(Bloomberg) -- Deadly bird flu was detected in an egg-laying chicken flock in Iowa, affecting 4.2 million birds in the biggest US outbreak since 2022.

The detection is the first since December in Iowa, which is the top US egg producer with nearly 12% of the country’s layer hens, according to US Department of Agriculture data. The findings come as the virus has been infecting dairy cows across the US.

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The Iowa Department of Agriculture and Land Stewardship said the site is in Sioux County, according to a statement Tuesday.

It’s the worst outbreak since more than 5 million birds were affected at a farm in Iowa’s Osceola County in March 2022, when the virus was first spreading in the country. Prices for eggs surged to a record as tens of millions of birds were killed to slow the spread. 

The virus now has moved to dairy cattle as well, prompting dairy farmers to seek compensation for milk losses. 

(Updates with details on outbreak in fourth and fifth paragraphs.)

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