migratory Salmonidae even at this early stage of their adult life in 

 fresh water cannot be accepted as any indication of a marine source of 

 the disease. 



The FxiTunculos is Committee (1930) does not believe that salmon 

 contract the infection in fresh water prior to smolt migration and 

 continue to harbor it in a latent form during their life in the sea_„ 

 developing active disease on return to fresh water. As noted above they 

 have not found B. salmonicida in members of the Salmoni.dae netted at or 

 near the mouths of rivers emptying into the sea. Their examination of 

 large numbers of smolts (1339) from the Coquet proved negative. Although 

 it is not impossible that a few smolts may carry the infection to the 

 sea, it is improbable in view of the selective distribution of the 

 disease (see above) that enough do so to be of serious epizootiological 

 importance. In view of viability studies it is most certain that 

 furunculosis does not, spread in the sea. 



Other sources 



Although susceptible by inoculation,-, attempts to infect eels by 

 contact and by feeding infected material have been unsuccessful. Other 

 experiments have indicated that in nature eels do not readHj-- contract 

 the infection or spread it. It has been shotim that B, salmonicida may 

 be carried on the surface of ova. 



Factors Favoring the Prevalence of the D i sease 



Water temperature and floods 



The factors predisposing to epizootic prevalence of the disease 

 constitute a question of utmost importance in its prophylaxis. In 

 Great Britain the disease has generally proved seasonal in its 

 incidence, outbreaks usually occur from the end of May to October,, there 

 being a relative quiescence during the winter and spring seasons 

 (Furunculosis Committee 1930). In a hatchery outbreak recorded by 

 Belding (1927), the disease appeared in winter but the mortality was 

 slight. However, when the temperatxire rose above 55° Fo, deaths 

 increased and when it reached 60° F. the infection became epizootic, 

 spreading from pond to pondo Observations tend to show that warm dry 

 weather favors the prevalence of disease, and that the occurrence of 

 floods brings to an and outbreaks. 



Further observations by the Furunculosis Committee in 1933 have 

 indicated that water temperatures must be within the range of 55'^ Fo to 

 66° Fo before an epizootic of important dimensions is likely to take 

 place among members of the Salmonidae in nature. Experimentation has 

 further supported this view (see below). In one case it was found that 

 until water temperature had reached over 55° F., in the begiming of 

 June, no deaths from furunculosis occurred, but thereafter, until the 

 night temperatxires remained consistently below 55° F. in the latter half 



45 



