60 U. S. BUREAU OF* FISHERIES 



in their turn invade uninfected cells and repeat the cycle. Under 

 certain conditions the intiacellular parasites may multiply very 

 rapidly, so that a large percentage of the cells become infected. 

 Since the infected cells are destroyed eventually, this can not fail to 

 result in serious injury to the host, (^)n the other hand, some of 

 the intracellular forms undergo a quite different course of develop- 

 ment and eventually make their way into the lumen of the intestine 

 as typical flagellates. The flagellates multiply by binary fission and 

 often become enormously abundant, especially in young fish. In 

 fact, in some hatcheries it is the excej^tion to Hnd trout less than 

 one 3^ear old which do not harbor considerable numbers of the 

 flagellates in their intestines. 



The spread of the parasite from fish to fish probably is accom- 

 plished by means of cj^sts, which can ordinarily be found in small 

 numbers in the intestines of infected fish. These cysts usually are 

 ovoidal in form and at first contain a single flagellate surrounded 

 by a thin, transparent membrane. Soon after encystment the in- 

 closed organism divides into two, and in this condition the cysts 

 pass from the body with the excrement. They no doubt remain 

 alive for some time in the water, and when acciclentally ingested by 

 another fish presumably pass to the intestine, where they set up a 

 new infection. 



The injurious effects of the parasite on the host may take two 

 quite different forms, which, it is believed, are correlated with the 

 two cycles of development previously referred to. The most com- 

 mon result of infection by Octomitus is the production of thin, 

 emaciated fish known to fish culturists as " pinheads." Such fish 

 usually are found to be infected with large numbers of flagellates, 

 which evidently seriously interfere with nutrition. This forn; of 

 octomitiasis is common in many hatcheries, and while not neces- 

 sarily accompanied by serious mortality, does much harm by inter- 

 fering with the normal growth of the fingerlings. On the other 

 hand, there is reason to believe that serious epidemics accompanied 

 by heavy mortality may be traced to Octomitus. In such cases there 

 is a very rapid increase in the intracellular stages, which ordinarily 

 are not abundant, and this results in extensive destruction of the 

 epithelium and more or less congestion of the intestinal wall. 



Since the parasite is widel}^ distributed and occurs in adults as 

 well as in the young fish, it is believed that it will be impracticable 

 to eradicate it from the hatcheries. However, there is every reason 

 to believe that much can be clone by improving the general condi- 

 tions under which fish are held so as to increase their hardiness 

 and vigor. The evidence at hand indicates that octomitiasis is 

 likely to occur whenever trout fingerlings are subjected to unfavor- 

 able conditions which tend to lower their vitality, but if the fish 

 are ke])t healthy and vigorous there is usually little trouble from 

 the disease. 



In addition to Octomitus, a species of aiueba is very common in 

 trout, although in spite of its abundance it apparently does little 

 harm. Unlike Octomitus, the vegetative stages of the ameba occur 

 only in the stomach, where they can be found crawling slowly about 

 over the epithelial lining. This species is of considerable interest 

 from a scientific standpoint, since it differs radically in many re- 



