We characterized six filamentous, gram-negative bacterial isolates, each recovered from a naturally occurring salmonid bacterial gill disease (BGD) outbreak in Ontario. The morphological, physiological, biochemical, and antigenic traits of the Ontario isolates were similar to those of Flavobacterium branchiophilum (ATCC 35035, ATCC 35036) recovered from salmonid BGD in Japan and Oregon. We conclude that the Ontario BCD isolates are strains of F. branchiophilum.
Three trials were conducted to determine if feeding influenced the development of bacterial gill disease (BGD) in fingerling rainbow trout Oncorhynchus mykiss following bath exposure to Flavobacterium branchiophilum (LAB 4a). In each trial (Trial 1. n = 70 X 3; Trial 2, n = 140 X 3 ; Trial 3, n = 250 X 3), the comparison groups (fed, unfed, negative controls) were held in similar conditions and (where applicable) fed prior to exposure to the pathogen. Severe clinical signs and high rates of mortality developed within 24 h in fish which were provided feed following the bath exposure. Cumulative mortalities ranged from 43 to 63% by 12 d post-exposure Unfed fish demonstrated only mild and transient clinical signs of BGD, and cumulative mortalities of 0 to 2 % 12 d post-exposure. In one trial, resumption of feeding 6 d post-exposure resulted in 26% cumulative mortality within 7 d. Estimation of the quantity of F branchiophilum antigen by enzyme-linked immunosorbent assay (ELISA) indicated that feeding facilitated 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.
The ability of selected strains of Flavobacterium branchiophilum to attach to and colonize the gills of five species of teleosts (four salmonid and one cyprinid) and cause mortality was investigated. In virulence studies with F. branchiophilum strain LAB4a, cumulative mortality was dose-dependent in rainbow trout, Oncorhynchus mykiss (Walbaum), and ranged from 0 to 75%. However, regardless of dose, the relative amount of gill-associated F. branchiophilum antigen inereased lh after challenge. The gill-associated F. branchiophilum antigen in fish which survived (moribund fish) increased by four to six times compared to that detected on the gills lh after challenge. The gill-associated antigen on moribund fish was not significantly different from that on the gills of dead fish. Flavobacterium branchiophilum strain LAB4a also attached to the gills of brook trout, Salvelinus fontinalis (Mitchell), rainbow trout, chinook salmon, Oncorhynchus tshawytscha (Walbaum), Arctic charr, Salvehnus alpinus (L.), and common shiners, Luxilus cornutus (Mitchell), following a 1-h bath exposure, and caused mortality in all species. The virulence of eight strains of F. branchiophilum for rainbow trout was examined. Both virulent and avirulent strains adhered to the gills following bath exposure (fimbriae were observed on all strains), but only virulent strains had the capacity to further colonize the gills and cause mortality.
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