Highly pathogenic avian influenza is back killing domestic poultry and wild birds in Minnesota again this spring as huge flocks of migratory birds carry it north for another season.

But, in fact, the deadly bird disease never left the state, even over our long winter, with birds dying in December and January and some new research showing the killer flu virus may survive even in cold Minnesota lake water — with no host bird — during the winter.

That’s the update from wildlife biologists as the great spring migration descends on Minnesota, as the snow line recedes north and ice on lakes and rivers begins to let loose.

Since the disease was first reported in Minnesota just over a year ago, some 566 birds have been tested and confirmed carrying the H5N1 strain of bird flu that’s been expanding worldwide since 2020, according to data from the U.S. Department of Agriculture. Wisconsin has had 211 confirmed cases and North Dakota 310 as of April 11.

Nationally, more than 6,500 wild birds have been confirmed dead from the virus over the past year, some from all 49 continental states, although wildlife experts say that’s likely a gross under counting of the total number, most of which die and are never found by people.

Minnesota has the highest number of wild birds confirmed dead from H5N1 of any U.S. state, although it’s not exactly clear why. It may be because Minnesota is a focal point for the northward migration, with many species of waterfowl and raptors — both highly susceptible to the disease — stopping over here on their way north.

Or, it could be that Minnesota has just turned in more birds for sampling than other states. Some states have had much larger die-off episodes than Minnesota, such as terns in Wisconsin and snow geese in Washington, but not all of the dead birds were tested and thus aren’t officially counted.

In Minnesota, the Department of Natural Resources, U.S. Fish and Wildlife Service, wild bird rehabilitation centers and others collect fresh carcasses, responding to reports of sickly or dead birds.

But no single agency is keeping track of how many are found and sent for testing by the other agencies. We only know how many positive results come back from the federal laboratory in Ames, Iowa, where testing occurs.

“We’ve asked the same question, why we are highest? … We don’t know how many samples were collected in other states, and we don’t even know how many were collected in our state, so we don’t know if we have more disease, more birds dying, or just more birds submitted” for testing, said Eric Hildebrand, the DNR’s wildlife health supervisor.

Michelle Carstensen, the DNR’s wildlife health group leader, said she’s cautiously optimistic that if birds winging north right now are bringing the same strain of influenza back with them from wintering grounds, more surviving wild birds have by now developed a resistance and this year’s carnage won’t be as great as last spring. (Domestic poultry develop little or no resistance and almost always die if they contract the disease.)

“The weaker, more susceptible birds may have died last year. And now the ones that have survived, they’ve been around it (H5N1) for a while, maybe they won’t succumb to it now,” Carstensen said. “Of course, that’s only if it’s the same strain. The problem is that this disease can mutate, very quickly. And if it gets more virulent, all bets are off.”

Carstensen said that, while it’s clear the disease can impact large numbers of individual birds and flocks, there’s still no evidence — but also no real research — on the broader, population-level impact H5N1 is having on hard-hit species like snow geese, ducks and eagles.

The disease seems to flourish most in the spring, with most deaths reported in Minnesota from March into May.

By late May last year, DNR officials noticed the number of wild birds reported and testing positive for H5N1 dropped dramatically, likely in part because migrating waterfowl kept flying north, out of the state, to their nesting grounds. While there was a resurgence of the virus in domestic poultry during fall 2022, biologists did not detect big impacts to wild birds in the fall and winter compared to spring 2022.

Still, birds did keep dying from bird flu in Minnesota over the winter, including four crows found dead in the Twin Cities in January. In March, a red-tailed hawk in Wright County was confirmed dead from the disease.

Bird flu hit its first domestic flock earlier this month, confirmed in a backyard poultry flock in Le Sueur County, where 114 birds were put down to prevent the disease from spreading. (Over the past 12 months, nearly 60 million domestic chickens and turkeys have been killed by the disease or put down to prevent its spread across the U.S.)

In Minnesota and nationally, the disease hit waterfowl and the raptors that eat waterfowl especially hard. Waterfowl flock tightly and in large numbers, each infected bird shedding the disease for others to pick up. Raptors like eagles find the dead, infected fowl easy pickings.

Species impacted in Minnesota include crows, pelicans, bald eagles, barred owls, broad-winged hawks, Canada geese, common goldeneyes, loon, ravens, Cooper’s hawks, dark-eyed juncos, great-horned owls, hooded mergansers, mallards, northern harriers, red-shouldered hawks, red-tailed hawks, Ross’s geese, rough-legged hawks, snow geese, trumpeter swans, turkey vultures, white-fronted geese and wood ducks.

“It really seemed to hit the cavity-nesting birds like wood ducks hard last year,’’ Hildebrand noted. “We aren’t sure why.”

Across the U.S., more than 300 bald eagles tested positive and died in 2022 after eating smaller birds infected with the virus. A study reported an unusually poor eagle nesting success in avian flu hotspots in Georgia and Florida. The authors said H5N1 may pose an “impending threat” to a species only recently rescued from the brink of extinction.

Even more concerning to some health experts, the disease has now been confirmed in several mammals across the U.S., including foxes and skunks in Minnesota, otters and a bobcat in Wisconsin and a black bear in Colorado.

Last year, only one human case of bird flu was confirmed by the Centers for Disease Control and Prevention in the U.S. during the ongoing outbreak. The case affected a commercial poultry industry worker in Colorado who later recovered. On Monday, another case was reported in a child in China.