A Hong Kong-born infant has contracted a mild form of bird flu, the fifth case of its type in the territory, health authorities said Tuesday.

The two-month-old girl, who lives with her parents in China's southern city of Shenzhen, was diagnosed with the influenza A (H9N2) infection after she developed vomiting, a cough, and a runny nose, according to the city's Centre for Health Protection.

Thomas Tsang, controller of the centre, said the virus was detected in a specimen taken from the girl's respiratory system when she was admitted to Hong Kong's Tuen Mun Hospital on December 22.

She was discharged on December 23, but re-admitted to the hospital six days later because of a high white blood cell count, which may be suggestive of an underlying haematological condition, the centre said.

The girl is staying in the hospital for further investigation.

"Further genetic sequencing is being conducted to determine if the virus is completely of avian origin," Tsang said.

"As the girl was staying in Shenzhen for the whole incubation period, we have informed the Guangdong health department of this case and they will carry out necessary investigation and follow up actions."

The department will inform the World Health Organization and health authorities in Macao of the findings, he said.

H9N2 is an avian flu virus that mainly affects ducks and chickens, the centre said. Infection in humans is rare, and presents as a mild disease.