INDIANAPOLIS – The fast-spreading avian flu virus, detected last week in southern Indiana, has prompted officials to order the killing of more than 400,000 exposed turkeys and chickens.

“Speed is of essence, to stamp it out,” said Denise Derrer, spokeswoman for the Indiana Board of Animal Health, which is coordinating the effort involving local, state and federal agencies.

It is said to the the first mass eradication of a poultry flock in state history.

Birds have been killed on 10 farms in Dubois County, Indiana’s top producing poultry county, west of Louisville.

The virus flourished there in sub-zero wind chills and snow, since ventilation in reduced in barns as temperatures fall.

Cold weather later slowed the eradication process. Officials at first planned to asphyxiate poultry using a water-based, firefighting foam – a procedure recommended by the American Veterinary Medical Association as most humane.

But hoses and valves froze over the weekend as temperatures dropped, Derrer said.

That was worrisome for animal and health officials, who feared exposing workers to the virus that is commonly spread on shoes and clothes. While highly contagious for birds, the virus poses minimal risk to humans, according to the U.S. Centers for Disease Control and Prevention.

To hasten eradication, the state brought in two-dozen prisoners who volunteered to help veterinarians and farmers manually kill birds. The process employs devices, approved for use by the American Veterinary Medical Association, that deliver a fatal head injury or break an animal’s neck.

“It’s not pretty, but it’s humane,” Derrer said.

The depopulation effort was launched after a fast-spreading strain of the virus was found by a veterinarian on a Dubois County commercial turkey farm.

Testing on neighboring farms found nine more flocks of chickens and turkeys exposed to the virus, with some infected by a slower spreading strain.

State officials said all birds had to be destroyed, however, because even the less-aggressive strain of avian flu has the potential to mutate.

State Sen. Mike Braun, R-Jasper, whose district includes the affected farms, said the eradication is hard on farmers, even though they’re eligible for a federal program that covers their financial loss.

“They feel responsible for those birds,” said Braun, who spent 33 years as a turkey farmer in Dubois County.

“They’re some of the best poultry producers in the state, but there’s nothing else they could have done to stop that virus,” he said.

Derrer said the state provides counseling services to farmers and workers involved, as part of a larger emergency response plan to fast-spreading viruses that threaten livestock.

She noted that, in the United Kingdom, the suicide rate among farmers spiked 10 times during an outbreak of foot-and-mouth disease in 2001, when the government required the slaughter of animals.

“We’ve known for a long time the kind impact this can have on farmers, and on veterinarians who take an oath to protect an animal’s life,” she said. “(Counseling) is now just part of the response plan.”

Maureen Hayden covers the Indiana Statehouse for CNHI’s newspapers and websites. Reach her at mhayden@cnhi.com. Follow her on Twitter @MaureenHayden.