More than 4 million chickens in Sioux County, Iowa are to be culled after the flock reportedly came into contact with bird flu.

According to the Iowa Capital Dispatch, 4.2 million chickens are affected, the largest number since the virus impacted 5 million chickens in 2022.

The Iowa Department of Agriculture and Land Stewardship said in a statement that the United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) “detected a case of Highly Pathogenic Avian Influenza (HPAI).”

The situation impacts “commercial layer chickens.”

Although wild birds can carry HPAI without “appearing sick,” according to the Iowa Department of Agriculture and Land Stewardship, cases of HPAI in domestic birds may be “fatal.”

An official told CBS News that “depopulation is ongoing.”

Governor Kim Reynolds (R) issued a disaster proclamation over the matter, as Iowa is the leading egg producer in the United States.

The proclamation permits numerous agencies to “implement … stop movement and stop loading restrictions, surveillance and sampling activities, and other control zone measures as are reasonably deemed necessary …”

Only two human cases of the H5 bird flu virus have been confirmed in the U.S.