Effects of hydrogen peroxide on clearance of formalin-killed Flavobacterium branchiophilum front the gills of rainbow trout, Oncorhynchus mykiss (Walbaum)

The effect of hydrogen peroxide (H2O2) treatment on the clearance of formal in-killed Flavobacterium branchiophilum from the gills of rainbow trout was examined. In untreated control fish, clearance was characterized by a rapi d initial phase, with 50% reduction in bacterial antigen in the first 12 h after exposure. The bacteria then cleared more gradually, with total cleara nce being achieved by 40 h. Treatment of fish with 100 p.p.m. H2O2 did not influence bacterial clearance compared to untreated controls. Exposure for 1 h to H2O2 concentrations less than or equal to 100 p.p.m. caused no detec table clinical signs or evidence of ultrastructural damage to the respirato ry epithelium. However, levels in excess of this caused significant gill da mage.