Mexico's animal health agency says a bird flu outbreak at seven farms in central Mexico has affected as many as 582,000 chickens.
The Agriculture Department says more than a half million birds were exposed, but the number that will have to be slaughtered has yet to be determined.
An outbreak of the H7N3 bird flu virus in western Mexico in 2012 led to the slaughter of more than 22 million hens and caused price increases in chicken and egg products.
But the department said Friday that the current outbreak has not affected the supply of chicken products.
It said tests were continuing to determine the exact strain of virus involved in the outbreak, but said it did not affect humans.
Mexico's nationwide flock amounts to 137 million birds.