Farmers tilling fields near poultry barns may have helped spread the bird flu that killed about 9 million birds in Minnesota last year and hammered the turkey industry, according to a University of Minnesota study.

The study, done by the U's Center for Animal Health and Food Safety and released Thursday, found that tilling could cause soil disruption, freeing airborne particles that could then carry the virus. The study also found that truck-washing stations aimed at stopping the bird flu's spread may have actually contributed to it.

Highly pathogenic H5N2 bird flu hit western Minnesota's Pope County in early March 2015 and struck over 100 poultry farms, the last one in early June. Minnesota, the nation's No. 1 turkey producer, lost 10 percent of its production or 5 million birds. About 4 million egg-laying chickens died, too.

The early period of last year's bird flu outbreak in Minnesota — March to the flu's peak around April 22 — coincided with wild bird migration.

Generally, animal health scientists believe the bird flu is transmitted by waterfowl, usually through their feces. Farm fields can be contaminated with bird droppings and research has shown influenza viruses can survive cold temperatures in soil. The U study found that active tilling of fields near turkey farms was a "risk factor" within 14 days before an actual outbreak.

However, risks associated with tilling were only observed early in the epidemic, suggesting that tilling played a role in the virus' introduction in Minnesota but not its "lateral spread," the study said.

Animal health specialists concluded that airborne transmission was likely one cause of the virus' spread and that a gusty, dry spring only made things worse. The U study reinforces the risks behind "the proximity of fields to barns," said Steve Olson, head of the Minnesota Turkey Growers Association. "It's a finding that gives us some strategies to mitigate future outbreaks."

The bird flu can also be spread by poultry barn workers and trucks and farm equipment traveling from farm to farm.

The study also found that farms with vehicle wash stations on site or nearby appeared to have higher odds of getting hit by the flu, though it's not clear why.

"It appears that at least it's possible that the [washing] practices done weren't adequate and may have concentrated the virus," said Scott Wells, the study's co-author and a professor in the U's veterinary medicine school, which houses the Center for Animal Health and Food Safety.

The U study covered 83 turkey farms, 43 of which were hit by the flu and 40 more "control" farms that weren't stricken.