The Gazette offers audio versions of articles using Instaread. Some words may be mispronounced.

Chickens in a massive, commercial egg-laying flock in northwest Iowa are infected by an avian flu that is highly transmissible and often deadly for them, according to the Iowa Department of Agriculture and Land Stewardship.

The detection of the virus in the Sioux County flock of about 4.3 million birds is the first in commercial birds in Iowa since July.

Entire flocks are culled to prevent the spread of highly pathogenic avian influenza. The virus is transmitted by migrating birds but can also be carried between farms by workers and equipment.

The state has had four infected flocks this year -- which is a small number in comparison to other recent years -- but the losses have been huge: a total of 8.7 million.

One of the previously infected flocks also had 4.3 million birds and was located in Sioux County, but state officials declined to say whether it was the same flock as the newly infected.

A version of the virus also infected several northwest Iowa dairy cattle herds in June.

On Friday, the U.S. Department of Agriculture announced a new milk-testing program that is meant to determine the scope of infections among dairy cattle.

Federal testing has confirmed that pasteurization kills the virus in milk, but farmworkers who are in close contact with infected animals are at risk. Nearly 60 people have been infected, and most of them were known to have been near infected cattle and poultry, according to the U.S. Centers for Disease Control and Prevention.

Human infections increase the likelihood for the virus to mutate to spread human-to-human and cause a pandemic.