Health investigators in Hong Kong have ruled out the possibility of human-to-human transmission in a case involving a nine-month-old baby who developed a mild form of avian influenza, Center for Health Protection Controller Dr Thomas Tsang said.

Speaking on a radio talk show on Saturday, Dr Tsang said genetic sequencing studies of the virus isolated from the nine-month-old girl - H9N2, a mild strain of bird flu - showed all genes from the virus are of avian origin.

Dr Tsang said the results suggested the virus was directly transmitted from a bird to the girl, without mingling with a human flu virus.

The girl had no contact wild birds before the onset of the symptoms. Dr Tsang said she may have contracted the virus at a neighborhood wet market in Hong Kong's New Territories, which she visited several times with her family.

He also reported that tests on respiratory specimens taken from a health-care worker and three children who were in the same cubicle in the United Christian Hospital with the girl in early March were all negative for the H9 virus. Also, her family members have no symptoms, Dr Tsang said, noting the case did not involve human-to-human transmission.

However, he said children with poorer immunity may be more susceptible to the virus. H9N2 cases recorded in 1999 and 2003 also involved children below age five.

On average, samples of 40,000 flu cases will be tested a year for surveillance. Dr Tsang said both H5 and H9 viruses are related to birds and can be transmitted from birds to humans, but the H5 virus has a mortality rate of 70 percent, while the H9 strain has symptoms and mortality similar to common colds.