By Maggie Fox, Health and Science Editor

WASHINGTON, April 7 (Reuters) - A 24-year-old Chinese man who died of bird flu in December passed the virus directly to his father in a rare case of human-to-human transmission of the virus, doctors reported on Monday.

Chinese officials had already said they believed the younger man infected his 52-year-old father, who survived, but genetic sequencing and other checks confirmed this was likely, the researchers said.

"In this family cluster of confirmed cases of infection with highly pathogenic avian influenza A (H5N1) virus in mainland China, we believe that the index case transmitted H5N1 virus to his father while his father cared for him in the hospital," they wrote in the Lancet medical journal.

H5N1 avian influenza is regularly breaking out in birds across Asia, the Middle East, Africa and Europe. It only rarely infects humans but has killed 238 out of 376 people known to have been infected since 2003.

Most have been directly infected by a sick bird, but in a few rare cases, one person appears to have infected another. These have been documented in Indonesia and, just last month, between two brothers in Pakistan.

Most have been among people who are genetically related and this also appears to be the case with the two Chinese men, the researchers said. Some experts believe there may be a genetic susceptibility to H5N1 infection.

The fear is that the virus will acquire changes that allow it to be passed from one person to another more easily. This could cause a pandemic that could kill tens of millions of people globally, so experts are carefully studying every case of transmission.

Yu Wang of the Chinese Centre for Disease Control and Prevention in Beijing and colleagues investigated the cases of the man and his son, who were diagnosed within a week of each other in December 2007 in Jiangsu Province.

They also tested 91 people the two men had come into close contact with. None of these people became infected.

The young man had a high fever, cough and watery diarrhea and his father nursed him intensively in the hospital.

The younger man died but his father got the flu drugs Tamiflu and rimantadine as well as serum from a woman inoculated with an experimental H5N1 vaccine and recovered.

"With the exception of occasional infection in health workers, all published incidents of possible or probable person-to-person transmission report transmission between genetically related individuals," Nguyen Tran Hien of Vietnam's National Institute of Hygiene and Epidemiology in Hanoi, and colleagues wrote in a commentary.

"Although this finding could be related to the intensity and intimacy of contact between family members, host genetic factors might also play a part in susceptibility to H5N1," they added. So anyone in close, prolonged contact with an H5N1 victim should get flu drugs just in case, they said.

Last week the World Health Organization said some human-to-human spread likely occurred when three brothers in Pakistan became infected with H5N1 last year.

The largest known cluster of human bird flu cases occurred in May 2006 in Indonesia when at least 7 family members died.

(Editing by Philip Barbara)

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