MINNEAPOLIS — Minnesota’s state veterinarian suggested Wednesday that bird flu may be spreading from farm-to-farm in the state’s top turkey-growing counties, a possibility they downplayed in the early days of the outbreak.

They said instead that they suspected the virus was being carried into the state by migratory waterfowl that would spread it via their droppings, then accidentally tracked into barns by farm workers or perhaps by rodents.

But U.S. Department of Agriculture officials, including the agency’s chief veterinary officer, have floated the theory in recent weeks that windy, dry conditions, which have been common this spring, could whip up dust and debris contaminated by infected waterfowl droppings and carry the virus short distances into barns and infect new flocks. If that’s what’s been happening, it raises the possibility that high winds could also blow contaminated dust from one farm to a nearby farm.

Confirmed and presumed outbreaks of H5N2 avian influenza have hit over 100 Midwest farms, affecting more than 28 million chickens and turkeys. Minnesota and Iowa have been the hardest-hit states;  Minnesota turkey and chicken producers have lost more than 5.5 million birds since early March.

State Veterinarian Bill Hartmann told reporters on a conference call that there’s no confirmation of lateral spread from farm to farm. But he said poultry industry veterinarians who’ve been working with the affected flocks now suspect the virus may be spreading that way via the wind.

“There is no evidence to substantiate that other than their observations,” he said. The latest epidemiology report that his agency received on Friday provided no fresh clues about how the disease is spreading and he didn’t have details on what those veterinarians might have observed.

Farm-to-farm spread might explain why so many farms in Kandiyohi and Stearns counties have been affected, but Hartmann said another reason might be that they simply have so many turkey farms. They’re the top two turkey producers in Minnesota, which is the country’s top turkey producing state.

Hartmann said the possibility of airborne spread is one reason why the state and USDA are looking seriously at a vaccine to protect poultry from the H5N2 virus, as well as whether turkey farms should use the kind of air filtration systems being used in the swine industry. He said those might not be practical for most existing turkey barns, which typically open up on the sides to allow in fresh air, but they might be economical for higher-value breeding operations.

Wednesday was Minnesota’s first day since mid-April with no new reports of outbreaks. Hartmann said it was a sign of progress but acknowledged it’s “too soon to say” whether it marks the start of a trend. Experts say warm, dry sunny weather should kill off the H5N2 virus, at least for the season.

“We’re hopeful that this weather will change the outlook for us,” Hartmann said.