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Influenza A virus subtype H5N1, also known as A(H5N1) or simply H5N1, is a subtype of the influenza A virus which can cause illness in humans and many other animal species. A bird-adapted strain of H5N1, called HPAI A(H5N1) for highly pathogenic avian influenza virus of type A of subtype H5N1, is the highly pathogenic causative agent of H5N1 flu, commonly known as avian influenza ("bird flu"). It is enzootic (maintained in the population) in many bird populations, especially in Southeast Asia. One strain of HPAI A(H5N1) is spreading globally after first appearing in Asia. It is epizootic (an epidemic in nonhumans) and panzootic (affecting animals of many species, especially over a wide area), killing tens of millions of birds and spurring the culling of hundreds of millions of others to stem its spread. Many references to "bird flu" and H5N1 in the popular media refer to this strain.〔 This was reprinted in 2005:〕 According to the World Health Organization and United Nations Food and Agriculture Organization, H5N1 pathogenicity is gradually continuing to rise in endemic areas, but the avian influenza disease situation in farmed birds is being held in check by vaccination, and so far there is "no evidence of sustained human-to-human transmission" of the virus.〔(Situation updates - Avian influenza ). World Health Organization.〕 Eleven outbreaks of H5N1 were reported worldwide in June 2008 in five countries (China, Egypt, Indonesia, Pakistan and Vietnam) compared to 65 outbreaks in June 2006 and 55 in June 2007. The global HPAI situation significantly improved in the first half of 2008, but the FAO reports that imperfect disease surveillance systems mean that occurrence of the virus remains underestimated and underreported.〔(【引用サイトリンク】title=October 11, 2010 FAO Avian Influenza Disease Emergency Situation Update 70 )〕 In July 2013 the WHO announced a total of 630 ''confirmed'' human cases which resulted in the deaths of 375 people since 2003.〔http://www.who.int/influenza/human_animal_interface/EN_GIP_20130604CumulativeNumberH5N1cases.pdf〕 Several H5N1 vaccines have been developed and approved, and stockpiled by a number of countries, including the United States (in its National Stockpile),〔(H5N1 Influenza Virus Vaccine, manufactured by Sanofi Pasteur, Inc. Questions and Answers ), U.S. Food and Drug Administration.〕〔(Availability of a new recombinant H5N1 vaccine virus ), June 2010, World Health Organization; (Availability of a new recombinant H5N1 vaccine virus ), May 2009, World Health Organization.〕 Britain, France, Canada, and Australia, for use in an emergency.〔(UK to buy bird flu vaccine stock ), BBC, February 4, 2009.〕 Research has shown that a highly contagious strain of H5N1, one that might allow airborne transmission between mammals, can be reached in only a few mutations, raising concerns about a pandemic and bioterrorism.〔(Fears of bioterrorism or an accidental release | Harvard Gazette )〕 ==Overview== HPAI A(H5N1) is considered an avian disease, although there is some evidence of limited human-to-human transmission of the virus. A risk factor for contracting the virus is handling of infected poultry, but transmission of the virus from infected birds to humans has been characterized as inefficient. Still, around 60% of humans known to have been infected with the current Asian strain of HPAI A(H5N1) have died from it, and H5N1 may mutate or reassort into a strain capable of efficient human-to-human transmission. In 2003, world-renowned virologist Robert G. Webster published an article titled "The world is teetering on the edge of a pandemic that could kill a large fraction of the human population" in ''American Scientist''. He called for adequate resources to fight what he sees as a major world threat to possibly billions of lives. On September 29, 2005, David Nabarro, the newly appointed Senior United Nations System Coordinator for Avian and Human Influenza, warned the world that an outbreak of avian influenza could kill anywhere between 5 million and 150 million people. Experts have identified key events (creating new clades, infecting new species, spreading to new areas) marking the progression of an avian flu virus towards becoming pandemic, and many of those key events have occurred more rapidly than expected. Due to the high lethality and virulence of HPAI A(H5N1), its endemic presence, its increasingly large host reservoir, and its significant ongoing mutations, the H5N1 virus has recently been regarded to be the world's largest current pandemic threat, and billions of dollars are being spent researching H5N1 and preparing for a potential influenza pandemic. At least 12 companies and 17 governments are developing prepandemic influenza vaccines in 28 different clinical trials that, if successful, could turn a deadly pandemic infection into a nondeadly one. Full-scale production of a vaccine that could prevent any illness at all from the strain would require at least three months after the virus's emergence to begin, but it is hoped that vaccine production could increase until one billion doses were produced by one year after the initial identification of the virus.〔(Science and Development Network ) article ''Pandemic flu: fighting an enemy that is yet to exist'' published May 3, 2006.〕 H5N1 may cause more than one influenza pandemic, as it is expected to continue mutating in birds regardless of whether humans develop herd immunity to a future pandemic strain. Influenza pandemics from its genetic offspring may include influenza A virus subtypes other than H5N1.〔(CDC ) ARTICLE ''1918 Influenza: the Mother of All Pandemics'' by Jeffery K. Taubenberger published January 2006〕 While genetic analysis of the H5N1 virus shows that influenza pandemics from its genetic offspring can easily be far more lethal than the Spanish flu pandemic,〔(Informaworld ) article ''Why is the world so poorly prepared for a pandemic of hypervirulent avian influenza?'' published December 2006〕 planning for a future influenza pandemic is based on what can be done and there is no higher Pandemic Severity Index level than a Category 5 pandemic which, roughly speaking, is any pandemic as bad as the Spanish flu or worse; and for which ''all'' intervention measures are to be used. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Influenza A virus subtype H5N1」の詳細全文を読む スポンサード リンク
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