OCTOMITUS SALMONIS, A PARASITIC FLAGELLATE OF TROUT 
11 
greater abundance of the parasites in hatchery fish being due to the exceptionally 
favorable conditions for its spread and development. This conclusion is borne 
out by the fact that allied species of flagellates occur in other fishes, both fresh- 
water and marine. 
Not only is Octomitus salmonis widely distributed geographically, but it has 
also been found in a considerable variety of hosts. It evidently occurs in all species 
of trout, having been reported from the brook trout (Salvelinus fontinalis) , rainbow 
trout ( Salmo sJiasta), brown trout {Salmo fario) , Loch Leven trout ( Salmo leven- 
ensis ), and lake trout ( Cristivomer namaycush). The writer has also found the 
flagellate in considerable numbers in fingerlings of the chinook salmon ( Oncorhyn - 
chus tschawytscha ) and the silver salmon ( Oncorhynchus Icisutch) . 
SYMPTOMS OF THE DISEASE 
The disease caused by infection with Octomitus salmonis and known as octo- 
mitiasis is not characterized by well-defined symptoms by means of which it can 
be readily distinguished from other ailments of trout. There are no external 
lesions, and the most common indication of the presence of the disease is the 
appearance of thin, emaciated fish, usually somewhat darker in color than normal, 
although in some instances they may be lighter. Owing to the relatively large 
head in comparison with the emaciated body, such fish are commonly referred to as 
“pinheads” by fish-culturists. Many of the “pinheads” may improve after a 
time and eventually resume their normal rate of growth, but others gradually grow 
weaker and weaker until finally death supervenes. 
More rarely the disease may occur in an acute form accompanied by a heavy 
mortality. Dr. Emmeline Moore (1923) refers to the disease as the “whirling 
sickness” and states that “Balance seems easily lost and the fish turn over repeatedly 
with a ‘whirling’ or ‘corkscrew’ motion in the water; too weak to make headway 
against the current, numbers of them are found in the corners at the foot of the 
trough or nosing along the sides near the surface; they lie on their backs with gills 
distended and in feverish action.” This whirling motion has been observed only 
rarely in our investigations, and in no case was it a prominent feature of the disease. 
It should be remembered in this connection that whenever fish become so weakened 
from any cause as to be unable longer to maintain their balance they turn on their 
sides, and any effort to swim necessarily results in their taking a spiral course. In 
one instance where the disease was exceptionally severe the dying fish were observed 
to he on the bottom of the trough and bend the body from side to side in quick 
spasmodic movements. Similar movements were also noticed when the fish were 
swimming at or near the surface, but in no case did they exhibit a whirling or spiral 
motion. 
Doctor Moore also refers to diarrhoeal symptoms as being specific for the 
disease, but this has not been noticeable in any of the diseased fish observed by us. 
It is true that the intestinal contents of infected fish are usually more fluid than 
normal, but this is believed to be due simply to the fact that such fish take com- 
paratively little food. 
The simplest and most reliable method of diagnosing the disease consists of a 
microscopical examination of the contents of the anterior end of the intestine. 
