DES MOINES, Iowa — Bird flu could cost nearly $1 billion in the economies of the two states hardest hit, Minnesota and Iowa, agricultural economists said Monday, and the virus is still spreading.

Iowa, the nation’s leading egg producer, has lost about 20 million chickens that lay eggs for food use, more than a third of the total. Minnesota, the top turkey state has lost more than 8 million birds.

So far the U. S. Department of Agriculture has confirmed the bird flu has claimed nearly 37 million birds in 15 states but the number is significantly larger because additional farms in Iowa and Minnesota recently discovered are not yet on the list. The figures include birds killed by the virus as well as those killed to prevent its spread.