More than 8 million turkeys and chickens have died in Minnesota this year in the bird flu outbreak: They've died either from contracting the flu or were euthanized to prevent the spread of the disease.

The state has verified nearly 100 outbreaks among domestic and commercial chicken, turkey and mixed-poultry flocks across 22 counties.

Investigators still aren't exactly sure what's behind the outbreak. Scientists suspect the infection of domestic poultry traces back to migrating waterfowl — such as ducks and geese — which can carry the virus without getting sick. Investigators believe it's those wild birds that carried the virus into the state.

What happens once the bird flu is detected on a farm? How are the birds destroyed and disposed of? And — something we've all wondered — just how bad does it smell?

Detection and quarantine

When the flu is detected on a farm, the Minnesota Board of Animal Health places the flock under quarantine. Commercial and backyard flocks located within a six-mile radius of the infection site are tested for the virus.

Symptoms include: lethargy and a lack of appetite; diarrhea; and swelling of the head, eyelids, comb, wattles and hocks.

Carol Cardona, a bird flu expert from the University of Minnesota, told MPR News that scientists haven't seen such a highly pathogenic — fatal — virus since the early 1960s.

"It kills 100 percent of the birds that it infects," she said. "They don't form antibodies before they die."

Euthanasia

A flock of turkeys at a Minnesota poultry farm. Bethany Hahn via AP

Turkeys at risk of infection — which is any turkey that lives in the same barn as an infected bird — are killed to prevent the disease from spreading.

Because turkeys aren't caged, a water-based foam containing carbon dioxide is pumped into an infected barn. The bubbles rise until they reach about 2 1/2 or 3 feet from the ground, enough to go over the birds' heads, said Steve Olson, executive director of the Minnesota Turkey Growers Association. The foam eventually dries into a dust.

Depending on the size of the flock, the euthanasia process usually lasts a few hours.

Foam is not used in chicken barns, mainly because of the way they are set up: Chickens are kept in cages stacked on top of one another, about 8-10 feet high, and the barns have narrow walkways.

"Trying to get (the foam) up high enough is too difficult," Olson said.

The water-based foam would also destroy the cages.

Instead, birds are placed in carts that are pushed up and down the walkways, which are then filled with carbon dioxide.

It's a longer process than using foam — a good crew can get through about 300,000-400,000 birds a day, said Dr. Jack Shere, associate deputy administrator for veterinary services at the U.S. Department of Agriculture's Animal and Plant Health Inspection Service.

Is it humane?

Because the bird flu is so deadly, some have wondered whether it would make more sense — or be more humane — to simply close up the barns and let the birds die from the virus.

"If you've seen these birds die of the disease, you'd say it's more humane to euthanize them," Shere said. "It's very cruel to sit there and let them suffer."

Shere and Olson said the birds aren't in distress during the process. "They go to sleep, pass out and die," Shere said.

One other advantage of euthanasia is that it stops the spread of the virus.

"The euthanization is happening for other barns on that farm," Olson said. "We want to make sure there is nowhere else for that virus to go. ... We want to react as quickly as we can to putting birds down."

Composting

Two of the Willmar Poultry Company's farms were infected with avian influenza. Jackson Forderer | File for MPR News

In Minnesota, dead birds are left in the barns to decompose. There's a base layer of bedding — which varies from farm to farm, but commonly includes wood chips — placed on the floor of the barns after the birds are destroyed. The birds are placed in the middle of the piles and then covered up with another layer of bedding.

Crews check the piles daily with thermometers. Once they've reached about 140 degrees, the piles are turned or mixed again to reintroduce oxygen and reinvigorate microbes to continue breaking down the compost piles.

The heat generated in the composting process kills the virus.

It's a technique that takes about a month.

The compost, which Shere said looks similar to mulch, is then moved out of the barns and can be safely spread over cropland as fertilizer.

Does it smell?

It shouldn't, Olson said.

"The compost pile is going to be like any other compost; it's going to have an earthy smell," he said.

MPR News' Mark Zdechlik reported that in Swift and Kandiyohi counties, "despite all of the rotting carcasses in the area, there's no discernible odor."

The same can't be said in parts of Iowa, the country's top egg-producing state, which has also been hit hard by the avian influenza outbreak. About 26 million birds have been affected at nearly 70 sites.

Dawn Cronk, who lives near a farm where birds were destroyed told Harvest Public Media:

"I have the window down and all of a sudden there's just that distinct dead animal smell," she says. "And it's not just one dead animal, it's like you walked into a ... a decomposing lot. It's just that strong."

Olson said the vast majority dead birds in Minnesota are being composted, which appears to be playing a big part in the lack of major stench in Minnesota.

Iowa, though, has struggled with disposal options.

According to The Des Moines Register, Iowa producers are typically composting, burying on site or incinerating the dead birds. They also have the option to landfill them.

But egg-laying barns aren't built the same as turkey barns, so farmers can't compost in the ground, Shere said. And many farmers haven't wanted to bury dead birds on their farms, in part because of land-use contracts: They'll have to disclose whether anything is buried on the farm when it's time to sell.

So, in many cases, dead birds in Iowa have been placed in sealed bio-bags and held in dumpsters waiting for disposal — some for a couple of days, others for several weeks.

Big incinerators that can handle millions of birds take about two weeks to set up. And many landfills either can't accept the vast quantity of dead birds — some commercial chicken farms have millions of poultry — or have been reluctant to take them because of fears over contamination. But officials have said the birds can be safely transported from farms to landfills because of strict biosecurity measures.

Disinfection and repopulation

A Minnesota Board of Animal Health worker carried a barrel out of the back of a pickup truck at a turkey farm in Nobles County that had been affected by the avian flu. Jackson Forderer | File for MPR News

Once the birds have been destroyed and disposed of, the barns get a thorough cleaning and disinfection (Shere said the best method is sealing up the cleaned-out barn, cranking up the heat and killing the virus that way). Once officials determine the virus is no longer present, barns must sit empty for 21 days before being released from quarantine.

State and federal regulators must re-certify the barns for production before poultry can re-enter the buildings.

In the best-case scenario, barns can be repopulated within two months. Realistically, it's more like two to three months, Olson said.

Why you should care: Bird flu in Minnesota's poultry farms

The Associated Press contributed to this report.