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Evidence of Human / Human Transmission of Bird Flu
January 24, 2005
>>Health officials have said the family had eaten raw duck blood pudding in late December, linking their infections to poultry.
"From the H5N1 virus infected cases in Thai Binh recently, there is no evidence of human-to-human transmission," Liem was quoted as saying.<<
The evidence for human to human transmission is quite strong, based on disease onset dates. When WHO issued a statement on two hypotheses being investigated, they indicated that the hospitalization of the third brother (36M) had not been confirmed. However, media reports quoted WHO officials as acknowledging that the onset date of Jan 10 for the second brother (42M) was an indication of human to human transmission, since the meal was in December and the index case (47M) developed symptoms Jan 1. The third brother (36M) developed symptoms about 3 weeks after the index case (47M) indicating that the link to poultry goes from the meal to the index case (47M), to the middle brother (42M), to the youngest brother (36M).
Although this is the longest reported chain, there are several additional examples. These clusters heightened alert levels and test results (or lack thereof), indicate that most or all of these cases would have gone unreported if there was not clustering.
The first cluster was in Vietnam in Hau Giang province in late July. Three patients died within a few days of each other. Although all three had bird flu symptoms, samples were not collected for testing. When the older sister of one of the fatalities, who was also a cousin to another fatality, developed symptoms, she was tested and was H5N1 positive. Her case was added to the confirmed list and the three earlier cases were not because no samples were collected, even though there were bird deaths in the province at the time. The bird deaths had not been reported, but subsequently H5N1 was isolated from at least 2 chickens and sequence data indicated the virus was virtually identical to earlier isolates from Vietnam. However, without the cluster, it seems that none of the patients would have been tested.
Several weeks later, a similar cluster arose in Thailand. An 11 year old girl developed symptoms after handling a sick chicken. She was living with her aunt in northern Thailand while her mother was in Bangkok. The daughter was diagnosed with dengue fever, so when the mother visited there was close contact. After the daughter died and the mother returned to Bangkok, she developed symptoms, as did the aunt. There was no confirmation for the index case, but sequences from the mother showed that the H5N1 was virtually the same as earlier isolates from Thailand. The aunt initially tested negative. Thus, without the cluster the index case would have been a dengue fever death, tissue from the deceased mother would not have been used to isolate sequences, and the aunt would either not be tested, or not retested.
The same situation developed with the three brothers. The index case tested negative twice, H5N1 was confirmed at autopsy. The middle brother also tested negative initially and the third brother was just confirmed. Without the cluster, it seems that the negative results on the first two brothers would have stood and they would not have been retested.
Another cluster is being analyzed in the south. A fatal infection in a 17 year old was confirmed and his 22 year old sister developed symptoms after he died. She is in critical condition with avian influenza symptoms, but has not yet been confirmed.
Thus, in all four clusters, the second case developed symptoms after the index case died. The evidence for human to human transmission is quite strong for three clusters and overwhelming for the 3 brothers.
If the 22F above is confirmed, then the two recent clusters in Vietnam will involve 5 of the 12 confirmed cases, suggesting more efficient human to human transmission of H5N1 this season.