OCTOMITTJS SALMONIS, A PARASITIC FLAGELLATE OF TROUT 19 
PATHOGENESIS 
The effects of the parasite on the host undoubtedly vary greatly under different 
conditions, and there is still much to learn in this connection. The evidence at 
hand is quite contradictory in some respects, but it is believed that much of this 
apparent discrepancy can be explained on the basis of the two cycles of develop- 
ment within the host. Since the extracellular and intracellular stages develop 
under such different conditions, it is to be expected that the effects on the host 
will show corresponding variations. 
There can he no doubt that fish may harbor large numbers of flagellates in 
the intestine without exhibiting any noticeable injurious effects. In fact, it has 
been a source of wonder to the writer that fish on occasion may contain such num- 
bers of flagellates and still appear to be in a healthy, vigorous condition. However, 
this appears to be largely a matter of age, and probably also of acquired immunity 
on the part of the host. Ordinarily Octomitus does not cause serious injury after 
the first summer, and it is a common occurrence to find yearling fish in a healthy 
condition in spite of the fact that they are infested with large numbers of flagel- 
lates. Since at most hatcheries practically all the fish become infected during the 
fingerling stage, it is reasonable to assume that these yearlings have developed 
more or less immunity to the effects of the parasite. However, the writer does not 
mean to imply that under such circumstances the parasites are harmless, for they 
probably interfere with the growth of the host — possibly to a greater extent than 
is realized. 
Among the fingerhngs the effects of a severe infestation by the flagellates are 
usually quite marked. The fish lose their appetite and become thin and emaciated, 
the large head and attenuated body suggesting the term “pinhead,” by which 
they are commonly known among fish-culturists. Such fish are usually weak and 
listless and in late stages of the disease may become too feeble to fight the cur- 
rent, being finally carried against the screen at the lower end of the trough, where 
they soon expire. 
This chronic form of octomitiasis is usually most prevalent during the spring 
and early summer when the fingerhngs are about 2 to inches long. While the 
mortality is usually not very heavy at any one time, this condition may persist 
for several weeks or months, with the result that the total loss may be consider- 
able, sometimes reaching as high as 50 or 75 per cent. 
Of course it is not contended that all “pinheads” are due to Octomitus, since 
anything that will interfere with nutrition will tend to have the same effect. How- 
ever, the examination of a large number of “pinheads” at several hatcheries has 
shown that such fish are almost invariably infected with the flagellate stage of 
Octomitus salmonis, and, furthermore, that on the average the number of flagel- 
lates present is much greater in the “pinheads” than in other fish from the same 
lot that are not so emaciated. 
No doubt there are a number of environmental factors that tend to bring on 
the chronic form of octomitiasis. Am ong these a rise in temperature, even though 
comparatively slight, and overcrowding are believed to be especially important. 
There is also reason to think that the character of the food may exert considerable 
