An outbreak of a strain of avian flu has affected as many as 24 million birds in the U.S. Midwest, and officials say they don't know how it's spreading. Above, an egg-producing chicken farm in Iowa, April 23, 2015. Reuters/Joe Ahlquist

Every spring, a wide swath of sky over the Midwest known as the Mississippi flyway becomes a highway for millions of birds heading north after spending winter months in the warmth of Central and South America. But this year, this mass migration over central states in the U.S., including Iowa and Minnesota, has caused significant problems for bird farmers in its path. Some of the migrating ducks and geese are carrying a deadly flu and their droppings are somehow sickening millions of turkeys and chickens being raised in commercial birdhouses for food.

So far, roughly 25 million turkeys and chickens have died or been euthanized in the bird flu outbreak, the largest in U.S. history. The crisis has prompted poultry farmers in Iowa, Minnesota, Wisconsin and surrounding states to embrace heightened biosecurity -- a set of tight sanitation measures that range from changing clothing before entering birdhouses to cleaning vehicles driven onto farms -- to prevent droppings from wild birds, or any other germs, from being tracked into poultry houses. Yet birds have continued to fall sick, with flocks totaling more than 2 million turkeys and chickens reported infected Monday and five more farms suspected of having the virus on Wednesday.

That has left government officials, farmers and researchers alike grasping for answers as to how the flu has continued to infiltrate birdhouses. The deadly outbreak does not affect humans or food produced by the farms, but could eventually cause food prices to jump.

The U.S. Department of Agriculture, which coordinates with states in responding to the outbreak and also conducts research on bird flu, has no clear answers so far as to why the virus is spreading. "We cannot say definitely how specific poultry operations are becoming infected at this point," Ed Curlett, a spokesman for the Animal and Plant Health Inspection Service at the Department of Agriculture, said in an email. "We are examining affected poultry operations in order to learn more about how they are becoming infected and have yet to come to conclusions." He added, "Sound biosecurity practices are essential to keep operations from becoming infected."

Dead Within 48 Hours

The current outbreak of bird flu began in March, when it was detected in a flock of turkeys on a commercial farm in Minnesota. Since then, the virus has most heavily affected commercial hen and turkey farms in Minnesota, Iowa and Wisconsin, but has also hit flocks in Arkansas, South Dakota and North Dakota. The strain is highly pathogenic, meaning it spreads and kills quickly. Once infected, a bird can die within 48 hours.

Bird flu, also known as avian influenza, has made headlines and sparked fears in the past. In 2011, 34 people, out of 62 reported cases from five countries, died from a different strain of the flu, H5N1, that can infect humans. But H5N2, the strain currently hitting the Midwest, poses a low threat to humans, say the U.S. Centers for Disease Control and Prevention. While the virus could mutate, for now, the biggest concern for people appears to be whether the prices of eggs, chicken and turkey will rise.

Nearly 6 percent of egg-laying hens in the U.S. have been killed by the virus and prices could rise by 15 cents per dozen eggs, one estimate says. Some farmers and trade groups have said that the U.S. turkey supply could also be affected, depending on how long the outbreak lasts.

Meat industry employees are seeing the ramifications of the deadly outbreak, as well, after 233 workers at a Jennie-O turkey processing plant in Minnesota were temporarily laid off Tuesday.

Farmers Can't Contain Virus

As the bird death toll mounts, poultry farmers have continued to follow a set of best practices to control disease, known as biosecurity. Specific requirements for biosecurity vary by state and by product, such as whether hens are being raised for eggs or chickens for slaughter, but generally these practices involve steps to stop germs from entering birdhouses. Before driving onto a farm, visitors, who should be kept to a minimum, are supposed to wash their cars, for instance. Farm employees, meanwhile, are supposed wear special clothing and boots and step through disinfectant foot baths before entering birdhouses. Upon leaving, they are supposed to follow the same steps in reverse, to prevent them from tracking germs out. Ideally, biosecurity programs also keep out pests, like insects and rodents, that could carry diseases.

While wild birds have been blamed for carrying the virus, it's not clear how it has continued to spread. The fact that the majority of birds struck by the flu have been hens and turkeys kept indoors on commercial farms and not those in backyard flocks indicates that wild birds are no longer the primary culprit.

"Migrating waterfowl don't get into these large commercial operations," said Jim Roth, director of the Center for Food Security and Public Health at Iowa State. "Something has to carry the virus in." He suggested that even just one minor lapse in biosecurity could potentially allow a small dose of the virus into birdhouses. It was possible, he said, that "once one bird becomes infected, the virus can spread throughout the house."

It's also possible that the virus was passed through infected feed -- if corn was stored outside and was contaminated with wild bird droppings, for instance -- or maybe droppings were somehow tracked into birdhouses, despite biosecurity measures.

"There's some recognition that maybe [there are] some other avenues of transmission," said Dustin VandeHoef, communications director for the Iowa Department of Agriculture and Land Stewardship. "These are all theories at this point."

Turkey houses, which tend to be the length of a football field, are not made to be airtight, Keith Williams, a spokesman for the National Turkey Federation, acknowledged. Still, he said that employees and visitors on turkey farms are required to follow standard biosecurity procedures to keep out any debris that could possibly be contaminated. "Could it come in on the feathers of a migratory bird? Could it come in on some other means, like dust?" Williams said. "Those are things we're looking at very carefully."

More Birds Could Die

Nevertheless, Williams argued that for the most part, biosecurity measures were working, citing that only 1.5 percent of U.S. turkey production had been affected by the flu outbreak, or 4 million turkeys out of roughly 240 million raised annually.

While farmers try to protect their flocks, researchers aim at learning more about the virus in the hopes of determining how it infects birds. Health and agriculture officials are also trying to help farmers contain the virus and investigate how it is spreading, before more birds die.

"We don't have a lot of information on the characteristics of the spread of the virus and how it's transmitted," Dr. Sally Noll, a professor at the University of Minnesota who is researching turkey nutrition and management, said. "Everything we're thinking about what's going on is based on historical information."

Several experts said they were hopeful that the flu would die down during the summer months, as the weather warms. Even then, however, it could return with the cooler temperatures of the fall.

"I don't know that anybody knows," said Roth, from the Center for Food Security and Public Health. "It's more difficult to stop with biosecurity than we anticipated."