Avian influenza

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For the H5N1 subtype of Avian influenza see Influenza A virus subtype H5N1.

Avian influenza, sometimes avian flu, and commonly bird flu, refers to "influenza caused by viruses adapted to birds."[1][2][3][4][5][6][7][clarification needed] 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 that the "A" does not stand for "avian").

Adaptation is non-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.

Nomenclature and taxonomy


Genetic factors in distinguishing between "human flu viruses" and "avian flu viruses" include:

PB2: (RNA polymerase): Amino acid (or residue) position 627 in the PB2 protein encoded by the PB2 RNA gene. Until H5N1, all known avian influenza viruses had a Glu at position 627, while all human influenza viruses had a Lys.
HA: (hemagglutinin): Avian influenza HA bind alpha 2-3 sialic acid receptors while human influenza HA bind alpha 2-6 sialic acid receptors. Swine influenza viruses have the ability to bind both types of sialic acid receptors. Hemagglutinin is the major antigen of the virus against which neutralizing antibodies are produced and influenza virus epidemics are associated with changes in its antigenic structure. This was originally derived from pigs, and should technically be referred to as "Pig Flu" (see ref. 7a)


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.[8]

Examples of avian influenza A virus strains:[9]

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)
H10 N7 A/chick/Germany/N/49(H10N7)
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)

Influenza pandemic

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.[10] The Spanish flu virus strain may have been transmitted directly from birds to humans.[11] 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 doesn't 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 in days over 90% of an entire flock and 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 flu virus strain, is deadlier than any previously known flu virus strain, and continues to evolve becoming both more widespread and more deadly causing 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.[12] 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 produced in quantities sufficient to protect more than a tiny fraction of the Earth's population in the event that an H5N1 pandemic breaks out. 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 virus 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 co-existence 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.[13]

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. However, because there is a significant species barrier that exists between birds and humans, 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.[10] 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.[14]

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[15].

Although millions of birds have become infected with the virus since its discovery, 262 humans have died from the H5N1 in twelve countries according to WHO data as of August 31, 2009.[16]

The avian flu claimed at least 200 humans in Indonesia, Vietnam, Laos, Romania, China, Taiwan, Turkey and Russia. Epidemiologists are afraid that 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.[17]

The Thailand outbreak of avian flu causes massive economic losses especially among poultry workers. Infected birds were culled and sacrificed. The public loss its confidence with the poultry products and thus decreasing the consumption of chicken and its products. This also elicited a ban from importing countries. There were however, factors which aggravated the spread of the virus which includes bird migration, cool temperature (increases virus survival) and several festivals at that time.[18]

In domestic animals

Several domestic species have been infected with and shown symptoms of H5N1 viral infection including cats, dogs, ferrets, pigs,and birds.


Attempts are made in the United States to minimize the presence of highly pathogenic avian influenza (HPAI) in poultry in thorough routine surveillance of poultry flocks in commercial poultry operations. Detection of a HPAI virus may result in immediate death 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).[19]

See also


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External links

United Nations System Coordinator for Avian and Human Influenza (UNSIC).
World Health Organisation (WHO)
Food and Agriculture Organization of the UN (FAO)
World Organisation for Animal Health (OIE)
United States

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  1. "Avian influenza strains are those well adapted to birds"EUROPEAN CENTRE FOR DISEASE PREVENTION AND CONTROL.
  2. Chapter Two : Avian Influenza by Timm C. Harder and Ortrud Werner in Influenza Report 2006
  3. Large-scale sequencing of human influenza reveals the dynamic nature of viral genome evolution Nature magazine presents a summary of what has been discovered in the Influenza Genome Sequencing Project.
  4. Avian Influenza A (H5N1) Infection in Humans by The Writing Committee of the World Health Organization (WHO) Consultation on Human Influenza A/H5 in the September 29, 2005 New England Journal of Medicine
  5. The Threat of Pandemic Influenza: Are We Ready? Workshop Summary (2005) Full text of online book by INSTITUTE OF MEDICINE OF THE NATIONAL ACADEMIES
  6. [1] CDC has a phylogenetic tree showing the relationship between dozens of highly pathogenic varieties of the Z genotype of avian flu virus H5N1 and ancestral strains.
  7. Evolutionary characterization of the six internal genes of H5N1 human influenza A virus
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  10. 10.0 10.1 Blanchard, Ben. "China says son likely infected father with bird flu." Reuters 10 Jen 2008 10 Jen 2008 <http://www.reuters.com/article/healthNews/idUSPEK27288320080110>.
  11. Chapter Two : Avian Influenza by Timm C. Harder and Ortrud Werner
  12. Webster, R. G. and Walker, E. J. (2003). "The world is teetering on the edge of a pandemic that could kill a large fraction of the human population" ([dead link]). American Scientist. 91 (2): 122. doi:10.1511/2003.2.122. 
  13. Food Safety Research Information Office. "A Focus on Avian Influenza". Created May 2006, Updated November 2007.
  14. World Health Organization. (2006). Avian influenza (" bird flu") – The Disease in Humans. Retrieved April 6, 2006.
  15. "Chinese say bird flu claims second victim". CNN. Retrieved April 26, 2010. 
  16. World Health Organization. Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO. August 31, 2009. http://www.who.int/csr/disease/avian_influenza/country/cases_table_2009_08_31/en/index.html.
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