H5N1 In Humans Evolved
Into 2 Separate Strains
By Jim Loney and Maggie Fox
(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 could die.
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, Tropical Agricultural
Economics Univ of West Indies
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