Highly pathogenic avian influenza
In
response to the implications of the avian influenza on economically significant
chicken farms, a classification scheme developed in 1981 divided avian virus
strains into either highly pathogenic (and possibly warranted vigorous control
measures) or low pathogenic. Testing for classification is only based on the
effect on chickens – a strain of virus is highly pathogenic avian influenza
(HPAI) if 75% or greater of chickens die from experimental infection; otherwise
it is classified as low pathogenic avian influenza (LPAI). This classification
scheme has been updated to consider the virus' haemagglutinin protein
architecture. Other species of birds, especially water birds, can become
infected with HPAI virus without exhibiting severe symptoms and they can then
disseminate the infection over some distance; the symptoms will be species and
strain of virus specific. Therefore, the classification of an avian virus
strain as HPAI or LPAI does not predict how grave disease would be should it
infect humans or other mammals.
open next page to continue reading
<><>
Virology
Avian influenza is a disease that primarily
affects birds but may also infect humans and other mammals caused by the
influenza A virus. Influenza A is an RNA virus with a genome based on
negative-sense, RNA segmented genome that encodes 11 viral genes. The virus
particle (or virion) is approximately 80-120 nanometers in diameter and may be
elliptic or filamentous. Studies suggest the virus can remain infectious for
long periods when released in feces by its avian host and can survive thawing
from prolonged freezing.
The viral envelope contains two proteins:
hemagglutinin (H) and neuraminidase (N). H and N are the most significant
antigens of the infectious agent to which neutralizing antibodies are produced.
Outbreaks and epizootics of influenza viruses have been associated with changes
in the antigenic properties of H and N.
H is an antigenic glycoprotein that facilitates
binding and the entry of the virus into a host cell. N is an antigenic
glycosylated enzyme that allows the release of progeny viruses from infected
cells. There are 18 known types of hemagglutinin, of which H1 through H16 have
been identified in birds, and 11 types of neuraminidase.
open next page to continue reading
<><>
Subtypes
open next page to continue reading
<><>
Influenza virus nomenclature
- A stands for the genus of influenza (A,B,C)
- chicken - which animal species, the isolate is found in? (Human isolates do not have this component term and are therefore identified by default as human)
- Nakorn-Patom/Thailand the location the specific virus has isolated
- CU-K2 the laboratory reference, to identify it from, other influenza viruses isolated from the same location, in the same year
- 04, the year of isolation in this or context, of 2004.
- H5- stands for the fifth of several known types of the protein hemagglutinin
- N1- stands for the first of several known types of the protein neuraminidase.