Chinese Health Ministry spokesman Mao Qunan said yesterday that it was likely the father, aged 52, had caught the H5N1 strain of bird flu from his 24-year-old son.

'The initial judgment is that it was an infection from close contact,' Mr Mao told a news conference. He said the father, who lives in the eastern city of Nanjing, had recovered.

Mr Hans Troedsson said in an e-mail message: 'Human-to-human transmission through close contact between the son and the father cannot be ruled out in this family cluster.

'However, the biological findings at this stage show that the virus has not mutated to a form that can be transmitted from human to human efficiently.'

Mr Mao said the ministry still did not know the cause of the initial infection, as neither man had had any contact with sick or dead birds.

This rare case of two family members being struck by the disease is of concern to the health authorities because humans almost always contract H5N1 from infected birds.

Experts fear the virus could mutate into a strain that jumps easily from person to person, risking wider outbreaks. The virus has a limited capacity for human-to-human transmission, and other cases have been reported in Asia.

REUTERS