Nearly 1 million chickens on a Minnesota egg farm will be slaughtered to help limit the spread of the highly contagious bird flu after it was confirmed there, officials said Monday.

The U.S. Department of Agriculture announced that the virus was found at a farm in Wright County, Minnesota, as well as in three smaller flocks in South Dakota and Iowa. Whenever the highly pathogenic avian influenza virus is detected on a farm, the entire flock is killed as to keep it from spreading to other farms.

In addition to the Minnesota case, the USDA said some 26,800 turkeys will be killed on a farm in McPherson County in South Dakota and nearly 17,000 birds will be slaughtered on two farms in Iowa’s Clay County.

The egg and poultry industry has been dealing with a bird flu outbreak since last year. In 2022, nearly 58 million birds — mostly chicken and turkeys — were slaughtered to deal with the virus, contributing significantly higher egg and turkey prices. The Minnesota farm is the first egg-laying operation where bird flu has been found this year.

The toll overall has been much lower in 2023 than in 2022 as the number of cases found in wild birds plummeted and farmers redoubled their efforts to prevent any contact between their birds and the ducks and geese migrating past their farms. Even after 940,000 chickens on the Minnesota farm are slaughtered, there will only have been about 3.4 million birds killed this year.

Minnesota has now lost a total of more than 5.5 million birds since the outbreak began. Iowa, which is home to many massive egg farms, has been the hardest hit with more than 16 million birds slaughtered, including one case where 5 million egg-laying chickens had to be killed. Egg farms like the one in Minnesota tend to have the most birds on any one farm. Turkey and chicken operations usually involve fewer birds.

There have been a number of cases reported over the past month, mostly at turkey farms in Minnesota, South Dakota and Iowa, as wild birds began to migrate south for the winter. But the virus can be found on any farm because it is spread easily, primarily through the droppings of wild birds or direct contact with them.

Egg and poultry farmers take steps like requiring their workers to change clothes and sanitize their boots before stepping inside their barns, limiting the sharing of tools between barns, and sealing up their facilities to prevent wildlife from getting inside.

Officials stress that bird flu isn’t a threat to food safety because all the birds on a farm where the disease is found are slaughtered before they reach the food supply, and properly cooking poultry and eggs to 165 degrees Fahrenheit (73.89 degrees Celsius) will kill any viruses. Infections in humans are rare and usually come only in people with prolonged exposure to sick birds.