MEXICO CITY – Animal health officials began vaccinating approximately 10 million poultry birds against H7N3, an avian flu strain that has led to millions of birds becoming ill or being culled.

The agency detected the virus at 33 of 253 chicken farms in the western state of Jalisco during inspections in late June. Another 82 farms were free of the virus, and diagnostic tests are being conducted at the remaining 138 facilities, according to Senasica, Mexico’s food safety agency. The virus was first uncovered in Jalisco on June 20. Mexico’s government issued a national animal health emergency on July 2.

Vaccines were imported from Pakistan, and Mexico's agriculture officials said they have developed a seed-based vaccine to be delivered to laboratories in preparations to make more than 80 million doses.