Intra-Annual Changes in Waterborne Nanophyetus salmincola.

An analysis of daily water samples collected from an index site on Big Soos Creek, Washington indicated intra-annual differences in the concentrations of waterborne Nanophyetus salmincola. Waterborne concentrations, quantified as gene copies/L, peaked during the fall (October-November 2016), decreased to very low concentrations over the winter (January-March 2017), and then increased in the spring and throughout the summer. High waterborne concentrations of N. salmincola DNA (2 × 106 gene copies/L) corresponded with live N. salmincola cercariae (mean = 3 cercariae/L) that were detected in companion water samples. Spikes in waterborne N. salmincola concentrations in October and November typically coincided with increases in streamflow; this combination resulted in elevated infection pressures during high water events in the fall. The peak in waterborne N. salmincola concentrations corresponded with an accompanying peak in tissue parasite density (metacercariae/posterior kidney) in Coho Salmon Oncorhynchus kisutch that were reared in the untreated water.

Susceptibility of Nanophyetus salmincola 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 cercariae survived for extended periods in freshwater, with 53-73% survival occurring in negative control groups after 24 h. Exposure to dilutions of formalin reduced this survival time, with 0% of cercariae surviving after 30 min in 450 µL/L, 40 min in 225 µL/L, and 300 min in 113 µL/L. Exposure to PEROX-AID (hydrogen peroxide) for 1 h resulted in reduced cercarial survival (16.4%) only at the highest concentration (100 µL/L), compared with 100% survival in the untreated controls and all lesser concentrations. Exposure to dilutions of seawater resulted in reduced cercarial survival only at high salinities (15.2-30.3‰), where 10-min exposures resulted in 0-20% survival. These results provide insights into options for prophylactic water treatment at salmonid enhancement facilities that experience high mortalities due to infections with Nanophyetus salmincola. Further, the intolerance of live cercariae to high salinities indicates that exposure to fish occurs primarily in the freshwater portions of watersheds.