Lethal bird flu has spread to four more Minnesota turkey farms and three additional southwest counties, state animal health officials said ­Friday.

The outbreak, first reported in early March in Pope County, has now struck 13 farms in nine ­counties, affecting 872,000 turkeys.

Gov. Mark Dayton issued an emergency executive order allowing heavier trucks and emergency equipment to respond to the turkey farms. The goal is to reduce the number of such trips, lowering the odds that vehicles will spread the H5N2 avian virus.

The virus turned up at a fourth farm in Stearns County, the state's second-largest turkey-producing county behind Kandiyohi, where two farms earlier had outbreaks. New outbreaks also hit farms in ­Cottonwood, Lyon and Watonwan counties. Together, the four new farms have 189,000 birds, all of which are expected to be killed.

"For these companies, and obviously for the farmers and their families that have been impacted by the H5N2 virus, there are some really difficult times ahead," Minnesota Agriculture Commissioner Dave Frederickson said Friday on a conference call with reporters.

Fredrickson said he isn't calling the outbreak an epidemic. "I don't think we have all settled on a term to describe it," he said. "Obviously this is an event, an ongoing event here in Minnesota. We can put whatever title we want on it, but at this point, we are just responding across the state to the influenza, and we'll ­continue to do that."

State experts, assisted by a 40-person team from the U.S. Department of Agriculture, have not determined the source of the outbreak. The chief suspects are spring-migrating wild birds, which can spread the virus through their droppings. It poses a low health risk to humans and is not a food safety threat since all birds on affected farms are destroyed.

The highly lethal bird flu has surfaced in at least nine states, but has hit hardest in Minnesota, the nation's largest turkey producer with 450 farms raising about 46 million turkeys a year.

The virus has raced through the state over the past two weeks. In the latest cases, the number of affected turkeys are 48,000 in ­Cottonwood; 66,000 in Lyon; 30,000 in Watonwan; and 45,000 in Stearns. Each farm has multiple barns, but only one barn at each location had sick turkeys.

Health regulators routinely kill all turkeys on an affected farm as a precaution. That's planned for the latest stricken farms. About one-third of the turkeys typically die from the disease, and the rest are euthanized. With the new cases, 872,000 turkeys have died or will die.

State officials have established quarantine zones with a 6-mile radius around each of the 13 farms. Two of the Stearns County zones partly overlap, officials said Friday. In these areas, authorities restrict movement of poultry and equipment.

No cases of H5N2 have turned up in people in Minnesota or across the United States, said Dr. Joni Scheftel, state public health veterinarian with the Minnesota Health Department. She said the department is monitoring farmworkers who worked among stricken turkeys.

Minnesota DNR wildlife experts have collected wild birds to test for the virus. Lab results from the first survey, in Pope County, turned up no positives. Samples from wild birds near affected farms in Stearns, Nobles and Lac qui Parle counties were collected this week, with lab results expected next week, said Michelle Carstensen, DNR wildlife health program supervisor. More wild bird sampling is planned, she added.

Carstensen said the DNR is asking people who hunt wild turkeys — the season opens Wednesday — to report sick or dead birds they find. The agency also is interested in reports of dead or ill raptors found by anyone. Hunters can get the numbers and other avian virus information at strib.mn/hunt-H5N2.

Investigators are looking at whether people and vehicles transport the virus among farms.

"There is a lot of traffic that comes into and out of a farm every day, so we are taking a very close look at that right now," said Dr. Beth Thompson, assistant director of the Minnesota Board of Animal Health.

Thompson said weather probably is a factor in the outbreak, and may offer relief.

"This virus, like any virus, likes cold and damp weather, and that is exactly what we have in Minnesota at this point in the year," she said. "We are hopeful that when the weather warms up and the environment starts drying out, we will see the death of this virus in the environment."