Virulence of Flavobacterium branchiophilum in experimentally infected salmonids

Abstract

Abstract. 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 increased 1 h 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 1 h 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, Salvelinus 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.