The avian flu outbreak that has killed poultry in 10 states, including Minnesota, is unprecedented in this country. In the past five months two highly pathogenic strains of bird flu have been detected in two dozen domestic flocks, killing hundreds of thousands of birds.
Previous outbreaks of lethal bird flu among U.S. poultry have occurred just a few times. But those outbreaks largely were limited to a single state and a small number of farms.
Agriculture officials and poultry farmers hope quarantines and culling measures will stop the current outbreak and protect the state's $800 million turkey industry.
But some avian researchers fear the new viruses may be here to stay.
The strains of bird flu currently circulating in the United States contain genes from viruses found in Europe and Asia.
That's very unusual, given that the oceans and north-south migration patterns are a barrier to significant mingling between birds from those continents, said David Halvorson, an avian influenza researcher.
"With the exception of Siberia and Alaska, where they can have a little bit of exchange of genetic material of their viruses," said Halvorson, a retired University of Minnesota veterinary medicine professor.
Halvorson suspects the new flu viruses were introduced to North America near the Bering Sea in the northern part of the Pacific Ocean between North America and Asia. Now that they're here, he believes it will be hard to get rid of them.
Wild waterfowl are a natural reservoir for flu viruses, Halvorson said.
The wild birds can be infected with flu and not appear ill. So far, none of the waterfowl fecal samples tested in Minnesota has tested positive for the flu strain that's killing domestic turkeys. But some wild birds in other states have tested positive for the new flu strains.
Halvorson said there's little doubt that the flu virus started in wild birds. Even though it has been hard to find any traces of that connection in Minnesota, he thinks the new flu strains could already be widespread in wild ducks, geese and other waterfowl.
"It's either an overwhelming amount of virus [in waterfowl] or the virus that we're dealing with now is much more infectious than we're used to seeing because we're not used to seeing anything like this," he said. "Getting bombed with virus from wild birds in the spring like this — we've never seen that before."
Halvorson predicts these highly pathogenic viruses will squeeze out less deadly strains of North American avian flu. If so, he said, the threat to domestic flocks could intensify and continue indefinitely.
Hon Ip, director of the Diagnostic Virology Lab at the National Wildlife Health Center in Madison, Wis., agrees with that assessment.
As it is not possible to eliminate the infections in wild birds, poultry producers must figure out how to keep the viruses out of their facilities, Ip said.
"That to me is the thing that hopefully we can learn best practices from and enable the industry to do biosecurity when we have this potentially continuing threat of the virus being in wild birds," he said.
The Minnesota Board of Animal Health and the U.S. Department of Agriculture have assembled teams of investigators who will search each outbreak site in the state to try to figure out how the virus is getting into turkey barns.
Biosecurity practices will be intensely scrutinized, said Beth Thompson, assistant director of the Board of Animal Health.
"We are doing all that we can," Thompson said. "A lot of us have been putting in long hours and long weeks. But we hope in the future that we're going to get ahead of this."
But getting ahead of this outbreak may be a big challenge.
Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said the idea that wild birds are spreading the infection is the easy, popular theory right now. But it's not helpful because it doesn't shed light on why the new flu viruses have so easily slipped through biosecurity measures that worked well in the past.
Osterholm suspects other factors are at play. But he said it will take a rigorous, expensive investigation to reveal what they are.
"We're now stretched so thin with the available resources to even respond to the number of birds that are infected in the poultry industry, that I'm not sure that we're getting the kind of information that we need right now to understand this situation," he said.
Osterholm said it's crucial to answer the specific question of how the virus is getting into poultry barns because today's biosecurity systems are based on a model where these types of outbreaks shouldn't happen so frequently. Without a specific answer, he said, poultry producers may not be able to stop future outbreaks.