Causes of Emergence of a Novel Virus in Washington Coastal Steelhead Trout

A novel disease threat has recently emerged in steelhead and rainbow trout stocks of Washington coastal rivers on the Olympic Peninsula.  In 2007 and 2008, epidemics of infectious hematopoietic necrosis (IHN) occurred in juvenile steelhead at multiple sites in Grays harbor watersheds that have never had IHN before, and also in tribal facilities in the Quinault and Queets River watersheds.  Virus typing has shown that these epidemics are due to emergence of IHN virus strains that are in the MD subgroup of IHNV, which was previously limited to the Columbia River basin. Continued genetic typing of field isolates of IHNV collected through 2010 indicates that there have been three independent incursions of MD IHNV into coastal watersheds, most likely from sources within the Columbia River basin.  Unlike the IHN virus types that were previously in these watersheds, these MD strains are specifically virulent for steelhead and rainbow trout, presenting a novel threat to both wild and cultured steelhead populations.  In addition to analyses of virus sequences from strain typing, we are conducting controlled wetlab studies of the coastal MD virus in steelhead trout, to identify variation in susceptibility of different fish stocks, and assess virulence of this virus relative to other known IHNV strains.  The goal of this work is to define the natural and anthropogenic factors that contributed at this viral emergence, and provide scientifically sound information to assist tribal, state, and federal agencies with management decisions aimed at controlling the spread of this new virulent virus.