Flavobacterium psychrophilum, invasion into and shedding by rainbow trout Oncorhynchus mykiss

The infection route of Flavobacterium psychrophilum into rainbow trout Onco rhynchus mykiss was studied using bath and cohabitation challenges as well as oral challenge with live feed as a vector. Additionally, the number of b acterial cells shed by infected fish into the surrounding water was determi ned in the cohabitation experiment and in challenge experiments at 3 differ ent water temperatures. The experiments showed that skin and skin mucus abr asion dramatically enhanced the invasion of F. psychrophilum into the affec ted fish in bath and cohabitation challenges. Disruption of the skin is dis cussed as an important invasion route for F. psychrophilum into the fish. T he shedding rate of F. psychrophiIlun by infected fish was associated with water temperature and the mortality of the infected fish. High numbers of F . psychrophilum cells were released into the water by dead rainbow trout du ring a long time period compared to the numbers of cells shed by Live fish. The results emphasise the importance of removing dead and moribund fish fr om rearing tanks in order to diminish the infection pressure against uninfe cted fish in commercial fish farms. In immunohistochemical examinations of organs and tissues of orally infected fish, F. psychrophilum cells were det ected in only 1 fish out of 31 studied. Mortality of the orally challenged fish was not observed in the experiment.