It was still winter in Minnesota when state officials first heard about turkeys on a large farm that seemed to be a bit off. Some of the birds were unusually quiet, drank and ate little and seemed to have trouble moving. Within two weeks of exhibiting this odd behavior they were dying. The cause, laboratory tests soon confirmed, was H5N2, a mixed-origin avian flu that had never been seen in the U.S. before this year.

For the nation’s number-one turkey-producing state, this was horrible news. In states including California and Washington the virus and its close cousins had led to more than 250,000 poultry deaths (by disease or depopulation) in the prior three months. Officials in Minnesota steeled themselves for an onslaught of H5N2 cases. All 26,000 birds on the farm where the virus was found were killed as a precautionary measure, and for a few weeks all seemed quiet. Then reports from a dozen more farms started to pour in from far-flung parts of the state. The farms were not owned by the same companies, were not close to one another and did not have the same employees. It was evident state officials had a formidable outbreak on their hands. But where the flu was coming from remained unclear. Now almost 90 farms in Minnesota have been hit and 7.7 million birds have either been killed by the flu or culled as a precautionary measure.

Nationwide, the situation is even more ominous. Since December almost 34 million birds across 15 states have died or been killed because of one of three avian flu strains: H5N2, H5N8 or H5N1.

How is it getting around?
The H5 flu strains may not be spreading because of conditions on the farms themselves. Instead, wild ducks and geese appear to be harboring the virus. The birds have been natural reservoirs for other H5 viruses without getting sick, making them likely couriers of the current strains gripping the nation. Ducks and geese are also ubiquitous in the afflicted states. “Minnesota is the land of 10,000 lakes, and we do have a lot of water fowl that are either residents here or on their migratory path north this time of year and stop for awhile here or fly over,” says Beth Thompson, assistant director of the Minnesota Board of Animal Health. The state tells people to stay away from wild waterfowl but “it’s kind of hard to do if you have a natural pond or stream around you.”

Yet evidence that wild birds are carrying the viruses remains mixed. The Minnesota Department of Natural Resources has collected more than 3,000 fecal samples from wild ducks and geese for H5N2 testing. They also analyzed tissue samples from 75 birds killed by hunters and examined dead wild birds around the state. No waterfowl have yet tested positive for H5N2, says Lou Cornicelli, wildlife research manager for the department. Oddly enough, his team did identify one Cooper’s hawk infected with H5N2. The hawks typically eat small birds—not ducks or geese—and they don’t scavenge for dead birds. That raises questions about where the infected hawk contracted the virus, says Cornicelli, adding that at the first farm in Minnesota where turkeys tested positive for H5N2 there were no waterfowl on the property.

“If it’s not from the waterfowl where would the virus be coming from? It doesn’t come from the sky,” says Jack Shere, associate deputy administrator for veterinary services at the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. So how to explain the lack of H5N2 in Minnesota’s geese and ducks? “I think the birds are gone,” Shere says. “They migrated and that’s the difficulty of wild bird sampling. The constituents aren’t there anymore but they leave the virus in the water,” and if commercial farms use an infected pond to water their poultry or to clean out the barns, they could be spreading the virus, he says.

Although Minnesota, one of the hardest hit avian flu states, has not found any sign of the disease in waterfowl, other states have. There are more than two dozen confirmed cases of individual ducks and geese infected with the H5N2 virus spread through Washington, Oregon, Idaho, Missouri, Kentucky, Wyoming and Kansas. In some states there are also reports of the other two avian flu strains (H5N8 and H5N1) in waterfowl.

But how would the virus get from a goose or a duck to a commercial bird? At most farms, turkeys or chickens live in closed barns so disease-carrying wild birds can’t wander in and spread infection, Thompson says. Humans, however, could step in virus-laden feces from waterfowl—or come into contact with some dropped on their cars—and then carry it into barns. But Thompson and other animal experts believe this is not the sole mode of transmission.

Researchers at the University of Minnesota, led by animal health expert Montse Torremorell, recently completed an air-sampling study of four poultry barns at three Minnesota farms and detected bits of genetic material from H5N2 in air particles inside and immediately around the infected facilities. This suggests the virus could be transmitted through the air, at least over short distances. Yet the research is far from conclusive because detecting a virus in the air does not mean it’s a viable pathogen that could cause disease in birds. To gain clarity Torremorell says her team plans to conduct further analysis with the samples.

Shere also thinks the virus could be transmitted via the air. “With the proper environmental conditions—cool weather, wet weather, enough wind and some dust—this virus could move in particles,” he says, particularly with the help of fans that would blow it through the barns. “It doesn’t take a lot of virus to get these birds sick, and we have lots of birds in close proximity in certain complexes and it’s just like a day care center for small children—if one kid gets sick the rest can get sick.”

Will it infect humans?
So far, the three flu strains seem to be sticking to birds. The virus has not yet been detected in humans and public health officials say it’s unlikely the virus will make the leap. That judgment is based on experience with similar H5 strains and the lack of human cases from this outbreak so far, says Joni Scheftel, state public health veterinarian at the Minnesota Department of Health. Moreover, if a person was infected with the virus after working closely with a sick bird, experience suggests the virus would still be unlikely to be passed between humans, she says. In order for the virus to make that leap to humans at all a number of changes in the virus would need to take place.

Simply put, H5 avian flu is tailor-made for birds. In the human respiratory tract there are receptors that are well suited for seasonal flu, making it easy to contract. Avian flu requires different receptors, which are common in birds. “We do have some of the type of receptors that avian flu can latch onto but we don’t have a lot of them,” says Michael Jhung, a medical officer with U.S. Centers for Disease Control’s Influenza Division. The nightmare scenario, however, would be if pigs contracted the avian flu, because they have both the seasonal flu and avian flu receptors. “If pigs got infected with seasonal influenza virus and avian influenza virus at the same time, the viruses could mix. That’s called reassortment,” he says. Then, if the subsequent virus has genes that code for human receptors, it could be more transmissible to and between people. So far, however, there have been no reports of the flu or elevated levels of sickness in pigs.

Agriculture officials believe that U.S. waterfowl picked up H5N8 from Asian birds during migration. That strain then likely mixed with less-lethal North American avian influenza viruses to create new mixed-origin viruses with the “H5” part of the virus from Asia and the “N” part from North America. H5N2 and H5N1 are two of the new mixed-origin viruses. (The H5N1 virus in the U.S. is not the same as the H5N1 virus seen in Asia that has caused some human sickness since 2003.)

Because these H5 virus mixtures are somewhat unique, what little knowledge we have is based on experts’ knowledge of other H5 viruses. That’s why the CDC says it is unlikely humans may already have contracted the avian flu while exhibiting only mild symptoms that are flying below the radar. Typically, people with H5 viruses get very sick. Still, public health officials are asking people to self-monitor for 10 days following potential exposure to infected birds, watching for typical symptoms such as runny nose, difficulty breathing or conjunctivitis.

So far there have been no reported cases in humans. Minnesota officials have already monitored 290 people and advised more than 150 to take Tamiflu as a precautionary measure. The state has also tested more than a dozen people with suspicious symptoms to see if they had avian flu—all were negative.

It would be hard for potentially exposed farm workers to fall through the cracks in Minnesota, Scheftel says. In that state, when a laboratory confirms a bird is sick with H5N2 it sets off a chain of actions. The health department contacts the flock master of the farm and gets a list of every worker. Then state officials contact each individual for an interview to ask about potential symptoms. For the following 10 days health department employees contact all the potentially exposed workers daily via text, e-mail or phone to continue asking about symptoms. Meanwhile all other birds on the farm are killed by a mix of state, federal and farm employees or contract workers, all bedecked in the required head-to-toe personal protective equipment, including rubber boots or disposable boot covers, waterproof coveralls, N95 respirators, safety goggles, gloves and head coverings.

Whether the H5 strains are coming from waterfowl feces, virus-laden water, people tracking the virus into barns or something else altogether, one thing is clear: The viruses are spreading, and figuring out how will take more than a wild goose chase.