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Clusters of Human to Human Bird Flu Transmission in SE Asia

Recombinomics Commentary
February 5, 2005

>> Mr. Hung, a slim man with a steady gaze, told of how the ordeal started when he took a three-hour bus trip to his hometown, Thai Binh, on Dec. 24.

He had gone to attend a wake for his elder brother's 19-month-old son, who had accidentally drowned - an incident so painful that Mr. Hung, who has no children, goes silent when asked about it, tears coming to his eyes.

Mr. Hung's elder brother, 46, and younger brother, 36, bought a live duck in the village market. The younger brother held the bird while the elder brother slaughtered and cleaned it before Mr. Hung arrived. The elder and younger brothers then made a pudding of raw duck blood, Mr. Hung said.

The elder brother, his brother-in-law and Mr. Hung each ate some of the pudding but decided it was too salty. So nobody else at the large extended family gathering ate any of it. After the lunch, Mr. Hung rode the bus back to Hanoi, where he works largely from home as a cement trader, spending a few hours a day with his cellphone constantly pressed to his ear. He said he did not go near any live poultry after his return to Hanoi.

Mr. Hung's elder brother became feverish on Dec. 27, said Dr. Nguyen Thi Tuong Van, the physician who oversaw the elder brother's care and then Mr. Hung's. The brother's family looked after him at home until Dec. 31……

As is common in Vietnam, family members cared for the elder brother during his stay in hospital, feeding him and staying with him day and night. His wife provided most of the care, but Mr. Hung and his younger brother also came daily.

Mr. Hung's illness began the same day his brother died. Mr. Hung said he went immediately to the hospital where his wife worked. An X-ray showed a small shadow on his right lung, which was misdiagnosed as tuberculosis. <<

The description above provides more detail on the 3 brothers in the Hanoi cluster, although it is still unclear if the youngest brother, who had no symptoms, has tested positive for H5N1.  The added detail (onset date of Dec 27 for the index case and limiting of the blood pudding consumption to two brothers and a brother-in-law), helps the common source argument, but still does not fit the timing of each infection.  The slaughtering of the duck on Dec 24 is the more likely factor leading to the Dec 27 onset date for the index case, and care of the hospitalized index case in January is the more likely factor leading to a Jan 9 onset date for the middle brother.

However, while details on an individual cluster can be helpful, the details are most helpful when an individual case is an outlier and the case is studied to identify what factors cause it to be different from the rest.  Unfortunately for the victims, the above case is all too common and it matches 8 other H5N1 familial clusters in Vietnam, Thailand, and Cambodia beginning a year earlier in Dec 2003.

The nine clusters involve 21 patients and none had final lab results showing that they were not infected.  In 5 instances no sample was collected for testing and results from a 6th are pending, but all other 15 have been confirmed as H5N1 positive and each cluster has at least one lab confirmed result.  The clinical picture for all 21 is the same.  Most are previously healthy young adults who developed fever and respiratory problems and 18 of the 21 have died (both of the recovered patients tested positive for H5N1 and 1 patient was in critical condition Jan 22).

The lab test results were less than ideal.  For six no conventional samples were collected (one patient was confirmed by testing fixed tissues) and results on one are pending.  For the 14 who eventually tested positive, two were initially inconclusive, two were initially negative, and was initially negative twice.  However, all 21 had bird flu symptoms and 18 of the 21 died.

The analysis of the clusters as a group is much more informative concerning H5N1 transmitting human to human.  The data were quite conclusive.  Disease onset dates were bimodal in each cluster, strongly hinting at human to human transmission, rather than common source transmission.  Analysis of the individual cases was even more conclusive.  Of the 12 cases that were not the index cases, 11 had disease onset of a week or more after the onset of symptoms in the index case.  Thus, the data for the group of clusters was internally consistent with the data for each of the clusters, suggesting that each of the clusters represented one or more examples of human to human transmission. 

The 11 likely human to human transmissions would represent over 17% of the reported bird flu cases.  The 21 cases in the clusters would represent about 1/3 of all of the H5N1 avian influenza cases.  Thus, familial clusters involving human to human transmission are quite common, and statements regarding how rare human transmissions are, are both misleading and dangerous, especially to family members caring for bird flu patients.

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