(Bloomberg) -- Egg prices are skyrocketing in new US inflation data, pushed up by a deadly bird flu that’s killed almost 10% of the country’s hens, adding to already rising food costs.

A dozen eggs at grocery stores cost 23% more in April than in March, at $2.52. That’s the biggest increase among consumer products tracked by the US Bureau of Labor Statistics, in a report released Wednesday.

The jump comes as a highly infectious avian influenza sweeps through the nation, killing more than 37 million chickens and turkeys in what’s shaping up to be the worst outbreak of its kind. Of that, nearly 29 million egg-laying hens have died, which compares to the U.S.’s total flock of over 300 million.

The outbreak has become so severe that it’s confounding insurers of poultry farms. Some have even halted underwriting such businesses for now, according to Nevin Dourte, president of Ruhl Insurance, a family-owned brokerage agency in Central Pennsylvania. 

This particular outbreak “doesn’t seem to follow any past historical trends,” he said.

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