The virus appears to have moved South after being confirmed on a salmon farm in British Columbia two weeks ago.
IHN occurs naturally in wild sockeye salmon and can be carried by other fish, such as herring, which sometimes pass through fish net pens.
The Washington hatchery affected, owned by American Gold Seafoods, and affiliated with Icicle Seafoods of Seattle, reportedly has 120 pens near Rochester.
The company has said it will have removed all fish from the site, Orchard Rocks salmon farm on Puget Sounds Bainbridge Island, by June. Monitoring of pens in the surrounding area has increased to avoid another outbreak.
The disease was recognised after the company noticed high mortality rates in April.
John Kerwin, fish health supervisor for the Washington Department of Fish and Wildlife, said the virus is a big concern.
"Any first time it occurs, you don't fully understand the impact to wild fish," Kerwin told press. "We know it can impact (farm) fish. If we move fast, we can try to minimise the amplification."
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