A dangerous bird-flu strain that has already hit numerous turkey farms in the Midwest has now been identified in a Wisconsin chicken flock, marking the first case of the virus in a commercial chicken farm in the U.S. and its first appearance in Wisconsin, the U.S. Department of Agriculture said Monday.

Authorities stressed there was no risk to public health and no danger to the food supply from the highly pathogenic H5N2 strain, which was first detected in the region in Minnesota early last month. Animal health officials have long said the virus is dangerous to all commercial poultry. The only surprise of it turning up in chickens is that it took so long, said Raechelle Cline, a spokeswoman for Wisconsin’s agriculture department.

The USDA said tests confirmed that a flock of about 200,000 chickens in Jefferson County, in southeastern Wisconsin, has been infected. About 20,000 chickens have already died from the disease, and the remaining 180,000 will be killed to help prevent the disease from spreading, according to the Wisconsin Department of Agriculture, Trade and Consumer Protection.

The disease has cost turkey producers more than 1.2 million birds across the Midwest — including more than 900,000 in Minnesota, the nation’s No. 1 turkey-producing state. Still, that only accounts for about 0.5 percent of the 235 million turkeys produced nationally in 2014. The disease has also struck farms in Arkansas, Missouri, Kansas, South Dakota and North Dakota since early March.

Although the affected Wisconsin farm produces eggs, the broiler chicken industry, which produces chickens for meat, also has been bracing for the virus since it was detected in the Pacific Northwest late last year, said Tom Super, spokesman for the National Chicken Council. He noted that the chicken industry is much larger than the turkey industry, with Americans eating about 83 pounds of chickens annually compared with about 16 pounds of turkey.

“We’re certainly remaining on double-heightened alert,” he said, noting that most broiler production is in southeastern and mid-Atlantic states where the virus hasn’t appeared.

Scientists suspect the disease is being spread by migratory waterfowl, but that hasn’t been proven. They’re also trying to determine how the disease has been getting inside poultry barns despite strict biosecurity measures designed to keep it out.

H5N2 turned up on several chicken farms in British Columbia, Canada, late last year, and this month in Ontario. It also has been detected in some backyard flocks, but the Wisconsin case is the first detected in a U.S. commercial chicken operation, said Joelle Hayden, a USDA spokeswoman.

Wisconsin, which typically ranks around 18th among U.S. states in chicken production, exported poultry products worth $4.7 million in 2014.

No human cases have been found in the U.S. But as a precaution, the Wisconsin Department of Health Services is reaching out to workers who may have been exposed. Surveillance and testing also are underway at nearby farms.

About 40 countries have blocked imports of turkey or chicken products to varying degrees from the affected states, but many of those countries weren’t big customers. And some major importers, such as Mexico, are scaling back their bans to specific counties.

The outbreaks have cost the industry a small fraction of its annual production, and they’re not expected to affect retail prices much.