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Avian influenza

Avlan influenza
From Wikipedia, the free encyclopedia.
This article is about the genus of influenza virus that is carried by birds. For the strain that is being widely reported, see H5N1.
Influenza A virus, the virus that causes Avian flu. Transmission electron micrograph of negatively stained virus particles in late passage. (Source: Dr. Erskine Palmer, Centers for Disease Control and Prevention Public Health Image Library).
Avian influenza (also known as bird flu or avian flu) is a type of influenza virus that is hosted by birds, but may infect several species of mammals. It was first identified in Italy in the early 1900s and is now known to exist worldwide. Avian influenza viruses compose the Influenzavirus A genus of the Orthomyxoviridae family and are negative-stranded, segmented RNA viruses. A strain of the H5N1-type of avian influenza that emerged in 1997 has been identified as the most likely source of a future influenza pandemic.

Contents
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1 Infection
2 Subtypes pathogenic to humans
2.1 H1N1
2.2 H5N1
2.3 Other strains
3 Prevention and treatment
4 Potential pandemic
5 See also
6 External links
6.1 General
6.2 News
6.3 Preparing
6.4 Blogs
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Infection
Strains of avian influenza may infect various type of animals, including birds, pigs, horses, seals, whales and humans. However, wild fowl act as natural asymptomatic carriers, spreading it to more susceptible domestic stocks. Avian influenza spreads in the air and in manure. It can also be transmitted by contaminated feed, water, equipment and clothing; however, there is no evidence that the virus can survive in well cooked meat. The incubation period is 3 to 5 days. Symptoms in animals vary, but virulent strains can cause death within a few days.
In humans, avian flu causes similar symptoms to other types of flu. [1] These 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. In one case, a boy with H5N1 experienced diarrhea followed rapidly by a coma without developing respiratory or flu-like symptoms, suggesting non-standard symptoms. [2]
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Subtypes pathogenic to humans
All avian influenza (AI) viruses are type A influenza in the virus family of Orthomyxoviridae and all known subtypes of influenza A virus can infect birds. Influenza type A is subdivided into subtypes based on hemagglutinin (H) and neuraminidase (N) protein spikes from the central virus core. There are 16 H types, each with up to 9 N subtypes, yielding a potential for 144 different H and N combinations. In addition, all AI viruses fall into one of 2 pathotypes: low (LPAI) and high (HPAI) pathogenicity, based on their virulence in poultry populations.
It is feared that if the avian influenza virus undergoes antigenic shift to the point where it can cross the species barrier (e.g., from birds to humans), the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global pandemic similar to the Spanish Flu that killed up to 50 million people in 1918.
However, there are substantial genetic differences between the subtypes that typically infect both people and birds. Within subtypes of avian influenza viruses there also are different strains. Avian influenza H5 and H7 viruses can be found in both "low pathogenic” or “high pathogenic” forms depending on genetic features and the severity of the illness they cause in poultry; influenza H9 virus has been identified only in a “low pathogenic” form.
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H1N1
Main article: H1N1
H1N1 was the first identified strain of Type A influenza. In early October 2005, researchers announced that they had successfully reconstructed the Spanish Flu virus. The gene sequence indicates that the 1918 epidemic was caused by H1N1, normally considered a strain of swine flu (which itself falls under avian influenza) that in this case passed directly from birds to humans. The reconstructed virus is very different from normal human viruses in that it infects lung cells which would normally be impervious to the virus [3].
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H5N1
Main article: H5N1
H5N1 is a highly pathogenic form of avian influenza. Since 1997, outbreaks of H5N1 have caused the death or culling of tens of millions of birds. Over 100 people have been infected by H5N1, with a mortality rate of over 50%. H5N1 has been the focus of much concern amid warnings that the H5N1 strain will likely evolve into a form that causes a global human pandemic with a very high mortality rate.
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Other strains
Since 1997 the H5N1, H7N2, H7N3, H7N7, and H9N2 have been confirmed in humans.
H2N2
Responsible for the Asian Flu pandemic of 1957 and 1958 that killed at least a million people worldwide.
H3N2
Evolved from H2N2 by antigenic shift and caused the Hong Kong Flu pandemic of 1968 and 1969 that killed up to 750,000. This was the least deadly pandemic of the twentieth century.
H7N2
Following an outbreak of H7N2 among poultry in 2002, 44 people were found infected in Virginia, United States.
H7N3
In North America, the presence of avian influenza strain H7N3 was confirmed at several poultry farms in British Columbia in February 2004. As of April 2004, 18 farms had been quarantined to halt the spread of the virus. Two cases of humans with avian influenza have been confirmed in that region.
H7N7
In 2003 in Netherlands 89 people were confirmed to have H7N7 influenza virus infection following an outbreak in poultry on several farms. One death has been recorded.
H9N2
The virus type has been documented only in low pathogenic form. Three infections in humans (China and Hong Kong) have been confirmed, all three patients recovered. In October 2005 an outbreak has been detected in the province of Tolima, central Colombia. No human infections have been reported.[4]
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Prevention and treatment
Although avian influenza in humans can be detected with standard influenza tests, these tests have not always proved reliable. In March 2005, the World Health Organization announced that seven people from Vietnam who initially tested negative for bird flu were later found to have carried the virus. All seven have since recovered from the disease. Currently (6/05) the most reliable test (microneutralization) requires use of the live virus to interact with antibodies from the patient's blood; because live virus is required, for safety reasons the test can only be done in a level three laboratory [5].
Antiviral drugs are sometimes effective in both preventing and treating the disease, but no virus has ever been really cured in medical history. Vaccines, however, take at least four months to produce and must be prepared for each subtype.
Further, as a result of widespread use of the antiviral drug amantadine as a preventive or treatment for chickens in China starting in the late 1990s, some strains of the avian flu virus in Asia have developed drug resistance against amantadine [6]. Chickens in China have received an estimated 2.6 billion doses of amantadine since early 2004. This use of amantadine for poultry goes against international livestock regulations, but China kept it secret until recently, in a manner reminiscent of the secrecy around the early spread of SARS.
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Potential pandemic
Main article: influenza pandemic
The World Health Organization (WHO) has warned of a substantial risk of an influenza epidemic in the near future, most probably of the H5N1 type of avian influenza. In response, countries have begun planning in anticipation of an outbreak. While short-term strategies to deal with an outbreak focus on limiting travel and culling and vaccinating poultry, long-term strategies require substantial changes in the lifestyles of the most at-risk populations.
The WHO divides a pandemic into six phases, ranging from minimal risk of an outbreak to full scale pandemic. Most health authorities categorize the situation as of 2005 at Phase 3, by which is meant that human infections of a new sub-type has occurred but there is little evidence of sustained human-to-human transmission.
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See also

External links

General

 

News

Preparing

 

Blogs


 

 

 

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