Authorities are on alert as this year’s outbreak of avian influenza sweeps the central region of the country a month earlier than in past years with an increased number of migratory birds.

A southward migration of the birds, the suspected source of the infections detected last month, as the weather cools later in November is fueling concern that the flu may spread nationwide.

The first case of the highly pathogenic H5N1 strain of the virus was reported on October 19 at a duck farm in Yecheon, North Gyeongsang Province where 98-hundred birds were culled, three days before another case was confirmed at a chicken farm in the area.

AI has since spread to the Chungcheong and Jeolla provinces while an egg farm in Gangwon Province also reported that 600 of its egg-laying chickens have perished.

Officials have yet to identify the source and infection route of the virus but suspect it may have spread to the farms by wild cats or field mice that made contact with the feces of migratory birds near local streams.

Meanwhile, the National Institute of Biological Resources said that over 553-thousand ducks have been observed at 200 migratory bird sanctuaries nationwide as of last month, some 26 percent more than last year, stoking concerns that the virus may spread if temperatures induce a migration.