Nearly 6 million laying hens in Sioux County have been affected by bird flu outbreaks over the past four days as wild birds that carry the virus migrate across Iowa.

The Iowa Department of Agriculture said Monday that a Sioux County commercial egg-laying operation with 1.6 million birds was infected with highly pathogenic avian influenza. It’s the second outbreak in Sioux County since Friday, when the infection was detected in a 4.3 million-bird flock.

The state reported Sunday that a Palo Alto County turkey operation with 31,000 birds tested also positive. The three detections followed a period since June when there were none.

All the birds are destroyed to prevent the spread of the highly contagious virus, which is deadly to poultry. The outbreak that began in 2022 has affected 29.2 million birds in Iowa, the nation’s leading egg producer, according to state and U.S. Department of Agriculture data. Nationwide, 115.3 million birds have been affected.

Gov. Kim Reynolds already had declared a disaster for Sioux County and extended it to Palo Alto County. The declaration allows the use of state resources to combat the spread of the disease.

It is believed to be spread by wild birds, which may show no apparent symptoms, and has been especially prevalent in Iowa during periods of migration. This fall’s above-normal temperatures may have delayed migration, with outbreaks recently reported in Minnesota, North and South Dakota and other Midwestern states.

The uptick in infections comes as the U.S. Department of Agriculture prepares to implement a national testing program for bird flu in milk after outbreaks of the virus, HN51, in dairy cattle in states including Iowa. Thirteen cases of H5N1 have been detected in dairy cattle in Sioux, O’Brien and Plymouth counties, though there have been none since June.

The first round of milk testing under the USDA order is set to begin Dec. 16.

While most infected dairy cattle experience mild symptoms from bird flu, there is concern among epidemiologists that it could be spread to humans through unpasteurized milk.

So far, no human cases of bird flu have been reported in Iowa.

Infected products do not enter the food supply and it remains safe to consumer poultry and eggs, as well as milk that has been pasteurized, a process that kills the virus, the Iowa Department of Agriculture has said.

The outbreak reported Monday was sixth in Iowa poultry this year.

Donnelle Eller covers agriculture, the environment and energy for the Register. Reach her at deller@registermedia.com.