Susceptibility of Nanophyetus salmonicola cercariae to formalin, hydrogen peroxide and seawater

The ability of formalin, PEROX-AID® (hydrogen peroxide), and seawater to kill waterborne Nanophyetus salmincola cercariae was evaluated in vitro. Newly emerged waterborne cercariae survived for extended periods in freshwater, with 50-74% survival occurring in negative control groups after 24 hr. Exposure to dilutions of formalin reduced this survival time, with 0% of cercariae surviving after 30 min. in 167 ppm, 40 min. in 83 ppm, and 180 min. in 42ppm. Exposure to PEROX-AID® (hydrogen peroxide) for one hour resulted in reduced cercariae survival (16.4%) only at the highest concentration (100 ppm), compared to 100% survival in the untreated controls and all lesser concentrations. Dilutions of seawater resulted in reduced cercariae survival only at high salinities (15.2 – 30.3 ‰), where 10 min. exposures resulted in 0 – 16% survival. These results provide insights into prophylactic water treatment options for salmonid enhancement facilities that experience high mortalities due to infections with Nanophyetus salmincola. Further, the intolerance of cercariae to high salinities indicates that exposures of fish to infectious waterborne cercariae occur primarily in the freshwater portions of watersheds.