- (Reuters) -- The H5N1 avian influenza
virus in humans has evolved into 2 separate strains, a development that
will complicate the search for a vaccine and the prevention of a pandemic,
U.S. researchers reported on Mon [20 Mar 2006].
- The genetic diversification of the pool
of H5N1 avian influenza viruses with the potential to cause a human influenza
pandemic heightens the need for careful surveillance, researchers said
at the International Conference on Emerging Infectious Diseases in Atlanta.
- "Back in 2003 we only had one genetically
distinct population of H5N1 with the potential to cause a human pandemic.
Now we have 2," said Rebecca Garten of the U.S. Centers for Disease
Control and Prevention (CDC), who helped conduct the study. One of the
2 strains, or clades, made people sick in Vietnam, Cambodia and Thailand
in 2003 and 2004 and the 2nd, a cousin of the 1st, caused the disease in
people in Indonesia in 2005.
- 2 clades may share the same ancestor
but are genetically distinct -- as are different clades, or strains, of
the AIDS virus, the team from the CDC found. "This does complicate
vaccine development. But we are moving very swiftly to develop vaccines
against this new group of viruses," said Dr. Nancy Cox, chief of the
CDC's influenza branch.
- The H5N1 strain of bird flu has spread
across Europe, Africa and parts of Asia and killed nearly 100 people worldwide
and infected about 180 since it reemerged in 2003. Although it is difficult
to catch bird flu, people can become infected if they come into close contact
with infected birds. Scientists fear the virus could mutate into a form
that could pass easily between humans, triggering a pandemic in which millions
- All influenza viruses mutate easily,
and H5N1 appears to be no exception. But Cox said the evolution of a 2nd
clade does not move the virus closer to human-to-human transmission. "Like
the group one or clade one viruses, the group 2 or clade 2 viruses are
not easily transmitted from person to person," she said. "It
really doesn't take us closer to a pandemic. It simply makes preparing
for the pandemic a bit more difficult."
- The U.S. Health and Human Services Department
has already recognized the 2 strains and approved the development of a
2nd H5N1 vaccine based on the 2nd clade. Several companies are working
on H5N1 vaccines experimentally, although current formulations are not
expected to protect very well, if at all, against any pandemic strain.
A vaccine against a pandemic flu strain would have to be formulated using
the actual virus passing from person to person.
- Researchers said while vaccines were
needed against different strains of the virus, a vaccination against one
clade could provide partial protection against another. "We would
expect the priming (of a patient) with a clade one (vaccine) could potentially
reduce the severity of disease," Cox said.
- For their study, Garten and colleagues
analyzed more than 300 H5N1 virus samples taken from both infected birds
and people from 2003 through the summer of 2005. Garten said the bird flu
strains being detected in Europe were generally clade 2 strains.
- ProMed Mail
- The term "clade" is derived
from the style of presentation of phylogenetic data, which are most conveniently
presented in the form of trees consisting of 2 elements; nodes and branches.
A branch is a line which connects 2 nodes. Nodes can be external (the tips
of the branches) or internal (representing the common ancestor of 2 nodes)
A cladogram shows only the branching order of nodes, and the lengths of
the branches contain no information. All of the descendents of a common
ancestor represented by a common node belong to the same clade. A clade
is a monphyletic group.
- A phylogram, on the other hand, displays
both branching order and distance information, where distance provides
an indication of the number of amino acid changes (or nucleotide substitutions)
from the ancestral node (i.e. giving an indication of the extent of relative
evolution from a common node). - Mod.CP
- Patricia A. Doyle, DVM, PhD- Bus Admin,
- Economics Univ of West Indies
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