LOS ANGELES, March 20 (Xinhua) -- Whole genome analysis of the H5N1 avian influenza virus samples revealed that the virus has two related but different strains, scientists at U.S. Centers for Disease Control and Prevention reported.

    Both strains could infect humans, and might cause a pandemic, scientists noted Monday at the International Conference on Emerging Infectious Diseases in Atlanta.

    "As the virus continues its geographic expansion, it is also undergoing genetic diversity expansion," said Rebecca Garten, a researcher on the study.

    "Back in 2003 we only had one genetically distinct population of H5N1 with the potential to cause a human pandemic, now we have two," she added in a statement.

    Garten and colleagues conducted a phylogenetic analysis of over 300 H5N1 virus samples taken from both avian and human sources from 2003 through the summer of 2005.

    They correlated the genetic makeup of viruses to the physical characteristics of viruses, including hemagglutinin (HA), a key surface protein, and receptor sites, to determine where each virus should be placed on the H5N1 family tree.

    Not all H5N1 viruses are genetically the same. Over the years researchers have identified different genetic groups called genotypes.

    The majority of the viruses, including all the human cases, belonged to genotype Z. There were also small numbers of viruses isolated from avian populations that were genotype V or W or recently identified genotype G.

    That is where the similarity between the two human strains ended. Previous research published by the World Health Organization has further classified genotype Z into subgroups called clades based on different hemagglutinin sequences.

    In 2003 and 2004, clade 1 viruses were primarily responsible for outbreaks, including all infections in human, in Vietnam, Cambodia and Thailand.

    But in 2005 a second strain of H5N1 began causing disease in humans in Indonesia. Analysis of the Indonesia strain found that it belongs to genotype Z clade 2, a subgroup of the virus that previously was not known to cause human disease.

    Clade 1 and clade 2 viruses may share the same ancestor, but are different and can be likened to cousins, the researchers said.

    "Human and avian isolates from Vietnam, Thailand, Cambodia, Malaysia and Laos are genotype Z and have a clade 1 HA," they saidin the research paper.

    "Viruses with clade 2 HA belong to genotypes Z, V or Z/W reassortants, and are responsible for the recent outbreaks of avian influenza in Indonesia, China, Japan, South Korea, Vietnam and Eastern Europe and human cases in Indonesia."

    What this means, the researchers said, is that the pool of H5N1candidates with the potential to cause a human influenza pandemic is getting more genetically diverse, which makes studying the virus more complex and heightens the need for increased surveillance.

    "Since 1997, H5N1 viruses which have infected humans have included HA from several clades and variable genotypes," they concluded in the paper.

    "Therefore, all H5N1 viruses must be considered a potential threat to public health. This increases the scope of viruses with pandemic potential and the importance of continued surveillance ofH5N1 avian influenza outbreaks." Enditem