Development of an enzyme-linked immunosorbent assay for Flavobacterium branchiophilum detection and research on bacterial gill disease

An enzyme-linked immunosorbent assay (ELISA) was developed to estimate the quantity of Flavobacterium branchiophilum antigen in gill extract from rainbow trout (Oncorynchus mykiss) following bath exposure. The assay utilized polyclonal antiserum raised against the LAB 4a strain of F. branchiophilum, and was specific for F. branchiophilum antigen in a comparison using other common gill isolates. The detection threshold was {dollar}1times 10sp3{dollar} bacteria ml{dollar}sp{lcub}-1{rcub},{dollar} and the mean intra-assay and inter-assay variation were 6.7% and 8.1%. ELISA absorbance (405 nm) was proportional to the amount of F. branchiophilum present (within a range of antigen concentration of 0-80,000 cells ml{dollar}sp{lcub}-1{rcub}).{dollar}; F. branchiophilum colonization of the gills was examined in three groups of rainbow trout (n = 250) following bath exposure. The resulting bacterial gill disease (BGD) was not treated in one group, but the others received a 1 h bath of chloramine-T (10 mg L{dollar}sp{lcub}-1{rcub}){dollar} 20 h or 7 d following exposure. The mean quantity of bacterial antigen on the gill increased rapidly following exposure and prior to the onset of high rates of mortality. Clinical recovery of surviving fish preceded a reduction in the antigen quantity in both treated and untreated groups. Chloramine-T treatment produced a marked decline in the quantity of antigen which was attributed to a bactericidal mode of action. A single chloramine-T treatment did not result in clearance of F. branchiophilum antigen.; Three trials were conducted to determine if feeding after exposure to F. branchiophilum (LAB 4a) influenced the development of BGD in fingerling rainbow trout. Severe clinical signs and high mortality rates developed within 24 h in fed fish. Cumulative mortalities ranged from 43 to 63% after 12 d. In contrast, unfed fish demonstrated cumulative mortalities of 0-2% 12 d post-exposure. Resumption of feeding 6 d post-exposure resulted in 26% cumulative mortality within 4 d. ELISA estimation of the quantity of F. branchiophilum indicated that feeding facilitated the colonization of the gill. The evaluation of water quality and direct observations suggest that the influence of feeding on BGD is linked to the consumption of feed by the fish, rather than to environmental changes arising from feeding. Alterations in the unstirred layer on the gill, secondary to feed consumption and waste excretion, may aid bacterial colonization.