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Diseases of Aquatic Organisms

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DAO 43:27-38 (2000)  -  doi:10.3354/dao043027

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

Jari Madetoja1,*, Pia Nyman1, Tom Wiklund2

1Institute of Parasitology, Department of Biology, Åbo Akademi University, BioCity, Artillerigatan 6, 20520 Åbo, Finland
2Danish Institute for Fisheries Research, Fish Disease Laboratory, Stigbøjlen 4, 1870 Frederiksberg C, Denmark

ABSTRACT: The infection route of Flavobacterium psychrophilum into rainbow trout Oncorhynchus mykiss was studied using bath and cohabitation challenges as well as oral challenge with live feed as a vector. Additionally, the number of bacterial cells shed by infected fish into the surrounding water was determined in the cohabitation experiment and in challenge experiments at 3 different water temperatures. The experiments showed that skin and skin mucus abrasion dramatically enhanced the invasion of F. psychrophilum into the affected fish in bath and cohabitation challenges. Disruption of the skin is discussed as an important invasion route for F. psychrophilum into the fish. The shedding rate of F. psychrophilum 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 during 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 from rearing tanks in order to diminish the infection pressure against uninfected fish in commercial fish farms. In immunohistochemical examinations of organs and tissues of orally infected fish, F. psychrophilum cells were detected in only 1 fish out of 31 studied. Mortality of the orally challenged fish was not observed in the experiment.

KEY WORDS: Flavobacterium psychrophilum · Rainbow trout · Bath challenge · Cohabitation challenge · Oral challenge · Shedding rate

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