Analyzing pathogens as contributing factors, with a focus on Nanophyetus

Several pathogens are known to be present in Puget Sound. Many infections from these pathogens result in marine mortality, and we do not know broadly what infection rates are for specific salmon and steelhead species throughout Puget Sound. Doing blind surveillance, broad spectrum pathogen studies is currently very expensive. New technologies are being developed in Canada, supported in part by the Salish Sea Marine Survival Project. Once these technologies evolve, the U.S. scientists will reconsider the value of broad spectrum analyses. In the interim, U.S. disease experts correlated what we know about Puget Sound salmon and steelhead marine survival patterns with the suite of potential pathogens. Through this process, they determined that Nanophyetus salmincola is the primary disease candidate given the apparent alignment between its high prevalence and low marine survival rates for specific salmon and steelhead populations, especially in south Puget Sound.

Nanophyetus was found at significantly higher prevalence and loads in steelhead from the Green/Duwamish and Nisqually river systems in central and southern Puget Sound than in steelhead from the Snohomish or Skagit watersheds in Northern Puget Sound, or the Tahuya watershed in Hood Canal. In Nanophyetus-infected systems, prevalence and parasite load increased from in-river smolt traps to the estuary. All steelhead collected in marine waters of the central and southern basins of Puget Sound tested positive for Nanophyetus. High loads of Nanophyetus were significantly associated with microscopic lesions in kidney, gill, and heart tissue; inflammation and fibrosis (scarring) were also observed. Tissue damage was especially prevalent in wild Nisqually steelhead: up to 69% carried heart lesions.

Comparisons between groups of infected and uninfected steelhead were conductedto determine whether Nanophyetus affects swimming ability and survival.  Infected fish had slightly higher mortality and slightly lower critical swim velocities than uninfected fish; however, parasite loads achieved in the laboratory were 3.5 to 10 times lower than those observed in central and southern Puget Sound estuaries. A dose-response relationship was identified: higher exposure to Nanophyetus produces higher parasite loads. Additional studies are being conducted at higher exposures to produce infections consistent with parasite loads observed in the wild and further understand impacts to swim performance and survival.

Researchers developed a qPCR tool to detect the presence of Nanophyetus in fish, snail, and water samples and are using it  to map spatial and temporal patterns of Nanophyetus presence. Water treatment options were also tested for managing the impact of the parasite on hatchery-reared steelhead. Formalin was most effective at eliminating Nanophyetus. Hydrogen peroxide and salt (salinity) also showed promise.