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Avian influenza — known informally as avian flu or bird flu — refers to "influenza caused by viruses adapted to birds."[1][2][3][4][5][6][7] Of the greatest concern is highly pathogenic avian influenza (HPAI).
"Bird flu" is a phrase similar to "swine flu," "dog flu," "horse flu," or "human flu" in that it refers to an illness caused by any of many different strains of influenza viruses that have adapted to a specific host. All known viruses that cause influenza in birds belong to the species influenza A virus. All subtypes (but not all strains of all subtypes) of influenza A virus are adapted to birds, which is why for many purposes avian flu virus is the influenza A virus. (Note, however, that the "A" does not stand for "avian").
Adaptation is not exclusive. Being adapted towards a particular species does not preclude adaptations, or partial adaptations, towards infecting different species. In this way, strains of influenza viruses are adapted to multiple species, though may be preferential towards a particular host. For example, viruses responsible for influenza pandemics are adapted to both humans and birds. Recent influenza research into the genes of the Spanish flu virus shows it to have genes adapted to both birds and humans, with more of its genes from birds than less deadly later pandemic strains.
While its most highly pathogenic strain (H5N1) had been spreading throughout Asia since 2003, avian influenza reached Europe in 2005, and the Middle East, as well as Africa, the following year.[8]
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Genetic factors in distinguishing between "human flu viruses" and "avian flu viruses" include:
There are many subtypes of avian influenza viruses, but only some strains of four subtypes have been highly pathogenic in humans. These are types H5N1, H7N3, H7N7 and H9N2.[9]
Most human contractions of the avian flu are a result of either handling dead infected birds or from contact with infected fluids. While most wild birds mainly have only a mild form of the H5N1 strain, once domesticated birds such as chickens or turkeys are infected, it could become much more deadly because the birds are often within close contact of one another. There is currently a large threat of this in Asia with infected poultry due to low hygiene conditions and close quarters . Although it is easy for humans to become infected from birds, it's much more difficult to do so from human to human without close and lasting contact.
Spreading of H5N1 from Asia to Europe is much more likely the cause of both legal and illegal poultry trades than dispersing through wild bird migrations, being that in recent studies, there were no secondary rises in infection in Asia when wild birds migrate south again from their breeding grounds. Instead, the infection patterns followed transportation such as railroads, roads, and country borders, suggesting poultry trade as being much more likely. While there have been strains of avian flu to exist in the United States, such as Texas in 2004, they have been extinguished and have not been known to infect humans.
Examples of avian influenza A virus strains:[10]
| HA subtype designation |
NA subtype designation |
Avian influenza A viruses |
|---|---|---|
| H1 | N1 | A/duck/Alberta/35/76(H1N1) |
| H1 | N8 | A/duck/Alberta/97/77(H1N8) |
| H2 | N9 | A/duck/Germany/1/72(H2N9) |
| H3 | N8 | A/duck/Ukraine/63(H3N8) |
| H3 | N8 | A/duck/England/62(H3N8) |
| H3 | N2 | A/turkey/England/69(H3N2) |
| H4 | N6 | A/duck/Czechoslovakia/56(H4N6) |
| H4 | N3 | A/duck/Alberta/300/77(H4N3) |
| H5 | N3 | A/tern/South Africa/300/77(H4N3) |
| H5 | N4 | A/jyotichinara/Ethiopia/300/77(H6N6) |
| H5 | N9 | A/turkey/Ontario/7732/66(H5N9) |
| H5 | N1 | A/chick/Scotland/59(H5N1) |
| H6 | N2 | A/turkey/Massachusetts/3740/65(H6N2) |
| H6 | N8 | A/turkey/Canada/63(H6N8) |
| H6 | N5 | A/shearwater/Australia/72(H6N5) |
| H6 | N1 | A/duck/Germany/1868/68(H6N1) |
| H7 | N7 | A/fowl plague virus/Dutch/27(H7N7) |
| H7 | N1 | A/chick/Brescia/1902(H7N1) |
| H7 | N3 | A/turkey/England/639H7N3) |
| H7 | N1 | A/fowl plague virus/Rostock/34(H7N1) |
| H8 | N4 | A/turkey/Ontario/6118/68(H8N4) |
| H9 | N2 | A/turkey/Wisconsin/1/66(H9N2) |
| H9 | N6 | A/duck/Hong Kong/147/77(H9N6) |
| H9 | N6 | A/duck/Hong Kong/147/77(H9N6) |
| H9 | N8 | A/manishsurpur/Malawi/149/77(H9N8) |
| H9 | N7 | A/turkey/Scotland/70(H9N7) |
| H10 | N8 | A/quail/Italy/1117/65(H10N8) |
| H11 | N6 | A/duck/England/56(H11N6) |
| H11 | N9 | A/duck/Memphis/546/74(H11N9) |
| H12 | N5 | A/duck/Alberta/60/76/(H12N5) |
| H13 | N6 | A/gull/Maryland/704/77(H13N6) |
| H14 | N4 | A/duck/Gurjev/263/83(H14N4) |
| H15 | N9 | A/shearwater/Australia/2576/83(H15N9) |
Pandemic flu viruses have some avian flu virus genes and usually some human flu virus genes. Both the H2N2 and H3N2 pandemic strains contained genes from avian influenza viruses. The new subtypes arose in pigs coinfected with avian and human viruses, and were soon transferred to humans. Swine were considered the original "intermediate host" for influenza, because they supported reassortment of divergent subtypes. However, other hosts appear capable of similar coinfection (e.g., many poultry species), and direct transmission of avian viruses to humans is possible.[11] The Spanish flu virus strain may have been transmitted directly from birds to humans.[12] In spite of their pandemic connection, avian influenza viruses are noninfectious for most species. When they are infectious, they are usually asymptomatic, so the carrier does not have any disease from it. Thus, while infected with an avian flu virus, the animal does not have a "flu". Typically, when illness (called "flu") from an avian flu virus does occur, it is the result of an avian flu virus strain adapted to one species spreading to another species (usually from one bird species to another bird species). So far as is known, the most common result of this is an illness so minor as to be not worth noticing (and thus little studied). But with the domestication of chickens and turkeys, humans have created species subtypes (domesticated poultry) that can catch an avian flu virus adapted to waterfowl and have it rapidly mutate into a form that kills over 90% of an entire flock in days, can spread to other flocks and kill 90% of them, and can only be stopped by killing every domestic bird in the area. Until H5N1 infected humans in the 1990s, this was the only reason avian flu was considered important. Since then, avian flu viruses have been intensively studied; resulting in changes in what is believed about flu pandemics, changes in poultry farming, changes in flu vaccination research, and changes in flu pandemic planning.
H5N1 has evolved into a flu virus strain that infects more species than any previously known strain, is deadlier than any previously known strain, and continues to evolve, becoming both more widespread and more deadly. This caused Robert G. Webster, a leading expert on avian flu, to publish 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.[13] Since the article was written, the world community has spent billions of dollars fighting this threat with limited success.
Vaccines have been formulated against several of the avian H5N1 influenza varieties. Vaccination of poultry against the ongoing H5N1 epizootic is widespread in certain countries. Some vaccines also exist for use in humans, and others are in testing, but none have been made available to civilian populations, nor are produced in quantities sufficient to protect more than a tiny fraction of the Earth's population in the event of an H5N1 pandemic outbreak. The World Health Organization has compiled a list of known clinical trials of pandemic influenza prototype vaccines, including those against H5N1.
The highly pathogenic influenza A virus subtype H5N1 is an emerging avian influenza virus that has been causing global concern as a potential pandemic threat. It is often referred to simply as "bird flu" or "avian influenza", even though it is only one subtype of avian influenza-causing virus.
H5N1 has killed millions of poultry in a growing number of countries throughout Asia, Europe and Africa. Health experts are concerned that the coexistence of human flu viruses and avian flu viruses (especially H5N1) will provide an opportunity for genetic material to be exchanged between species-specific viruses, possibly creating a new virulent influenza strain that is easily transmissible and lethal to humans.[14]
Since the first H5N1 outbreak occurred in 1987, there has been an increasing number of HPAI H5N1 bird-to-human transmissions, leading to clinically severe and fatal human infections. Because a significant species barrier exists between birds and humans, though, the virus does not easily cross over to humans, though some cases of infection are being researched to discern whether human to human transmission is occurring.[11] More research is necessary to understand the pathogenesis and epidemiology of the H5N1 virus in humans. Exposure routes and other disease transmission characteristics, such as genetic and immunological factors that may increase the likelihood of infection, are not clearly understood.[15]
On January 18, 2009, a 27-year-old woman from eastern China died of bird flu, Chinese authorities said, making her the second person to die from the deadly virus at that time. Two tests on the woman were positive for H5N1 avian influenza, said the ministry, which did not say how she might have contracted the virus.[16]
Although millions of birds have become infected with the virus since its discovery, 306 humans have died from the H5N1 in twelve countries according to WHO data as of February 2, 2011.[17]
The avian flu claimed at least 300 humans in Azerbaijan, Cambodia, China, Egypt, Indonesia, Iraq, Laos, Nigeria, Pakistan, Thailand, Turkey, and Vietnam. Epidemiologists are afraid the next time such a virus mutates, it could pass from human to human; however, the current A/H5N1 virus does not transmit easily from human to human. If this form of transmission occurs, another pandemic could result. Thus, disease-control centers around the world are making avian flu a top priority. These organizations encourage poultry-related operations to develop a preemptive plan to prevent the spread of H5N1 and its potentially pandemic strains. The recommended plans center on providing protective clothing for workers and isolating flocks to prevent the spread of the virus.[18]
The Thailand outbreak of avian flu caused massive economic losses, especially among poultry workers. Infected birds were culled and sacrificed. The public lost confidence with the poultry products, thus decreasing the consumption of chicken products. This also elicited a ban from importing countries. There were, however, factors which aggravated the spread of the virus, including bird migration, cool temperature (increases virus survival) and several festivals at that time.[19]
Several domestic species have been infected with and shown symptoms of H5N1 viral infection, including cats, dogs, ferrets, pigs,and birds.[20]
Attempts are made in the United States to minimize the presence of HPAI in poultry thorough routine surveillance of poultry flocks in commercial poultry operations. Detection of a HPAI virus may result in immediate culling of the flock. Less pathogenic viruses are controlled by vaccination, which is done primarily in turkey flocks (ATCvet codes: QI01AA23 for the inactivated fowl vaccine, QI01CL01 for the inactivated turkey combination vaccine).[21]
Orent, Wendy. "The Science of Avian Flu, Answers to Nine Frequently Asked Questions." Discover. February 2006. 59-61.
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