MINNEAPOLIS - Animal health experts and poultry growers are scrambling to determine how a dangerous new strain of bird flu infected poultry flocks in four states - and to stop it from spreading.

Avian influenza is common in wild migratory waterfowl but doesn't usually harm them. But the H5N2 strain is deadly when it spreads to commercial poultry. It can wipe out a flock of tens of thousands of birds in a few days, as it did at a farm last month in Minnesota, the nation's top turkey-producing state. The same strain soon turned up on two farms in Missouri and one in Arkansas.

The vast majority of turkeys and chickens in the U.S. spend their lives confined indoors to protect against disease. Yet, as the infections show, viruses can still reach them - tracked in by humans or rodents; carried on trucks, equipment, crates and egg flats; passed from waterfowl to shore birds that find their way into a barn.

"This new guy is a bad actor," said Dr. Carol Cardona, an avian influenza specialist at the University of Minnesota.

Minnesota confirmed its outbreak March 4, the first H5N2 found in the Mississippi flyway, a major bird migration route. The Missouri and Arkansas cases were confirmed last week. The only known commonality among those states is the flyway. Meanwhile, samples from a Kansas backyard flock of chickens and ducks tested positive for the strain late this week; the affected flock is in a county just west of the Mississippi flyway.

Why it showed up at these locations simultaneously is a mystery, though Cardona and other experts suspect waterfowl or other wild birds. Meanwhile, officials are keeping an eye on the workers who had contact with the infected flocks, and producers are tightening their standard biosecurity measures, which include putting on sanitary clothing and showering on their way in and out of barns.

The new strain - along with other recent outbreaks of highly pathogenic strains in commercial turkey and chicken farms, backyard flocks and wild birds in Oregon, Washington state, Idaho and other states in the Northwest's Pacific flyway - also affects the industry's bottom line in the affected states. Dozens of countries have banned poultry imports from affected states, a common practice done mainly to protect their own flocks.

The World Health Organization says avian influenza viruses can survive in contaminated raw poultry, so it's possible to spread them via fresh or frozen products. But government and industry officials say the danger to humans is very low, and people can't catch it from properly cooked poultry or eggs. Cardona said the likelihood of bird flu entering a country through imported products is "very, very low."

Some countries also use those bans to protect their markets from cheaper foreign products, according to Dr. Donna Carver, extension veterinarian at North Carolina State University. "There's not always a scientific reason," she said.

Minnesota, Missouri and Arkansas followed rapid response protocols from the federal government and poultry industry. Once H5N2 was confirmed at the farms, surviving turkeys were killed and kept out of the food supply. Officials quarantined the facilities and declared six-mile-radius restricted zones. They tested nearby farms but found no signs of other infected flocks.

"At least right now we're breathing a little easier," Missouri Department of Agriculture Director Richard Fordyce said.

To infect a commercial flock, the virus still needs to get into a barn. It spreads through bird-to-bird contact or via bird feces, so either a person, a bird or a rodent has to physically bring it inside. Modern biosecurity makes that difficult, so outbreaks in North America are rare.

The few people allowed to enter poultry farms typically must wear sanitary foot coverings, coveralls, bonnets and gloves, Carver said. They may need to shower coming in, change into fresh clothing and shower before leaving. Keeping rodents and wild birds out is critical, too, she added.

The goal is to "keep what's in the barn in and what's out of the barn out," said Steve Olson, executive director of the Minnesota Turkey Growers Association and the Chicken and Egg Association of Minnesota. He added that many farms also rely on disinfectants to try to kill viruses before they're tracked into or out of a barn.

Arkansas last year edged North Carolina as the No. 2 turkey producer in the U.S. North Carolina, which like Arkansas is among the top chicken producers, isn't in an affected flyway. But Carver said producers there are on high alert.

"I think the whole country is in lockdown mode in terms of trying to make sure that the biosecurity that they have is working the best it can," she said.