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Human Transmission in One Third of Bird Flu Cases

Recombinomics Commentary
February 5, 2005

>> Medical investigators said it is the first documented case of human-to-human transmission of the Avian flu virus. Health officials in Thailand said an 11-year-old girl who died from Avian flu last fall most likely transmitted the disease to members of her family.

Avian flu is typically transmitted from infected birds to humans, but health investigators said, in this case, the young girl may have passed on the virus to her mother and aunt when they hugged and kissed.

World health officials are concerned the virus could mutate, allowing for widespread human to human transmission, but said this small scale transmission is not an indicator the virus is mutating. <<

Although the case in Thailand led to the first published paper documenting human to human transmission of H5N1 avian influenza in the recent outbreak, there are at least 8 other familial clusters that are similar.  The cluster in Thailand stood out because the mother was several hundred miles away from her daughter when her daughter developed symptoms and the mother had no history of exposure to birds.  She developed symptoms after her daughter died, suggesting she was infected by her daughter.  In addition, the aunt also developed symptoms.  Thus, it was the lack of bird contact by the mother and a bimodal distribution of onset dates that made human to human transmission in the cluster convincing. 

The lab data supported the conclusion, but only after considerable effort.  The index case was initially diagnosed as dengue fever and was never tested to show that she was infected with H5N1.  Samples were also not collected for the mother, but after her death H5N1 sequences were identified in fixed tissue.  The aunt did test positive, although she was negative on her initial test.

An equally compelling case can be made from a much earlier cluster in Vietnam.  The index case developed symptoms on Jan 3, 2004, was hospitalized on Jan 7 and died on Jan 12.  There was no sample collected for testing.  However, his two sisters who cared for him both developed symptoms on Jan 10, both were hospitalized on Jan 13, both were inclusive on initial tests, both were H5N1 positive on subsequent tests, and both died on Jan 23.  One sister had no history of contact with birds so the cluster was very similar to the Thailand cluster.  One patient had no history of contact with birds, but developed a fatal infection after caring for a close relative.  Although the disease onset was bimodal for the cluster, it was unimodal for the two sisters, indicating they were infected by a common source, who was almost certainly their brother.

The other 7 clusters had the same type of bimodal distribution for disease onset, indicating they were all likely human to human transmission.  The 9 clusters involved 21 patients and 18 died.  These patients represent about 1/3 of all reported H5N1 avian influenza cases in Vietnam, Thailand, and Cambodia so the clusters as a group are not small scale. 

However, each of the 9 transmission chains was short, because the patients from the second round of infections sought medical treatment, and their caregivers knew from first hand experience that human to human transmission of the fatal illness was quite possible.

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