22 
BULLETIN OF THE BUREAU OF FISHERIES 
tion to Octomitus may not have been involved. Theoretically it is not improbable 
that under certain conditions the intracellular forms may exhibit a sudden and 
rapid increase while the fish are still in the hatchery troughs which would result 
in a severe mortality. Fortunately, under ordinary circumstances there appears 
to be little danger of such an outbreak; otherwise it is difficult to account for the 
fact that at many hatcheries in which the fingerlings are known to be thoroughly 
infected with Octomitus there has been no sudden excessive mortality for a number 
of years. However, given the proper conditions there would seem to be no reason 
why the intracellular forms might not develop as rapidly at the hatchery as in cans 
during distribution. 
Although it is believed that there are thus two quite different forms of octo- 
mitiasis produced by different phases in the life history of the parasite, and that, 
as the result of environmental conditions, one or the other form may be more 
prominent, it is nevertheless true that in the great majority of cases we have to 
deal with a combination of the two types. In some instances the injurious effects 
appear to be due almost entirely to the flagellated stage, while in others it is the 
intracellular stages that are primarily responsible. Obviously the character and 
severity of the disease will depend on which stage of the parasite is the more abun- 
dant, although in most cases both stages are not without their influence. It is 
very probable that even in the chronic, wasting type of octomitiasis the mortality 
may to a considerable extent be due to the intracellular parasites, and a comparison 
of the dying fish with emaciated but fairly vigorous fish from the same lot has shown 
that in most instances the intracellular stages were distinctly more numerous in 
the former than in the latter. 
As previously pointed out, fish are sometimes infected with the intracellular 
stages when no flagellates can be found in the intestine. This is usually true only 
of an occasional fish, but in one notable instance a lot of brook-trout fingerlings 
1 to 1 Yl inches long were observed in which there was a heavy mortality, although 
no flagellates could be found in the intestine. The explanation of this was not 
apparent until it was found that the intracellular stages were abundant in the 
csecal epithelium. 
While apparently all species of trout and salmon may be infected with Octo- 
mitus, not all species are equally susceptible to the injurious effects of the parasite. 
At the bureau’s hatcheries Octomitus appears to be more injurious to the rainbow 
trout than to other species, although according to Dr. Emmeline Moore in the New 
York State hatcheries it is the brook trout that are the principal sufferers. The 
explanation of this fact is not evident, although it is suggested that since the 
bureau’s hatcheries at which most of the investigations on this parasite have been 
carried on are primarily rainbow hatcheries, it may be that a strain of Octomitus 
has been developed which is especially adapted to this species. The fact that the 
brood stock at these hatcheries consists entirely of rainbow trout may be significant 
in this connection. 
No evidence is at hand regarding the effects of Octomitus salmonis on young 
salmon, but since considerable losses sometimes occur among the fingerlings in early 
spring, at about the time its effects are most noticeable in trout, it is not improbable 
that Octomitus may be responsible. 
