The USDA says biosecurity lapses by the poultry industry likely played a role in the spread of a lethal bird flu that has killed 47 million chickens and turkeys nationwide this year.

Airborne transmission of the virus may also play a role, the U.S. Department of Agriculture says in its initial look at the worst ever U.S. bird flu epidemic, which since surfacing last winter has hit hardest in Iowa and Minnesota.

After conducting investigations that spanned 80 commercial poultry farms, the USDA concluded there are several ways the virus could be transmitted, including through equipment and employees moving from an infected farm to a noninfected farm.

"Although [the USDA] cannot at present point to a single statistically significant pathway for the current spread of [bird flu], a likely cause of some virus transmission is insufficient application of recommended biosecurity practices," the USDA epidemiology report said.

As the bird flu spread, Minnesota turkey growers have improved their biosecurity, reducing the likelihood of lapses cited in the report, said Steve Olson, head of the state's poultry industry associations.

But guarding poultry farms against an airborne virus will cost growers tens of thousands of dollars in barn ventilation upgrades. "The physical part [of biosecurity] will be easier to deal with than the airborne," Olson said.

The H5N2 avian flu has been a disaster for the Upper Midwest's poultry industry, and a blight on its rural economy.

In Minnesota, 9 million turkeys and egg-laying chickens have been killed due to flu, and 108 farms have been afflicted. Most of them are turkey operations, and Minnesota — the nation's largest turkey producer — has seen about 10 percent of its annual production wiped out. In Iowa, the nation's egg capital, 29 million birds have been lost, mostly hens.

From the beginning, scientists have believed the lethal bird flu was introduced to poultry by wild waterfowl, who themselves are resistant to the virus, but shed it through feces. The USDA agrees with that theory but found that once in the environment, the virus was spread other ways, too, given the proximity of so many farms hit by the flu.

In other words, the flu appears to have hopped from farm to farm, not just from the wilderness to a nearby barn.

The USDA said it observed the sharing of equipment between an infected and noninfected farm; employees moving between infected and noninfected farms; and lack of cleaning and disinfection of vehicles moving between farms.

When the USDA surveyed infected turkey farms, it found that of 81 respondents — including 67 in Minnesota — only 46 percent said they had a vehicle "wash/spray area" on their farms. There also were reports inside poultry houses of rodents and small wild birds, which could spread the virus.

"We recognized fairly early on this is something we needed to change," Olson said about the USDA's observations on biosecurity. Employees and equipment are now dedicated to one farm, he said. Growers are also moving toward a "Danish entry system," in which workers change clothes before actually setting foot in a barn.

Growers also have improved the frequency of washing and disinfecting vehicles entering and exiting their farms, he said.

The airborne spread of the bird flu is another matter.

The USDA found that air samples collected outside of infected poultry houses contained virus particles, indicating that the bird flu could be transmitted by air. Also, preliminary analysis of wind data shows a relationship between sustained high winds and an increase in the number of infected farms five days later.

The report included airborne virus data for six specific flocks stricken with bird flu: three turkey flocks in Minnesota and three egg farms in Iowa and Nebraska. At least one air sample tested positive for the virus in five of the six flocks. Of the 382 total air samples, 26 percent were positive for the bird flu; 24 percent were "suspect;" and 50 percent were negative, the report said.

The bird flu, the report continued, "can be aerosolized from infected flocks and remain airborne."

Olson said that ventilation system improvements to stop airborne flu could cost turkey growers up to $60,000 per barn. A typical turkey farm has at least three to five barns.