72 Thirtieth Annual Meeting 



small wooden ponds where every fish is in plain sight at all 

 times. 



I do not forget that the source of disease may he looked for 

 in quite other directions than the one I have indicated. The food 

 would ])erhaps l)e the first to come under suspicion. It is very 

 dillieult of determination for the food which causes disease is 

 long past examination by the time the disease is manifest. Daily 

 examination hacteriologically, of a large number of rations,, 

 while involving a large amount of work, may be undertaken to 

 advantage when the more probable explanation is discredited. 

 Organisms to which the brook trout is susceptible can hardly be 

 habitually present in the livers fed, or the disease would be more 

 widespread than it is. 



Thus far in considering the sul)ject it has been assumed all 

 along that the parasitic organism invades the trout from without, 

 that is, is external to the fish. Xow it may conceivably l^e a per- 

 manent resident within the trout, even performing some n(U-mal 

 function, and not really foreign to it, though I expressed it that 

 way. and when the vitality of the trout is lowered, or the suscepti- 

 bility increased, under the conditions of domestication, it be- 

 comes a virulent parasite and destroys its host. A somewhat 

 analogous instance may be cited in man. Tlie cohui bacillus is a 

 normal and constant inhabitant of the human intestine. It 

 ordinai'ily does no harm aiiTl formerly was not reckoned a disease 

 ])roducer. It is now known to be frequently concerned in patho- 

 logic process. The intestines of the brook trout of course con- 

 tain always many bacteria, and it appears that the ])lood some- 

 times does in a])parent health. The identity and significance of 

 these organisms is little known at present. The qiu^stion cannot 

 l)e taken up to advaiitage until by experimental inoculation a 

 specific bacterium is established as the cause of this trout disease. 

 It will readily be seen however tliat should this hypothesis prove 

 true the disease is much mort' serious than at present appears,, 

 and that cement ponds will be cpiite useless in combating it, un- 

 less they happen to be more hygienic. The nuitter of prevention 

 would then resolve itself into a question of hygienic conditions 

 and keeping the hi'oods in a higli state of vigor. Presumably 

 this is best accomiilisbed hv imitation of th(> natural conditions,. 



