Detection of Flexibacter psychrophilus by immunofluorescence in fish suffering from fry mortality syndrome: a rapid diagnostic method

A rapid diagnostic method for fry mortality syndrome is described. The method is based on immunofluorescence analysis of spleen imprints from diseased rainbow trout fry and utilizes rabbit antlserum prepared against Flexibacter psychrophilus. A low level of cross reactivity with Flexibacter columnans was observed but this can be avoided by absorbing the diagnostic antiserum with F columnans. During the last 6 to 7 yr there has been considerable mortality among fry and fingerlings of rainbow trout Oncorhynchus mykiss in Danish hatcheries due to fry mortality syndrome (FMS) (Lorenzen et al. 1991). The fish suffer from a severe anaemia and splenomegaly, and numerous bacterial cells can be found wlthin the vascular system as well as in the haematopoietic tissue. The bacterium involved has been identified (authors' unpubl. data) as Flexibacter psychrophilus (formerly Cytophaga psychrophila; Bernardet & Gnmont 1989), the causative agent of cold water disease (Pacha & Ordal 1970). Infection experiments have shown that the isolated bacterium can induce FMS. In accordance with this, infected fish can be cured by being given oxytetracycline in the feed (Lorenzen et al. 1991). Diseases very similar to FMS appear to cause the same problems in rainbow trout hatcheries in other European countries [Baudin-Laurencin et al. (1989) and Bernardet & Kerouault (1989) (France); Hamlet Partnership pers, comm. (UK); Reyneri Marco pers.