H5N1
Influenza A virus subtype H5N1, also known as A(H5N1) or H5N1, is a subtype of the Influenza A virus that is capable of causing illness in many species, including humans. An avian-adapted, highly pathogenic strain of H5N1 (called HPAI A(H5N1), for “highly pathogenic avian influenza virus of type A of subtype H5N1″) is the causative agent of H5N1 flu, commonly known as “avian influenza” or simply “bird flu”, and is endemic in many bird populations, especially in Southeast Asia.
An Asian lineage strain of HPAI A(H5N1) is spreading globally. It is epizootic (an epidemic in nonhumans) and panzootic (a disease 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 other birds in an attempt to control its spread. Most references in the media to “bird flu” and most references to H5N1 are about this specific strain.
HPAI A(H5N1) is an avian disease and there is no evidence suggesting either efficient human-to-human transmission of HPAI A(H5N1) or of airborne transmission of HPAI A(H5N1) to humans. Those infected with H5N1 have had, in almost all cases, extensive physical contact with infected birds. However, H5N1 has the potential to mutate or reassort into a strain capable of efficient human-to-human transmission. Due to the high lethality and virulence of human HPAI A(H5N1) infection to date, its endemic presence, its large and increasing biological host reservoir, and its significant ongoing mutations, the H5N1 virus is today the world’s major current pandemic threat, and billions of dollars are being raised and spent researching H5N1 and preparing for a potential influenza pandemic.
In 2003, world-renowned virologist Robert 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 calling for adequate resources to fight what he saw 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 a pandemic flu virus, and many of those key events have occurred more rapidly than expected.
WHO pandemic phases
Low risk
New virus
Self limiting
Person to person
Epidemic exists
Pandemic exists
Symptoms in humans
Further information: Pneumonia
Avian influenza HA bind alpha 2-3 sialic acid receptors while human influenza HA bind alpha 2-6 sialic acid receptors. Usually other differences also exist. There is as yet no human form of H5N1, so all humans who have caught it so far have caught avian H5N1.
Humans who catch a humanized Influenza A virus (in other words a human flu virus of type A) usually have symptoms that include fever, cough, sore throat, muscle aches, conjunctivitis and, in severe cases, severe breathing problems and pneumonia that may be fatal. The severity of the infection will depend to a large part on the state of the infected person’s immune system and if the victim has been exposed to the strain before, and is therefore partially immune. No one knows if these or other symptoms will be the symptoms of a humanized H5N1 flu.
Highly pathogenic H5N1 avian flu in a human is far worse, killing 50% of humans that catch it. In one case, a boy with H5N1 experienced diarrhea followed rapidly by a coma without developing respiratory or flu-like symptoms.[23] There have been studies of the levels of cytokines in humans infected by the H5N1 flu virus. Of particular concern is elevated levels of tumor necrosis factor alpha (TNFα), a protein that is associated with tissue destruction at sites of infection and increased production of other cytokines. Flu virus-induced increases in the level of cytokines is also associated with flu symptoms including fever, chills, vomiting and headache. Tissue damage associated with pathogenic flu virus infection can ultimately result in death. The inflammatory cascade triggered by H5N1 has been called a ‘cytokine storm’ by some, because of what seems to be a positive feedback process of damage to the body resulting from immune system stimulation. H5N1 type flu virus induces higher levels of cytokines than the more common flu virus types such as H1N1.
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