An outbreak of a lethal bird flu strain spread Saturday when 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.

At the Lac qui Parle County farm, the virus killed 22,000 turkeys in one barn. That farm must kill 44,000 birds in two other barns as a precaution.

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

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.