An outbreak of a bird flu strain that's deadly to poultry deepened Saturday when state and federal officials confirmed a third Minnesota turkey farm has been infected, this time in one of the state's top poultry producing counties.

The federal Animal and Plant Health Inspection Service said a commercial flock of 39,000 turkeys in Stearns County of central Minnesota has been infected with the highly pathogenic H5N2 strain of avian influenza, which also killed tens of thousands of turkeys at two other farms in Pope and Lac qui Parle counties of western Minnesota.

Saturday's announcement came one day after officials announced the outbreak at the Lac qui Parle County farm, where the virus quickly killed 22,000 of the 12-week-old turkeys in one barn. That farm must kill 44,000 birds in two other barns as a precaution to prevent the disease from spreading.

The confirmation at the Pope County farm on March 5 marked the first detection of H5N2 in the Mississippi Flyway, a major bird migration route. H5N2 was also found within the next several days in commercial and backyard flocks in Arkansas, Missouri and Kansas. The same strain also has turned up in several western states in the Pacific Flyway.

The Stearns County farm has been quarantined and the remaining turkeys there will be killed and kept out of the food supply, according to the Minnesota Board of Animal Health.

State veterinarian Bill Hartmann said the outbreak in Stearns County is a challenge to contain since the infected flock is close to other poultry farms.

"When we find an infected flock we draw a 10 kilometer circle around it, which is the infected zone, and all of the flocks that are within that inner 10 kilometers will be tested and are under quarantine," Hartmann said.

More than 40 countries have banned poultry imports from Minnesota, the country's top turkey producing state, since the virus was first detected in the state.

According to the Minnesota Turkey Growers Association, Stearns County is the state's No. 2 turkey-producing county, behind only Kandiyohi County in western Minnesota, where the virus has not been reported. Stearns County is also one of the state's top chicken and egg producers, according to the Minnesota Department of Agriculture.

Scientists consider wild migratory waterfowl to be a natural reservoir for avian influenza. While they don't generally get sick from flu viruses, they can spread them through their droppings. But top researchers say they don't know how the virus got to Minnesota.

MPR News reporter John Enger contributed to this story