An 86-year-old Hongkonger living in Shenzhen has become infected with the H9N2 strain of bird flu - Hong Kong's first human case of the virus strain in four years.
The case, in light of the other bird flu strains hitting humans with increased frequency - including the deadly H7N9 and H10N8 subtypes - may signal a higher risk of virus mutation and genetic swapping, experts say.
The man was in a stable condition at North District Hospital, in Sheung Shui, after crossing the border into the city on Saturday to seek medical help, health authorities said.
He had neither consumed nor come into direct contact with live poultry, nor had he been in contact with bird-flu sufferers.
"This case has not raised the overall risk much, but it is worrying that we are recently seeing various avian flu subtypes infecting humans more frequently," microbiologist Dr Ho Pak-leung of the University of Hong Kong said yesterday.
The man arrived at the Lo Wu border crossing suffering from chills, a cough and excess sputum. He was then taken to a hospital in an ambulance. He developed a mild fever in the hospital.
More than 50 medical and ambulance workers at the hospital are now being monitored for symptoms. The man's relatives in Shenzhen had not displayed any symptoms of illness, the Department of Health said.
Both Centre for Health Protection controller Dr Leung Ting-hung and HKU microbiologist Professor Yuen Kwok-yung said that the H9N2 strain usually caused a mild infection.
The virus did not spread to humans easily and there was no evidence of human-to-human transmission, Leung said.
However, the strain could be serious for the elderly and those with a weak immune system, Yuen said.
The south of the mainland had seen sporadic cases of human infection with H9N2 since 1998, he said.
In Hong Kong, where the virus was a statutory notifiable disease, seven cases had been recorded since 1999, the department said, two of which were imported from the mainland and four of which were local infections. One case was of an unknown origin.
The last case, an imported infection, was in 2009. All of the infected recovered from the virus.
Leung said H9N2 had been detected around the world.
The emergence of different strains of bird flu in humans heightened the risk of genetic swapping, which might lead to mutated viruses, Yuen said.
Increased demand for chicken on the mainland had prompted traders to pack more birds into the same amount of space in wet markets, increasing the risk of cross infection and genetic adaptation, he said. "Biosecurity measures and regulatory controls on the mainland have not caught up with increasing poultry numbers," he added.
Ho said that before Hong Kong banned wet markets from keeping live poultry overnight in 2008, the H9N2 infection rate was 5 per cent to 6 per cent. The rate fell below 1 per cent after the ban. Leung urged the public to avoid contact with live poultry and to practise good personal hygiene. He expects more cases of avian influenza as temperatures fall.
BIRD FLU STRAINS
Differences between the H9N2, H5N1, H10N8 and H7N9 bird flu strains:
Sources of human infection
H9N2 chickens, ducks, magpies, sparrows, waterfowl
H5N1 first spread from poultry in Hong Kong in 1997
H10N8 detected in water birds on mainland in 2007
H7N9 detected in mainland live chickens and wet markets since first human infections in March
Possibility of human-to-human transmission
H9N2 no evidence yet
H5N1 appears low
H10N8 believed to be low, as most of its genes come from birds
H7N9 limited; only suspected to have been transmitted among people in four patient clusters
Number of cases
H9N2 11 in southern China since 1998
H5N1 at least 22 in Hong Kong since 1997; 648 cases globally since December 2003
H10N8 one (on the mainland)
H7N9 143 since February, all in China