PROGRESS IN BIOLOGICAL INQUIRIES, 1923. 21 



in different parts of the country. The parasite has been found to 

 be widely distributed and probably occurs at most of the bureau's 

 trout hatcheries. It is strictly an intestinal parasite, usually being 

 most abundant at the anterior end, but may be found throughout 

 the entire length of the intestine. It occurs in two quite distinct 

 forms that evidently represent different stages in the life cycle. 



The most connnon form of Octoniltus is the flagellate stage that 

 lives in the lumen of the intestine and under certain conditions may 

 be present in large numbers. In addition there is also an in- 

 tracellular stage that is quite different in appearance and occurs in 

 the epithelial lining of the pyloric ca3ca and the anterior end of the 

 intestine. This stage first makes its appearance as a small rounded 

 body lying in a vacuole in an epithelial cell. It grows rapidly, 

 producing hypertrophy of the infected cell, which eventually is 

 destroj'ed. After a time the parasite undergoes schizogony and 

 the resulting schizonts invade other epithelial cells and repeat the 

 process. On the other hand, some of the schizonts, instead of 

 again undergoing schizogony, after a short period of growth become 

 metamorphosed into the free-swimming flagellate. Usually both 

 intracellular and flagellated forms are found in the same host, but 

 under some conditions, which are not well understood, one form may 

 greatly predominate. This is especially true of the intracellular 

 stage, which may become very abundant without any corresponding 

 increase in the flagellated form. 



The effect of the parasite on the host varies greatly and is ap- 

 parently largely dependent on local conditions. Under ordinary 

 circumstances the injurious effects appear to be confined to the 

 fingerlings, older fish being affected little if at all. In the young 

 trout the parasite is often present in great numbers, the flagellates 

 usually being more abundant than the intracellular stages. The 

 flagellates do not ordinarily cause a severe mortality, but may seri- 

 ously interfere with the proper nutrition of the host. The fish 

 become greatly emaciated and instead of growing steadily may 

 even lose weight for a number of weeks. The disease usually is 

 more pronounced in the spring and apparently is associated with 

 a slight rise in the temperature of the hatchery water. In some 

 instances, for reasons not well understood, the intracellular stages 

 may become very abundant, producing an inflammation of the intestine 

 that often terminates fatally. There can be little doubt that many, 

 if not most, of the epidemics among young fish, which occasionally 

 break out in trout hatcheries in early spring, are due to this cause. 

 The intracellular stages are also the cause of an excessive mortality 

 that sometimes occurs when fish are being transported from the 

 hatcheries. Usually there is little loss for the first 24 hours, but 

 if the fish are held in cans for a much longer time the mortality 

 is sometimes very high. It has been found that for some reason 

 the intracellular stages multiply very rapidly when the fish are 

 placed in cans, producing a fatal enteritis. 



Since Octomitus is so widely distributed and occurs in the brood 

 fish as well as fingerlings it will doubtless be very difficult, if not 

 impossible, to eradicate it in the hatcheries. It is believed, how- 

 ever, that the parasite can be controlled and its injurious effects 

 reduced to a minimum. Although it has been found in wild fish, 



