Flavobacterium psychrophilum, cause of ... - cop.eXtension.org

of the tubules, and a slight increase in hemosiderin content (Bruno 1992). There is also
a consistent splenomegaly or hypertrophy of the spleen (Bernardet et al. 1988). In recent
years in the Pacific Northwest, juvenile rainbow trout undergoing BCWD epizootics
displayed hemorrhaged areas on the external body skin located near where the spleen is
found internally and the spleen was enlarged (Figure 8).The presence of the bacterium in
the retina and the subsequent inflammation associated with the
infiltration of the retina and choroid gland by polymorphic
granulocytes containing bacteria may account for why some of the
rainbow trout that survive RTFS are blinded (Evensen and
Lorenzen 1996).
Figure 8. Juvenile rainbow trout from a hatchery outbreak of bacterial cold-water disease. Hemorrhaged
area on the body is found over the spleen. The spleens were enlarged and contained many Flavobacterium
psychrophilum cells. Photograph by C. Banner.
Pathogenicity
The virulence of different isolates of F. psychrophilum can vary extensively (Madsen and
Dalsgaard 2000; Holt et al. 1993; Nematollahi et al. 2003a). Mortality in yearling coho
salmon injected subcutaneously with viable cells of different strains of F. psychrophilum
varied from 0-100% (Holt et al 1993). A number of virulence factors including adhesins,
exotoxins, proteases and endotoxin contribute to the pathogenicity of F. psychrophilum
(Dalsgaard 1993). Adhesins facilitate bacterial attachment to host cells and tissues,