There have been four reported cases of the avian disease and that is probably the "tip of the iceberg," said Dr. Philippe Buchy, head of the virology unit at the Institut Pasteur du Cambodge that tested the recent cases.
The girl, 15, was from the village of Snao in Takeo province.
On Saturday, more than 4,000 chickens and ducks were killed and burned outside her village, district Gov. Ith Sa said.
The village was sprayed and medicine was distributed to eradicate the virus, the governor said.
The other person killed by the disease since the beginning of the year was a 35-year-old man, The Phnom Penh Post reported.
In the latest bird-flu case, Health Ministry officials said a 2-year-old girl from Kampong Speu was diagnosed Saturday after being admitted to a Phnom Penh hospital.
More than 40 chickens at the child's house had become sick and died before she was hospitalized, health officials said.