A highly contagious and lethal strain of bird flu is spreading across parts of the U.S.

A deadly bird flu outbreak ripped through U.S. poultry 7 years ago. That’s when more than 50 million chickens and turkeys died and egg prices soared.Researchers believed migratory waterfowl like ducks and geese started the outbreak. They can carry the virus in their excrement or saliva.

But Iowa State University poultry veterinarian Yuko Sato says it’s still not quite clear how the virus got into barn after barn. And it makes it harder to stop it now.

“There's no concluded study on how it's coming in,” Sato said. “We can identify some weak links, right? For example, people tracking it in.”

Since February,more than 13 million chickens, turkeys and other birds have died because of the disease or have been culled to stop the spread.

Highly pathogenic avian flu has the ability to replicate in a bird's body and spread through the bloodstream to different organs, Sato said. Some of the signs in poultry include diarrhea, lack of energy and appetite, eyelids swelling, difficulty breathing and sudden death.

“If they’re coughing or sneezing, they’ll have droplets that are aerosolized and they’re spreading it that way,” Sato said. “If it’s manifested into diarrhea, then it’ll spread through feces.”

Since 2015, poultry producers across the country have strengthened their protocols to try to keep diseases away from their birds.

Ben Slinger stood in the town square of Ellsworth. Wild birds nested between buildings here. We met in the town square because like most producers, Slinger is limiting visitors on his nearby farms to prevent bird flu from reaching his flocks.

“It’s a very concerning time,” Slinger said. “We’re watching stuff extremely closely.”

Slinger and his family raise about 850,000 turkeys a year for an Iowa meat processor. The 2015 bird flu outbreak hit them hard. In May of that year, Slinger noticed some of his turkeys were really quiet, and not very active. He had to cull 38,000 turkeys to stop the disease from spreading. The family sat around for six months with no turkeys.

“We know what the aftermath of that is like, and it is pretty disheartening,” Slinger said.

So Slinger and his workers now wear dedicated boots into each barn to avoid tracking anything in. They also walk through disinfectant before entering the barns.

“And we've been in pretty good shape,” he said. “This year's definitely going to challenge us, it seems like in the coming weeks, as to if we're truly prepared again for what happened in 2015.”

Producers aren’t the only ones concerned. So are people with backyard flocks. Martin Hutchison raises nearly 50 adult hens, chicks and guinea fowl.

“It would devastate me to think about losing my birds,” Hutchison said.

Hutchison has been taking precautions with his birds. He doesn’t let anyone else near them. Whenever he’s with his birds, he monitors their health.

“I stand there and watch them and look over each individual bird to see if they’re active and seem healthy,” he said. “I look for any kind of change in their behavior.”

Poultry producers and backyard flock owners are on high alert as ducks and geese migrate north to their breeding grounds. Diann Prosser, a wildlife research biologist with the U.S. Geological Survey, says she’s closely watching the migration.

“There are a lot of moving targets,” Prosser said. “We know that this virus has been evolving over the past handful of years. And we keep watching and trying to add additional studies to understand what virus we're dealing with right now.”

Scientists are especially interested in where wild birds might have close contact with domestic birds. State agriculture officials are advising people to try to keep their flocks away from migrating birds this spring.

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