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H5N1 Bird Flu Onset Dates Confirm Human Transmission
May 25, 2006
Fuelling the suspicion hanging over person-to-person transmission is the unusually long time lag of 15 days between the first and the last person in the cluster falling ill.
The incubation period for the H5N1 is usually no more than seven days and if the family had been exposed to the same source, they would all have fallen ill at about the same time.
"You want to look at the dates of onset of the disease. If they are close together they may have had the same exposure," Thompson said.
The above comments by WHO on the importance of disease onset dates in determining to source of H5N1 bird flu are also used to identify human-to-human transmission clusters. The onset dates are the most important data point because most H5N1 infections have an incubation time of 2-5 days as indicated in the New England Journal of Medicine review of H5N1 cases, authored by physicians who are WHO consultants. The 2-5 dates contradict recent comments to the New York Times that the incubation time for H5N1 was normally 7-10 days. The shorter time explains why most of the H5N1 clusters reported since 2004 have a 5-10 day gap between the index case and other family members. Half of the gap is due to the incubation time in the newly infected patients, and half is due to the time it takes for the index case to be optimally contagious.
Because of the importance of disease onset dates, they are usually included in WHO updates. They were withheld from descriptions of the Turkey clusters involving the two largest families, who were cousins and also withheld in the Sumatra cluster. Although onset dates were given for the first and last victim, there were no dates for the other H5N1 confirmed cases.
Recently, the number of suspected human-to-human clusters acknowledged by WHO has been rising. However, the vast majority of prior clusters have the 5-10 gap in disease onset dates, indicating most are due to human-to-human transmission, since bird to human infections are very rare and two independent infections from birds are unlikely. Authors from the WHO and the CDC wrote a report on the first 15 clusters, and most had the 5-10 day gap. The report was on cluster through there first cluster in Indonesia. Subsequent clusters have shown the same pattern, and clusters are common in Indonesia.
However, WHO investigates these cases, focusing on non-human sources and if found assuming the non-human source infected all family members, instead of just the index case. Thus, the presence of infected birds in the area does not explain the time gap, which is most easily explained by human-to-human transmission.