in other months, unless there were factors during this period which 

 predisposed the population to the disease. Experimentally, it has not 

 been possible to demonstrate the existence of such factors. If the 

 disease were introduced by aquatic or airborne vectors, the higher in- 

 cidence during these months could possibly be correlated with an increase 

 in insects possibly acting as vectors during this period. It is inter- 

 esting to note that when the percentage of the troughs infected at the 

 Leavenworth station in 195>3 is plotted against time, a sigmoid curve is 

 obtained (fig. 9), which suggests a limiting factor during the course of 

 the epizootic. Possibly this limiting factor could be the increase and 

 decrease of an insect population which serves as a vector in the disease. 



It is not clear why only 69 of the 132 troughs at Leavenworth in 

 1953 became infected and why all but two of these troughs became infected 

 between June IS and July 1$. All viscera were removed from the diet on 

 June 22, but fed again to one-half of the remaining healthy fish starting 

 July 30. None of these troughs became infected though they were fed a diet 

 containing $0 percent viscera. It would seem likely that if the virus had 

 been introduced into the hatchery population through the viscera, some of 

 the viscera fed after this date would have been infective. It is possible 

 that by the end of July the resistance of the fish had increased enough to 

 make them immune to the amount of virus carried in the frozen viscera, or 

 it is possible that none of the viscera fed at this time contained virus. 

 This removal of viscera from the diet may be the limiting factor of the 

 sigmoid curve (fig. 9). If some insect were acting as a vector this 

 phenomenon might be explained by the increase of the population during the 

 period when the majority of the troughs became infected and the subsequent 

 decrease in the insect population during the period when only 2 of the 52 

 remaining healthy troughs became infected. 



Of possible significance is the climatic condition at the hatcheries 

 and the time of year during which the epizootics have occurred. Two of 

 the hatcheries are located in western Washington where the climate is 

 milder and wetter than in eastern Washington where three of the hatcheries 

 are located (figo 1). The onset of the epizootics in the three eastern 

 Washington hatcheries was almost simultaneous (see table l) . The 

 epizootic in the western Washington hatchery in 195 2 occurred 2 to 3 

 months after the onset of the epizootics in the eastern Washington hatch- 

 eries. In 195>3> the epizootics in western Washington hatcheries occurred 

 approximately 30 to UO days before the onset of epizootics in eastern Wash- 

 ington hatcheries. 



Control of the Disease 



Because viruses are intracellular parasites dependent upon the 

 metabolism of the host cell for their nutrition, most virus diseases 

 cannot be controlled, nor can the symptoms be alleviated by chemotherapy. 

 Because of this and because of the work done in the present investigation 

 where it was not possible to control the disease by chemotherapy, it is 



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