Saprolegnia parasitica S1 and S2 Strains Differ in Zoospore Transition Timing and Pathogenicity Against Juvenile Atlantic Salmon (Salmo salar).

S1 and S2 strains of S. parasitica are both common among diseased farmed salmonids in Nova Scotia, whereas globally S1 is rare and S2 is common. Following the initiation of asexual maturation and overnight incubation at 20°C then harvest, and incubation at 10°C in vitro, S2 secondary zoospores mostly transformed into cysts within 3 h, and by 6 h post-harvest > 80% had germinated. S1 zoospores, by contrast, exhibited persistent motility; at > 30 h post-harvest > 80% were in the motile stage, and < 10% were either cysts or germlings. The disease challenge test began with skin disruption by shaking pairs of fish in a net for 15 s, then stocking six fish in up to 12 aquaria (27 L water), addition of 400 zoospores/ml, held in static water for 11 h, then 10°C flow-through. Through smolting (January-June) at 10°C, S1 was consistently more virulent than S2 (p = 0.005), with gross disease signs evident among 50% of test fish in 40 h for S1 versus 72 h for S2. Susceptibility to disease was independent of smolt status (p = 0.512). Secondary motile zoospores, cysts, or germling stages of both strains all caused disease, but S2 germlings were the least pathogenic.