Fresh fears over the deadly H5N1 strain of bird flu returned last night when health officials in China confirmed a man had caught the disease from his son.
The incident has revived the spectre of human-to-human transmission, which has occurred around a dozen times in the past in countries including Cambodia, Thailand, Vietnam and Indonesia.
In nearly every case, transmission has occurred among blood relatives in close contact with one another, and the virus has not spread further.
The 52-year-old father and his 24-year-old son were diagnosed with the disease in the Jiangsu province of China within a week of each other last December.
The World Health Organization said it could not rule out human-to-human transmission.
The son's only exposure to the disease was at a poultry market, while his father had no direct exposure to sick birds – only to his ill son.
The son died in hospital. His father survived after being treated with antivirals and participated in an H5N1 vaccine trial. Some 91 friends, colleagues, and family members all tested negative for H5N1, proving that the virus is not casually transmitted.
In an article for The Lancet, researchers at Beijing's Chinese Centre for Disease Control and Prevention wrote: "Limited, non-sustained person to person transmission of H5N1 virus probably occurred in this family.
"There is no indication from this data that we are any nearer to a pandemic," said Ian Jones, a professor of virology at the University of Reading.
"An air of tension still surrounds this disease," wrote Dr Jeremy Farrar of the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam, along with colleagues. "Given that the species barrier can be breached, the intriguing question is why the transmissibility of H5N1 among people remains so low."
There are fears among some flu experts that H5N1 will spark a pandemic, potentially killing millions worldwide. However, cases of human infections remain relatively rare.
The chairwoman of Influenza Virology at Imperial College London, Wendy Barclay, urged calm, saying that there was "no virological evidence to support the idea that this strain of H5N1 virus has acquired mutations that allow it to pass readily from one person to another".
As of 3 April, WHO reported 378 cases and 238 deaths around the world.
In a separate incident, Indian officials said they will begin slaughtering poultry after confirming an outbreak of bird flu in the country's remote northeast.
Tests have revealed that several hundred birds who died in Tripura state were killed by the disease, according to Asim Roy Burman, a senior state Veterinary and Animal Resources Department official.
Mr Burman said authorities are also banning imports of poultry from neighbouring Bangladesh.
Twenty teams will start slaughtering the poultry once they obtained permission from federal authorities, Mr Burman added.Reuse content