DES MOINES, Iowa — Millions of dead chickens and turkeys lie in stinking, fly-swarmed piles near dozens of Iowa farms, casualties of a bird flu virus that’s swept through the state’s large poultry operations in the last month.

Contractors hired by the U.S. Department of Agriculture move methodically from barn to barn at the 28 farms, killing the animals that haven’t yet died from the H5N2 virus — a suffocating foam for turkeys, carbon dioxide gas to asphyxiate chickens. Neighbors say they understand the challenge the state’s producers face in disposing of more than 20 million bird carcasses, but are eager for quick action, especially as temperatures rise.

“People in low-lying areas near those facilities have contacted me complaining about a very large number of flies and, until the wind comes up and goes the right direction, a lingering odor,” said Sen. David Johnson, whose northwest Iowa district includes Sunrise Farms — where 3.8 million chickens in 20 barns are dead or dying.

There are few details about how the carcasses are being handled; some are piled up and covered with dirt or other material, turkeys are often composted inside barns and at least one chicken farm is burying them in trenches. The Iowa Department of Natural Resources is working with area landfills on agreements to dispose of poultry inside plastic bio-bags and has approved permits for a company to operate portable incinerators in three counties, with the possibility of a fourth roving incinerator.

But one thing is clear, according to Ken Hessenius, a DNR environmental supervisor: “We’ve got to look at all methods of disposal here to deal with this. It truly is a crisis. It’s an extremely difficult situation for all these farms.”

More than 100 farms in the Midwest have been hit, leading to the loss of more than 28 million chickens and turkeys — 5.5 million of those in Minnesota, the nation’s leading turkey producer.

Hessenius has said his office has received numerous calls from neighbors about increased flies and odor at Sunrise Farms, where the virus was confirmed on April 20.