VIRULENCE OF FLAVOBACTERIUM-BRANCHIOPHILUM IN EXPERIMENTALLY INFECTEDSALMONIDS

The ability of selected strains of Flavobacterium branchiophilum to at tach to and colonize the gills of five species of teleosts (four salmo nid and one cyprinid) and cause mortality was investigated. In virulen ce studies with F. branchiophilum strain LAB4a, cumulative mortality w as dose-dependent in rainbow trout, Oncorhynchus mykiss (Walbaum), and ranged from 0 to 75%. However, regardless of dose, the relative amoun t of gill-associated F. branchiophilum antigen increased Ih after chal lenge. The gill-associated F. branchiophilum antigen in fish which sur vived (moribund fish) increased by four to six times compared to that detected on the gills Ih after challenge. The gill-associated antigen on moribund fish was not significantly different from that on the gill s of dead fish. Flavobacterium branchiophilum strain LAB4a also attach ed to the gills of brook trout, Salvelinus fontinalis (Mitchell), rain bow 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 ah s pecies. The virulence of eight strains of F. branchiophilum for rainbo w trout was examined. Both virulent and avirulent strains adhered to t he gills following bath exposure (fimbriae were observed on all strain s), but only virulent strains had the capacity to further colonize the gills and cause mortality.