A 24-YEAR-OLD man in China probably infected his father with the H5N1 strain of bird flu before dying, renewing concerns that the disease could one day spread easily among humans, according to a study released yesterday.

The case is one of a handful over the past four years in which the H5N1 virus is suspected of having spread from one person to another, according to lead researcher Yu Wang of the Chinese Centre for Disease Control.

To date, however, all these cases have been what scientists call "limited, non-sustained, person-to-person transmission," meaning that contagion occurs only under specific circumstances.

The vast majority of the known 378 human cases of H5N1 bird flu diagnosed since 2003 were spread by domestic or wild fowl, according to the World Health Organisation. More than 60 percent proved fatal.

"It is not normal social contact that has led to the human transmission," epidemiologist Jeremy Farrar, a researcher at the national Institute of Hygiene and Epidemiology, said.

Another limiting factor may be genetic, the study found. The suspected cases of human-to-human transmission had all been "within the family, among blood relatives," said Farrar.

Besides the father, none of the 91 persons who came into contact with his son before he died showed any sign of infection, said the study, published in the British medical weekly The Lancet.

Nor was there any significant genetic variation between the viral strain in the father or the son.

Any new clusters of the virus "require urgent investigation because of the possibility that a change in the epidemiology of H5N1 cases could indicate that H5N1 viruses have acquired the ability to spread more easily among people," said Wang.

(SD-Agencies)