One year after America's deadliest avian flu outbreak on record, scientists are still flummoxed over the role of wild birds.
All the prior science had pointed to migrating ducks and geese spreading the virus to domestic turkey and chicken farms.
But none of the evidence in Minnesota implicates wild birds in last year's devastating outbreaks, and that's leading to new disputes about the virus.
Last year, in a massive hunt for the source of bird flu, the Minnesota Department of Natural Resources tested more than 6,200 wild bird samples, mostly droppings, for signs of the virus.
Grow the Future of Public Media
MPR News is supported by Members. Gifts from individuals power everything you find here. Make a gift of any amount today to become a Member!
The search turned up only two infected birds and neither was a waterfowl.
"We certainly did the best that we could using the tools that we had to describe infection. We just didn't find it," said Lou Cornicelli, wildlife research manager at the DNR.
The agency has spent several hundred thousand dollars on its wild bird surveillance — and that doesn't even include laboratory tests. The federal government has paid the $30 testing fee for most of the thousands of bird samples.
With nary to show from gathering bird poop on lake shores and river banks, Cornicelli said the DNR won't repeat the effort.
"We're not doing this again," he said.
The DNR plans to ratchet down its bird flu testing to pre-outbreak levels.
Cornicelli said the agency will quickly increase its surveillance if avian flu reappears in the Mississippi Flyway, or if a Minnesota poultry flock gets infected with the virus.
But Steve Olson, executive director of the Minnesota Turkey Grower's Association, said he fears the DNR's decision is a mistake that could further devastate the state's poultry operations.
"We want them to keep looking and we are committed to working with them," Olson said.
Olson says poultry producers will press the DNR to keep searching for avian flu in wild birds. He says the industry will ask the DNR to test different species of wild birds, collect samples for more consecutive days, and switch to testing blood instead of droppings in an effort to find the source of the virus.
"Despite the industry pointing its finger at the wild birds, it's just not there," said Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota. "It was not the source of widespread transmission to many operations throughout the Upper Midwest."
Osterholm said there are a couple of holes in the waterfowl theory.
Backyard poultry flocks should have been hit hardest because those birds were most exposed to migrating waterfowl. Instead, those birds were largely spared, he said.
Osterholm also said the lone Pope County case popped up before wild ducks and geese had even traveled through the area.
Disease investigators found poor biosecurity practices allowed the virus to move easily between farms.
Osterholm said that points to a different theory worth checking — that a non-fatal version of the virus circulated undetected among flocks that aren't tested frequently, such as layer hens. And then at some point mutated to the highly pathogenic H5N2 that laid millions of birds to waste.
University of Minnesota avian flu expert Carol Cardona has come to agree the idea that migrating ducks and geese brought bird flu to Minnesota isn't holding up.
On the other hand, Cardona rejects the notion that the deadly virus came out of the industry's poultry barns.
Under that scenario, she said there would have been much less genetic variation in the bugs that caused more than 100 flock infections.
"It wasn't just a static thing. There was evolution in the virus," Cardona said.
The U.S. Department of Agriculture identified dozens of different versions of the virus that popped up in Minnesota.
Cardona said it's still possible the virus traveled in another wild bird species that hasn't been tested yet.
But as Saturday's anniversary of the first outbreak approaches, the source of the disease remains a mystery, and fissures are forming in the alliance that battled the virus a year ago.