OCTOMITUS SALMONIS, A PARASITIC FLAGELLATE OF 
TROUT 
By H. S. DAVIS, Ph. D. 
Fish Pathologist, United States Bureau of Fisheries 
Introduction. . 
Nomenclature 
Distribution 
Symptoms of the disease 
Morphology of the flagellate 
Multiplication of flagellates. 
Development of cysts 
CONTENTS 
Page 
9 
10 
10 
11 
12 
14 
15 
Intracellular stages 
Pathogenesis 
Control measures 
Summary 
Bibliography 
Explanation of figures 
Page 
16 
19 
23 
24 
25 
26 
INTRODUCTION 
It is well known that when any animal is confined under unnatural conditions 
various difficulties are encountered, which are of little importance in nature. This 
principle is well illustrated in the artificial propagation of fish, which is becoming 
more and more important because of the rapid rate at which the fish life of lakes and 
streams is becoming depleted. As a result of herding together large numbers of 
fish at the hatcheries, various parasites, which cause little or no injury in nature, 
have taken advantage of the exceptionally favorable conditions for their rapid 
increase and now present one of the most important problems with which the 
fish-culturist has to deal. Unless methods for controlling the various parasites 
and diseases so prevalent at hatcheries can be developed, the artificial propagation 
and rearing of fishes is threatened with failure. This is especially true of trout, 
which, owing to the ease with which the eggs can be handled, are crowded together 
in large numbers in a very limited space. 
Among the parasites of trout probably none is more important to the fish-culturist 
than the flagellate Octomitus salmonis Moore, since this species infests the young 
fish at an early age before they are large enough to leave the hatchery. The con- 
ditions in the hatchery troughs, where thousands of fingerlings are crowded to- 
gether in a narrow space, afford an ideal environment for the development of this 
parasite at a time when the fish are least able to resist its ravages. These facts, 
coupled with its wide distribution, have made Octomitus a serious menace to the 
success of trout culture. 
9 
