- "Almost all cases of H5N1 human
infection appear to have resulted from some form of direct or close contact
with infected poultry, primarily chickens. In addition, a few persons may
have been infected through very close contact with another infected person,
but this type of transmission has not led to sustained transmission."
-
- The above comments from the prepared
statement for the Senate appropriations committee seriously underplay the
involvement of human-to-human transmission of H5N1. The role was
even more distorted in the actual testimony, which indicated that strong
evidence of human-to-human existed for only two familial clusters.
Familial clusters have made up an ever increasing percentage of the official
H5N1 cases and virtually all such clusters involve human-to-human transmission
among family members.
-
- Although exposure to sick or dying poultry
can be frequently linked to the index case in a cluster, this linkage does
not necessarily extend to other family members. These members frequently
have exposure to both the index case and poultry. To distinguish
between a common poultry source and a common family member, the dates of
disease onset are used. Since transmission from bird to human is
rare, the likelihood of two independent transmission is low. Therefore,
if the common source is poultry, the index case and other family members
would be expected to develop symptoms over a short time course (1-2 days).
If however, the index case transmitted the H5N1 to other family members,
the time interval between disease onset in the index case and disease onset
in other family members would be long (5-10 days).
-
- The number of family clusters in the
various countries reporting H5N1 outbreaks since 2004 has now exceeded
thirty. Almost all of these clusters have a time gap of 5-10 days
between disease onset of the index case and other family members.
This gap indicates that most of the familial clusters involve human-to-human
transmission.
-
- The clusters date back to early 2004
in Vietnam and later in Thailand. By early 2005, these clusters account
for almost one third of H5N1 cases. In Indonesia, the number of H5N1
patients in familial clusters grew to about two thirds of cases.
The initial 15 clusters were described in a recent CDC/WHO publication.
At that time, WHO changed wording in their characterization of the H5N1
outbreak. They had indicated that there was little evidence for human-to-human
transmission. This changed to little evidence for efficient human-to-human
transmission, acknowledging the growing number of familial cases which
involved human-to-human transmission.
-
- Recently, the size and number of these
clusters grew, and WHO again changed their description from a lack of evidence
for efficient human-to-human transmission to a lack of evidence for sustained
human-to-human transmission. Although this terminology suggests the
increased frequency has been noted by WHO, public comments and media reports
still leave the impression that human-to-human transmission of H5N1 is
rare or non-existent.
-
- This impression is particularly misleading
at the present time because a genetic change has been noted in H5N1 from
the index case in Turkey. The change in the receptor binding domain
of HA, S227N (also called S223N), increases the affinity of the HA for
human receptors. This change coupled with another change, PB2 E627K,
increases the efficiency of H5N1 infection in humans, especially in cold
weather. These genetic changes have led to very large clusters in
Turkey as well as linkage between clusters.
-
- The linked cluster included the index
case for Turkey. Index cases from familial clusters have in fact
been the index cases for countries since 2005. The index case for
Cambodia, Indonesia, China, Turkey, and Iraq all were familial index cases
and all clusters included a 5-10 day gap in disease onset dates.
-
- These data leave little doubt that human-to-human
transmission of H5N1 is quite common and now represent the majority of
human cases. Representations to the contrary are cause for concern.
-
- © 2005 Recombinomics. All
rights reserved.
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