of water. If, in 

 such a situation, the fish are fresh-run the mortality is more serious. 

 On the other hand, fish fresh-run from the sea is by no means a necessary 

 condition for an outbreak, nor does resistance for any particular length 

 of time in fresh water render them immune. 



The Furunculosis Committee (1933) also undertook experimental 

 work to determine the influence of tem-perature on the prevalence of the 

 disease. Tanks ivith thermostatic controls vrere used, and running v/ater 

 was well aerated and mixed ty compressed 6ir, Infection was produced by 

 contact with trout which had been inoculated intramuscularly with B. 

 salmonicida cultures. The occurrence and development of disease was 

 obs'erved at different temperatures — 5° C, (Al° F, ), 13° C. (55° F. ), 

 15° C, (59° F.), 180 c, (64° F.), and 21° C. (70° F.), 



Death from furunculosis resulted more rapidly at higher 

 temperatures (up to 21° C<, ), but there was an apparent optimum temperature 

 for the spread and development of the disease, the incidence being 

 greater at about 15° C« than at 21° C. or at 5° C. Thus, both very low 

 and unusually high temperat\ires may reduce the prevalence of the disease 

 among fish exposed to infection under experimental conditions (Blake and 

 Clark 1931^ Furunculosis Committee 1933)<> These findings accord in part 

 mth obser'/ations in nature that a favorable temperature range for the 

 spread of the disease is from 55° F. (13° C, ) to 66° F. (19° C). 



Blalce and Clark (1931) have suggested that it is possible that the 

 susceptibility of trout to the disease actually increases mth a rise in 

 temperature up to a point Tj-here the temperature itself is definitely 

 harmful to the fish. As the optijiTum temperature for growth of B. 

 salmonicida is about 20° C, the falling off in incidence of infection 



46 



