(CNN) -- Mexican authorities say they've slaughtered more than a million chickens infected with bird flu.
Investigators detected the virus in 18 farms in the central Mexican state of Guanajuato, Mexico's Agriculture Ministry said this week.
Chickens there were infected with the H7N3 virus, which Mexican authorities said does not pose a threat to humans.
"This virus is exclusive to birds, so there is no risk for public safety," the Agriculture Ministry said earlier this month.
Authorities offered different tallies of how many infected birds had been slaughtered.
Agriculture Minister Enrique Martinez said Monday that more than 2.1 million chickens had been killed -- including 519,000 egg-producing chickens, 722,265 breeding chickens and 900,000 chickens raised for meat.
On Tuesday, Javier Usabiaga Arroyo, a state agriculture official, said the total number of infected chickens killed was about 1.2 million, Mexico's state-run Notimex news agency reported.
Officials have vaccinated 1.9 million birds since the outbreak began earlier this month, and they plan to vaccinate millions more, the Agriculture Ministry said in a statement this week.
The outbreak has sparked concern about a possible spike in food prices, but authorities said Monday that the number of slaughtered chickens is a small fraction of the country's overall population and there is no reason for egg or chicken prices to increase.
"The outbreak of avian influenza is controlled," Mexico's food safety agency said in a statement.
Other strains of bird flu have spread to humans and prompted authorities to slaughter animals.
In 1997, authorities in Hong Kong killed about 1.5 million chickens after H5N1 avian influenza passed from birds to humans there.
Last year a new strain of H3N8 flu jumped from birds to mammals and was responsible for the death of more than 160 seals off the New England coast.
CNNMexico.com contributed to this report.