For the current concern about the transmission of an avian flu to humans see Transmission and infection of H5N1.
For the H5N1 subtype generating the concern see H5N1.
For its current status see Global spread of H5N1.
Avian flu (also “bird flu”, “avian influenza”, “bird influenza”), means “flu from viruses adapted to birds”, but is sometimes mistakenly used to refer to both other flu subsets (such as H5N1 flu) or the viruses that cause them (such as H5N1).[1][2][3][4][5][6][7]
“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 flu viruses such that the strain in question has adapted to the host. “Avian flu” differs in being named after an entire vertebrate class with 8,800–10,200 species. All known avian flu viruses belong to the species of virus called 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 sometimes partial or multiple so a flu virus strain can be partially adapted to a species or adapted to more than one species. Flu pandemic viruses are human adapted and also bird adapted. Being adapted to one species does not mean another species can not catch it; nor does it mean it can not adapt to another species.
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 lysine.
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.
The HA changes have not yet occurred in any sequenced H5N1 virus - even ones from humans that died from it and the PB2 changes don’t stop it from being a flu virus adapted to birds (the definition of “avian flu virus”).
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. The Spanish flu virus strain may have been transmitted directly from birds to humans.[8]
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 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.[9] Since the article was written, the world community has spent billions of dollars fighting this threat with limited success.
[edit] Illustrative examples of correct usage
H5N1
Influenza A virus subtype H5N1
Genetic structure
Infection
Human mortality
Global spread
in 2006
Social impact
Pandemic
WHO pandemic phases
Low risk
New virus
Self limiting
Person to person
Epidemic exists
Pandemic exists
Swans can carry highly pathogenic avian H5N1 and other avian flu virusesIn technical contexts, correct usage of terms is necessary because precise distinctions are the essence of the communication.
“Avian influenza strains are those well adapted to birds”[1]
“An outbreak of influenza A (H5N1), also known as ‘avian flu’ or ‘bird flu,’ has been reported in several countries throughout Asia.”[10]
“Avian influenza virus usually refers to influenza A viruses found chiefly in birds, but infections can occur in humans.”[11]
“Of the few avian influenza viruses that have crossed the species barrier to infect humans, H5N1 has caused the largest number of cases of severe disease and death in humans. Unlike normal seasonal influenza, where infection causes only mild respiratory symptoms in most people, the disease caused by H5N1 follows an unusually aggressive clinical course, with rapid deterioration and high fatality.” Seasonal influenza is human flu.[12]
“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.”[13]
Sometimes a virus contains both avian adapted genes and human adapted genes. Both the H2N2 and H3N2 pandemic strains contained avian flu virus RNA segments. “While the pandemic human influenza viruses of 1957 (H2N2) and 1968 (H3N2) clearly arose through reassortment between human and avian viruses, the influenza virus causing the ‘Spanish flu’ in 1918 appears to be entirely derived from an avian source (Belshe 2005).”[2]
[edit] Illustrative examples of imprecise usage
In nontechnical contexts, imprecise usage of terms is typical when discussing complex things.
“A 1,000 square mile quarantine zone to combat an outbreak of bird flu was lifted in Scotland today - despite the spread of a similar disease south of the border.” Here “bird flu” is used to mean “Asian lineage HPAI A(H5N1) flu” (which is a bird flu) and contrasted with flu from an avian adapted strain of H7N3 (which is also a bird flu).[14]
[edit] See also
Timeline data on avian flu
Timeline data on avian flu can be found at the article on the causative agent species called Influenza A virus.
Timeline data on the global spread of the strain that is the current pandemic concern (called Asian lineage HPAI A(H5N1)) can be found at Global spread of H5N1.
Timeline data on creation of a flu vaccine for H5N1 can be found at H5N1 clinical trials.
Subtypes of the causative agent species of avian flu include
H1N1
H1N2
H2N2
H3N2
H3N8
H5N1
H5N2
H5N3
H5N8
H5N9
H5N8
H7N1
H7N2
H7N3
H7N4
H7N7
H9N2
H10N7
Information concerning research about it can be found at
Center for Biologics Evaluation and Research
OIE/FAO Network of Expertise on Avian Influenza
Flu research
Flu vaccine
H5N1 genetic structure
ICEID
Influenza Genome Sequencing Project
Influenza pandemic
Cytokine storm
International Partnership on Avian and Pandemic Influenza
National Influenza Centers
Pandemic Preparedness and Response Act
Reporting disease cases
Transmission and infection of H5N1