felt that an extensive investigation of this subject offers little promise 

 for the control cf the disease . 



This investigation showed fairly conclusively that the disease spread 

 rapidly through a population in a trough or a pond, once one or more fish 

 became infected,, Once a trough or pond became infected, reducing the number 

 of fish per volume of water, chemotherapy,, or any other change in their en- 

 vironmental conditions did not alter the course of the disease significantly, 



Although Rucker et al. , (1953) indicated that the disease was easily 

 spread from diseased to healthy troughs,, neither experimental results nor 

 observations during the majority of epizootics substantiated those findings. 

 Except for the original epizootic (Leavenworth) described by Rucker et al ., 

 and the epizootic at Cook in 1953* it did not appear that the disease was 

 disseminated from trough to trough by fish-cultural procedures as carried 

 on by hatchery personnel. However, at these two hatcheries when the dis- 

 ease occurred, the fish were so small they could have been transferred un- 

 noticed into the healthy troughs in the bristles of the cleaning brush. 

 It is believed by the present investigators that once the fish become so 

 large that this danger no longer exists, the dissemination of the disease 

 by contaminated cleaning and feeding equipment is slight. However, it must 

 be emphasized that hatchery personnel should still take every sanitary 

 precaution to prevent the spread of the disease. 



The most promising method for control of the disease, at the present 

 time, is the elimination of the source. Should the source be found to be 

 aquatic or airborne vectors, or should it be demonstrated that the virus 

 is being transmitted from the adults through the eggs to the fingerlings, 

 the determination of a control measure will, no doubt,, be a laborious and 

 lengthy procedure. Should it be demonstrated that the virus is being in- 

 troduced by the feeding of fish products, the elimination of the disease 

 in hatchery -reared fish should be simple . 



ACKNOWLEDOffiNTS 



The authors thank all those to whom they are indebted for having made 

 this study possible. Part of this work was done at the Fisheries Center 

 at the University of Washington, while the remaining experiments were con- 

 ducted at the U. S. Fish and Wildlife hatcheries at Cook, Entiat, Leaven- 

 worth, and Winthrop, all located in the State of Washington. We thank 

 Dr. Van Cleve, Director of the School of Fisheries ", and Mssrs,. Hagen, 

 Garlick, Gastineau, Burrows, and Bernier of the U. S. Fish and Wildlife 

 Service for making these facilities available. Our thanks are also due 

 Mssrs. Johnson and Palmer who helped plan and execute some of the experi- 

 ments, and Mssrs. Soderstrom and Holliday who very kindly gave invaluable 

 technical assistance for which we are particularly grateful. 



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