The deadly flu virus that has wiped out nearly 5 percent of Minnesota's turkey industry is a part of a global disease outbreak, but scientists still don't understand it.
After emerging in Asia, the avian flu spread to poultry farms in the Netherlands, Great Britain, and Germany.
Its quick arrival in North America has alarmed scientists who are trying to unravel the mystery of how these deadly bugs have infected so many turkey farms in such a short period of time.
"It's been really troubling to understand how in the world this can possibly be happening," said Carol Cardona, a professor of avian medicine at the University of Minnesota.
A popular theory among many scientists suggests that wild waterfowl are depositing the virus across the landscape in their feces as they migrate north.
That's likely, Cardona said, as the virus has been detected in numerous wild ducks and geese, birds that can be infected with flu and not appear sick.
But Cardona said it's hard to fathom that waterfowl alone could explain such an unprecedented flu outbreak. She suspects another bird species is helping to spread the virus.
"This is so unusual that we can't help but think something different must be going on," she said.
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The North American version of the virus has changed slightly since arriving in the United States from Europe, but its lethal effects on poultry are the same.
Besides wild ducks and geese, the new viruses have also been detected in live coots and cranes. But Cardona said there are thousands of other bird species that have yet to be tested for the virus.
Another theory suggests the viruses may have changed in a way that makes it easier for wild birds to catch and carry the poultry flu strains, avian flu researcher Ron Fouchier said.
"It's possible upon genetic mixing with wild bird viruses, these viruses have become more capable of spreading through wild birds," said Fouchier, a professor of molecular virology at Erasmus University Medical Center in Rotterdam in the Netherlands. "But this is something that's still under investigation."
The outbreaks have occurred in many countries. But the presence of avian flu in Minnesota is particularly notable because the state's 26 cases are so geographically spread out they are seemingly unrelated.
Biosecurity mistakes are thought to be a likely method of disease introduction at many of the turkey farms. State and federal investigators also are looking at the possibility that strong spring winds could have made the flu problem worse — at least in some locations.
Avian flu isn't typically spread across great distances by wind, but some researchers say it's possible the virus could have been blown into turkey buildings, if it was attached to field debris or feathers.
Mick Fulton, an associate professor of avian diseases at Michigan State University, said wind has been known to spread infectious laryngotracheitis in his state. That's a viral disease that, like flu, is typically transported in barns by people or equipment.
"We had a very strong wind that went in the direction of another farm that was located two miles away and then soon after that real strong wind that farm broke with the disease," Fulton said.
However, Fulton doubts that wind is a primary cause of the Minnesota outbreak. He believes biosecurity failures probably account for most of the cases.
Mark Cook, a professor of animal sciences at the University of Wisconsin, Madison, agrees. Cook said it can be extremely challenging to keep a highly infectious disease out of an animal barn if it is widespread in the environment, as this virus is thought to be.
When Cook was a graduate student, his Ph.D. project tested his ability to keep an arthritic disease called reovirus from spreading from a sick flock in one barn to his healthy chickens in a different barn. He was successful for the first two and a half years of his three-year project.
"I don't know how it got from one barn to the other, and these were on opposite ends of the campus, but eventually I goofed and it did [spread]," he said. "But I had enough data to get my Ph.D."
That experience still influences Cook's approach to poultry biosecurity: producers cannot be too careful.
"Everything I ever wear gets washed every day to this day, for fear that I might move something from one place to another," he said.
Cook said he has no doubt that avian influenza can be kept out of most poultry barns. But he said it may require periods of total lockdown during the spring and fall wild bird migration seasons. That could last for years, as long as the new viruses are circulating.