Re: Times on line and bird flu

Jim Webster wrote:

this link might also be useful for background
http://www.cdc.gov/flu/avian/gen-info/flu-viruses.htm

Avian Flu: The Virus & Its Spread
Types, Subtypes, and Strains
There are three types of influenza viruses: A, B, and C. Only influenza A
viruses are further classified by subtype on the basis of the two main
surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). Influenza A
subtypes and B viruses are further classified by strains.

Human Influenza Viruses and Avian Influenza A Viruses
Humans can be infected with influenza types A, B, and C viruses. Subtypes of
influenza A that are currently circulating among people worldwide include
H1N1, H1N2, and H3N2 viruses.

Wild birds are the natural host for all known subtypes of influenza A
viruses. Typically, wild birds do not become sick when they are infected
with avian influenza A viruses. However, domestic poultry, such as turkeys
and chickens, can become very sick and die from avian influenza, and some
avian influenza A viruses also can cause serious disease and death in wild
birds.

Low Pathogenic versus Highly Pathogenic Avian Influenza A Viruses
Avian influenza A virus strains are further classified as low pathogenic
(LPAI) or highly pathogenic (HPAI) on the basis of specific molecular
genetic and pathogenesis criteria that require specific testing. Most avian
influenza A viruses are LPAI viruses that are usually associated with mild
disease in poultry. In contrast, HPAI viruses can cause severe illness and
high mortality in poultry. More recently, some HPAI viruses (e.g., H5N1)
have been found to cause no illness in some poultry, such as ducks. LPAI
viruses have the potential to evolve into HPAI viruses and this has been
documented in some poultry outbreaks. Avian influenza A viruses of the
subtypes H5 and H7,including H5N1, H7N7, and H7N3 viruses, have been
associated with HPAI, and human infection with these viruses have ranged
from mild (H7N3, H7N7) to severe and fatal disease (H7N7, H5N1). Human
illness due to infection with LPAI viruses has been documented, including
very mild symptoms (e.g., conjunctivitis) to influenza-like illness.
Examples of LPAI viruses that have infected humans include H7N7, H9N2, and
H7N2.

In general, direct human infection with avian influenza viruses occurs very
infrequently, and has been associated with direct contact (e.g., touching)
infected sick or dead infected birds (domestic poultry).
How Influenza Viruses Change: Drift and Shift
Influenza viruses are dynamic and are continuously evolving. Influenza
viruses can change in two different ways: antigenic drift and antigenic
shift. Influenza viruses are changing by antigenic drift all the time, but
antigenic shift happens only occasionally. Influenza type A viruses undergo
both kinds of changes; influenza type B viruses change only by the more
gradual process of antigenic drift.

Antigenic drift refers to small, gradual changes that occur through point
mutations in the two genes that contain the genetic material to produce the
main surface proteins, hemagglutinin, and neuraminidase. These point
mutations occur unpredictably and result in minor changes to these surface
proteins. Antigenic drift produces new virus strains that may not be
recognized by antibodies to earlier influenza strains. This process works as
follows: a person infected with a particular influenza virus strain develops
antibody against that strain. As newer virus strains appear, the antibodies
against the older strains might not recognize the "newer" virus, and
infection with a new strain can occur. This is one of the main reasons why
people can become infected with influenza viruses more than one time and why
global surveillance is critical in order to monitor the evolution of human
influenza virus stains for selection of which strains should be included in
the annual production of influenza vaccine. In most years, one or two of the
three virus strains in the influenza vaccine are updated to keep up with the
changes in the circulating influenza viruses. For this reason, people who
want to be immunized against influenza need to be vaccinated every year.

Antigenic shift refers to an abrupt, major change to produce a novel
influenza A virus subtype in humans that was not currently circulating among
people (see more information below under Influenza Type A and Its Subtypes).
Antigenic shift can occur either through direct animal (poultry)-to-human
transmission or through mixing of human influenza A and animal influenza A
virus genes to create a new human influenza A subtype virus through a
process called genetic reassortment. Antigenic shift results in a new human
influenza A subtype. A global influenza pandemic (worldwide spread) may
occur if three conditions are met:

a.. A new subtype of influenza A virus is introduced into the human
population.
b.. The virus causes serious illness in humans.
c.. The virus can spread easily from person to person in a sustained
manner.
Types, Subtypes, and Strains
Influenza Type A and Its Subtypes
Influenza type A viruses can infect people, birds, pigs, horses, and other
animals, but wild birds are the natural hosts for these viruses. Influenza
type A viruses are divided into subtypes and named on the basis of two
proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase
(NA). For example, an "H7N2 virus" designates an influenza A subtype that
has an HA 7 protein and an NA 2 protein. Similarly an "H5N1" virus has an HA
5 protein and an NA 1 protein. There are 16 known HA subtypes and 9 known NA
subtypes. Many different combinations of HA and NA proteins are possible.
Only some influenza A subtypes (i.e., H1N1, H1N2, and H3N2) are currently in
general circulation among people. Other subtypes are found most commonly in
other animal species. For example, H7N7 and H3N8 viruses cause illness in
horses, and H3N8 also has recently been shown to cause illness in dogs.

Only influenza A viruses infect birds, and all known subtypes of influenza A
viruses can infect birds. However, there are substantial genetic differences
between the influenza A subtypes that typically infect birds and those that
infect both people and birds. Three prominent subtypes of the avian
influenza A viruses that are known to infect both birds and people are:

Influenza A H5
Nine potential subtypes of H5 are known. H5 infections, such as HPAI H5N1
viruses currently circulating in Asia and Europe, have been documented among
humans and sometimes cause severe illness or death.

Influenza A H7
Nine potential subtypes of H7 are known. H7 infection in humans is rare but
can occur among persons who have direct contact with infected birds.
Symptoms may include conjunctivitis and/or upper respiratory symptoms. H7
viruses have been associated with both LPAI (e.g., H7N2, H7N7) and HPAI
(e.g., H7N3, H7N7), and have caused mild to severe and fatal illness in
humans.

Influenza A H9
Nine potential subtypes of H9 are known; influenza A H9 has rarely been
reported to infect humans. However, this subtype has been documented only in
a low pathogenic form.

Influenza Type B
Influenza B viruses are usually found only in humans. Unlike influenza A
viruses, these viruses are not classified according to subtype. Influenza B
viruses can cause morbidity and mortality among humans, but in general are
associated with less severe epidemics than influenza A viruses. Although
influenza type B viruses can cause human epidemics, they have not caused
pandemics.

Influenza Type C
Influenza type C viruses cause mild illness in humans and do not cause
epidemics or pandemics. These viruses are not classified according to
subtype.

Strains
Influenza B viruses and subtypes of influenza A virus are further
characterized into strains. There are many different strains of influenza B
viruses and of influenza A subtypes. New strains of influenza viruses appear
and replace older strains. This process occurs through antigenic drift. When
a new strain of human influenza virus emerges, antibody protection that may
have developed after infection or vaccination with an older strain may not
provide protection against the new strain. Therefore, the influenza vaccine
is updated on a yearly basis to keep up with the changes in influenza
viruses




regards
Jill Bowis

Pure bred utility chickens and ducks
Housing; Equipment, Books, Videos, Gifts
Herbaceous; Herb and Alpine nursery
Working Holidays in Scotland
http://www.kintaline.co.uk




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