Highly pathogenic avian influenza hit two more barns in Iowa last week, leaving state and federal officials struggling to determine how the deadly disease made its way into the three barns in northwest Iowa.

"The equipment, the feed, the personnel, you have to look at all of those factors," said John Clifford, chief veterinary officer for the U.S. Department of Agriculture (USDA) and the Animal and Plant Health Inspection Service (APHIS) deputy administrator for Veterinary Services. "And right now, we can’t just say that it was this or this."

Highly pathogenic H5N2 avian influenza was confirmed last week in a barn holding 3.8 million laying hens in Osceola County, forcing the farm to euthanize all of its birds.

The barn, Sunrise Farms, own­ed by Sonstegards Foods Co. in South Dakota, reported the single largest loss to avian influenza in the United...