86 American Fisheries Society. 



behavior of the similar organism, Giardia, but which from the 

 nature of the problem in the human host cannot be so readily 

 investigated. 



Other aspects of the study of parasitism in the fish of im- 

 portance in comparative medicine have to do with double or even 

 multiple infections. These, also, have been observed in the fish. 

 Octomitus is clearly associated with a species of Entamoeba, 

 similar to that occurring in man. Yeasts with branching habit 

 occur frequently and suggest, from their abundance, a possible 

 role in the progress or decline of octomitiasis. 



A hatchery in which infection is severe affords unlimited 

 opportunity for research in this field of parasitology and, because 

 of its bearing on the human problem, offers a fertile field both 

 for study and experimentation. 



SUMMARY. 



Trout hatcheries suffer annual losses of fish from octom- 

 itiasis, a disease caused by a minute protozoan flagellate {Octo- 

 mitus salmonis Moore) parasitic in the intestine of the fish. 



Lesions occurring internally in the intestinal wall and in 

 the caeca are occasioned by the penetration of the organism 

 into the wall tissue, where certain developmental stages take 

 place. 



All species of trout are susceptible to the disease, though 

 the susceptibility of brook trout over other species is pronounced 

 and doubtless explains the inability to rear this species at some 

 of the older hatcheries. 



Mortality decreases with increase in size, so that an epidemic 

 among the smaller fingerlings may result in heavy loss. The 

 survivors of the disease may become "carriers." 



Transmission of the disease probably occurs by cysts, highly 

 resistant structures, which pass to the exterior in the excrement 

 and become widely disseminated through various agencies, or 

 through lack of proper sanitary precautions. Although some- 

 what similar parasites occur in the frog, the observations incline 

 to the belief that each type is specific for its host. 



Octomitus salmonis Moore is a binucleate, bilateral, parasitic 

 octo-flagellate, belonging in the order Polymastigina and in the 

 family Hexamitidae. The genus has been previously described for 



