Characterization of isolates of Flavobacterium psychrophilum associated with coldwater disease or rainbow trout fry syndrome II: serological studies

The possibility of seroIogical differentiation between isolates of Flavobacterium psychrophilum was analyzed by ELISA and slide agglutination. Twenty-five Danish isolates and 20 isolates from other European countries were studied using polyclonal rabbit antisera and whole-cell preparations. Unabsorbed as well as reciprocally absorbed antisera and purified Ig preparations derived from the antisera were included. Most of the isolates originated from clinical outbreaks of rainbow trout fry syndrome (RTFS) or coldwater disease (CWD), but some were isolated from asymptomatic fish or from other fish species with different disease signs, The ELISA showed the existence of different serotypes most distinctly, but slide agglutination supported the ELISA results. Three serotypes were found among the isolates studied: 1 major serotype (serotype Th) represented most of the Danish isolates and isolates from other European countries; 2 minor serotypes (Serotypes Ed and Fp(T)) also occurred. Serotype Th could be further divided into a major subtype, Th-1, and a minor subtype, Th-2. Serotype Fp(T) was defined by the type strain F. psychrophilum NCIMB 1947(T), and seemed to include mostly isolates from asymptomatic fish or from fish species other than rainbow trout.