HUNTING -- An outbreak of avian influenza in a private game farm in Okanogan County is forcing federal and state agriculture officials to kill up to 5,000 ducks, geese, chickens, pheasants and turkeys.

About 40 birds at a game farm for private hunting and bird-dog training in Riverside, Washington, were sick and died over the weekend. The birds tested positive for bird flu on Tuesday.

The U.S. Department of Agriculture and the state of Washington announced it would kill birds from the flock and establish a six-mile quarantine around it to contain the disease.

The flock represents the largest number of birds the state has had to test and possibly kill during 2015 bird flu outbreaks.

"As recently as November, the flock owners had tests run on their birds which, at that time, showed no sign of avian influenza in the flock," Agriculture officials said in a statement.

Other outbreaks of the avian flu have been reported in Clallam, Benton and Franklin county, but involved much smaller numbers of poultry. Washington state has now lifted a quarantine in the Tri-Cities but one in Port Angeles remains. No new cases have been found in either location.

An outbreak in California led to 146,000 turkeys being killed at a commercial operation. Several countries including China have banned poultry and eggs from the United States.