Detection of the bacterium Flavobacterium psychrophilum from a natural infection in rainbow trout, Oncorhynchus mykiss (Walbaum), using formalin-fixed, wax-embedded fish tissues.

The bacterial pathogen Flavobacterium psychrophilum was successfully identified from formalin-fixed, wax-embedded tissue blocks of infected rainbow trout heart and spleen tissues, using a polymerase chain reaction (PCR)-based assay. Filamentous bacteria were observed in haematoxylin and eosin and Giemsa-stained sections but no bacteria were recovered from the diseased fish using standard bacteriology isolation techniques. All infected fish had histopathological evidence of myocarditis or rainbow trout fry syndrome. Immunohistochemistry was attempted using three different anti-F. psychrophilum sera but the results were inconclusive, and an alternative molecular approach was therefore attempted. This paper describes the use of a PCR-based assay to help identify bacteria present in formalin-fixed, wax-embedded tissue samples. This is the first time that this technique has been used for the detection of fish bacteria from diagnostic samples.