Validation of diagnostic assays to screen broodstock for Flavobacterium psychrophilum infections.

It is hypothesized that the frequency of bacterial coldwater disease outbreaks can be reduced through the detection of the aetiologic agent, Flavobacterium psychrophilum, in broodstock followed by culling of eggs from heavily infected broodstock. Before a culling programme can be instituted, however, it is necessary to determine the sensitivity and specificity of existing assays for the detection of F. psychrophilum. In this study, tissue and ovarian fluid samples were collected from 224 fish at five hatcheries and screened using an enzyme-linked immunosorbent assay (ELISA), a membrane-filtration fluorescent antibody test (MF-FAT), bacteriological culture and nested PCR. Latent class analysis was used to estimate sensitivity and specificity of kidney culture, kidney ELISA, nested PCR and MF-FAT. Analytical sensitivity of the ELISA varied but was greatest when bacteria were cultured under iron-limiting conditions. Diagnostic sensitivity estimates ranged from 0.02 (kidney culture) to 0.97 (kidney ELISA). Specificity estimates ranged from 0.02 (MF-FAT) to 0.98 (kidney ELISA). In a separate challenge experiment, the ELISA confirmed the presence of F. psychrophilum in sub-clinically infected fish. Results from this study demonstrate that the ELISA is an appropriate tool to screen broodstock and provides an indication of infection severity, which is crucial for implementation of a screening/culling programme.

Characterization of susceptibility and carrier status of burbot, Lota lota (L.), to IHNV, IPNV, Flavobacterium psychrophilum, Aeromonas salmonicida and Renibacterium salmoninarum.

In this study, susceptibility and potential carrier status of burbot, Lota lota, were assessed for five important fish pathogens. Burbot demonstrated susceptibility and elevated mortality following challenge with infectious haematopoietic necrosis virus (IHNV) by immersion and to Aeromonas salmonicida by intraperitoneal (i.p.) injection. IHNV persisted in fish for at least 28 days, whereas A. salmonicida was not re-isolated beyond 17 days post-challenge. In contrast, burbot appeared refractory to Flavobacterium psychrophilum following intramuscular (i.m.) injection and to infectious pancreatic necrosis virus (IPNV) by immersion. However, i.p injection of IPNV resulted in re-isolation of virus from fish for the duration of the 28 day challenge. Renibacterium salmoninarum appeared to induce an asymptomatic carrier state in burbot following i.p. injection, but overt manifestation of disease was not apparent. Viable bacteria persisted in fish for at least 41 days, and bacterial DNA isolated by diagnostic polymerase chain reaction was detected from burbot kidney tissue 90 days after initial exposure. This study is the first to investigate susceptibility of burbot to selected fish pathogens, and this information will aid in efforts to culture and manage this species.